Publications by authors named "Subhas Samanta"

48 Publications

Polymerisation of styrene using pincer type amine functionalized azo aromatic complexes of Co(II) as catalysts.

Dalton Trans 2022 Jan 6. Epub 2022 Jan 6.

Department of Chemistry, Indian Institute of Technology Jammu, Jagti, Jammu 181221, India.

In the present report, three mononuclear azo-aromatic complexes of Co(II), 1-3, and an imine-based Co(II) complex, 4, were synthesized through a reaction of respective amine-functionalized pincer-like ligands, HL1-4, with CoCl·6HO in the ligand-to-metal ratio of 1 : 1. All the complexes, 1-4, were thoroughly characterized using various physicochemical characterization techniques, single-crystal X-ray structure determination, and density functional theory (DFT) calculations. Complexes 1-4 were explored for the catalytic styrene polymerisation reaction separately in the presence of modified methyl aluminoxane (MMAO). All the complexes, 1-4, are indeed active for the polymerisation of styrene under mild conditions at room temperature upon activation with MMAO. Among the azo-aromatic complexes 1-3, complex 3 is the most efficient. The activity of the imine complex 4 is poor compared to those of the azo-aromatic complexes 1-3. The weight average molecular weight () of the isolated polystyrene ranges from 32.9 to 144.0 kg mol, with a polydispersity index () in the range of 1.1-1.8. Microstructural analysis of the isolated polymer from complexes 1-4 was carried out by C NMR spectroscopy, infrared spectroscopy, and powder X-ray diffraction studies. Their thermal properties were scrutinized by differential scanning calorimetry and thermogravimetric analysis. These studies have shown the atactic and amorphous nature of the polymers. The mechanical strength of the polymers was measured by a nanoindentation technique which has shown the good plastic/soft nature of the polymers.
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http://dx.doi.org/10.1039/d1dt02622dDOI Listing
January 2022

Azide-Alkyne "Click" Reaction in Water Using Parts-Per-Million Amine-Functionalized Azoaromatic Cu(I) Complex as Catalyst: Effect of the Amine Side Arm.

Inorg Chem 2021 Dec 21;60(23):17537-17554. Epub 2021 Nov 21.

Department of Chemistry, Indian Institute of Technology Jammu, Jagti, Jammu, India 181221.

A series of Cu(II) complexes, - and , were synthesized through a reaction of amine-functionalized pincer-like ligands, , , and a bidentate ligand with CuCl·2HO. The chemical reduction of complex using 1 equiv of sodium l-ascorbate resulted in a dimeric Cu(I) complex in excellent yield. All of the complexes, -, were thoroughly characterized using various physicochemical characterization techniques, single-crystal X-ray structure determination, and density functional theory calculations. Ligands and behaved as tridentated donors by the coordination of the amine side arm in their respective Cu(II) complexes, and the amine side arm remained as a pendant in Cu(I) complexes. All of these complexes (-) were explored for copper(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition (CuAAC) reaction at room temperature in water under air. Complex directly served as an active catalyst; however, complexes - and required 1 equiv of sodium l-ascorbate to generate their corresponding active Cu(I) catalyst. It has been observed that azo-based ligand-containing Cu(I)-complexes are air-stable and were highly efficient for the CuAAC reaction. The amine side arm in the ligand backbone has a dramatic role in accelerating the reaction rate. Mechanistic investigations showed that the alkyne C-H deprotonation was the rate-limiting step and the pendant amine side arm intramolecularly served as a base for Cu-coordinated alkyne deprotonation, leading to the azide-alkyne 2 + 3 cycloaddition reaction. Thus, variation of the amine side arm in complexes - and use of the most basic diisopropyl amine moiety in complex has resulted in an unique amine-functionalized azoaromatic Cu(I) system for CuAAC reaction upon sodium l-ascorbate reduction. The complex has shown excellent catalysis at its low parts-per-million level loading in water. The catalytic protocol was versatile and exhibited very good functional group tolerance. It was also employed efficiently to synthesize a number of useful functional triazoles having medicinal, catalytic, and targeting properties.
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http://dx.doi.org/10.1021/acs.inorgchem.1c02115DOI Listing
December 2021

Irreversible Resistive State Switching in Devices with a Homoleptic Cobalt(II) Complex Active Layer.

Chem Asian J 2021 Jun 10;16(12):1545-1552. Epub 2021 May 10.

Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, West Bengal, India.

Molecules with bi-stable electronic transport behaviour have been in upfront research topics of the molecular semiconductor devices in the past few decades due to the use of such materials in resistive data storage devices. Transition metal complexes (TMC) are expected to be potential candidates in regard to the tunable and manifold redox behaviour expecting multiple bulk transport states. Finding alternate mechanisms in such devices with TMC as the active layer materials would revoke the multifaceted approach to the functional gain. We have succeeded in demonstrating write once-read many (WORM) type of resistive memory device using a homoleptic Cobalt(II) (Co(II)) complex with large on/off current ratio ensuring the easy readout process at lower voltage. The advantage of this device was the turn on voltage was found to be the low (<2.7 V) operational voltage and the success ratio of the devices were more than 83%. The durability of the stored data was found to be more than 35,000 seconds which ensures the stability of the bistable state in the fabricated devices. Such ambient stable, solution processable devices are important for the large-scale printable devices. The manuscript describes the preparation, optical and electrochemical characterisation of the metal complex used along with a detailed mechanistic investigations and electrical characterisation of memory device obtained from a stable cobalt complex.
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http://dx.doi.org/10.1002/asia.202100152DOI Listing
June 2021

Robust bi-directional photoswitching of thiomethyl substituted arylazopyrazoles under visible light.

Chem Commun (Camb) 2020 Sep;56(70):10247-10250

Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata 700009, West Bengal, India.

Mono-ortho- and para-thiomethyl-substituted arylazopyrazoles show substantial absorbance in visible wavelengths, enabling very high to near-quantitative forward and reverse isomerization in DMSO and aqueous solution by visible light. cis isomers display excellent thermal stability (∼3 days, 27 °C). Additionally, these switches possess good photostability and are resistant to reduction by glutathione.
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http://dx.doi.org/10.1039/d0cc04098cDOI Listing
September 2020

Dehydrogenation of amines in aryl-amine functionalized pincer-like nitrogen-donor redox non-innocent ligands via ligand reduction on a Ni(ii) template.

Dalton Trans 2020 May;49(20):6816-6831

Department of Chemistry, Indian Institute of Technology Jammu, Jammu 181221, India.

We have synthesized a series of new redox non-innocent azo aromatic pincer-like ligands: 2-(phenylazo)-6-(arylaminomethyl)pyridine (HLa-c: HLa = 2-(phenylazo)-6-(2,6-diisopropylphenylaminomethyl)pyridine, HLb = 2-(phenylazo)-6-(2,6-dimethylphenylaminomethyl)pyridine, and HLc = 2-(phenylazo)-6-(phenylaminomethyl)pyridine), in which one side arm is an arylaminomethyl moiety and the other arm is a 2-phenylazo moiety. Nickel(ii) complexes, 1-3, of these ligands HLa-c were synthesized in good yield (approximately 70%) by the reaction of ligands : (NiCl2·6H2O) in a 1 : 1 molar ratio in methanol. The amine donor in each of the ligands HLa-c binds to the Ni(ii) centre without deprotonation. In the solid state, complex 3 is a dimer; in solution it exists as monomer 3a. The reduction of acetonitrile solutions of each of the complexes 1, 2 and 3a, separately, with cobaltocene (1 equivalent), followed by exposure of the solution to air, resulted in the formation of new complexes 7, 8 and 9, respectively. Novel free ligands Lx and Ly have also been isolated, in addition to complexes 7 and 8, from the reaction of complexes 1 and 2, respectively. Complexes 7-9 and free ligands Lx and Ly have been formed via a dehydrogenation reaction of the arylaminomethyl side arm. The mechanism of the reaction was thoroughly investigated using a series of studies, including cyclic voltammetry, EPR, and UV-Vis spectral studies and density functional theory (DFT) calculations. The results of these studies suggest a mechanism initiated by ligand reduction followed by dioxygen activation. A Cl-/I- scrambling experiment revealed that the dissociation of the chloride ligand(s) was associated with the one-electron reduction of the ligand (azo moiety) in each of the complexes 1, 2 and 3a. The dissociated chloride ligand(s) were reassociated with the metal following the dehydrogenation reaction to yield the final products. All of the newly synthesized compounds were fully characterized using a variety of physicochemical techniques. Single-crystal X-ray structures of the representative compounds were determined to confirm the identities of the synthesized molecules.
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http://dx.doi.org/10.1039/d0dt00466aDOI Listing
May 2020

Reversible and Tunable Photoswitching of Protein Function through Genetic Encoding of Azobenzene Amino Acids in Mammalian Cells.

Chembiochem 2018 10 2;19(20):2178-2185. Epub 2018 Oct 2.

University of Pittsburgh, Department of Chemistry, Pittsburgh, PA, 15260, USA.

The genetic encoding of three different azobenzene phenylalanines with different photochemical properties was achieved in human cells by using an engineered pyrrolysyl tRNA/tRNA synthetase pair. In order to demonstrate reversible light control of protein function, azobenzenes were site-specifically introduced into firefly luciferase. Computational strategies were applied to guide the selection of potential photoswitchable sites that lead to a reversibly controlled luciferase enzyme. In addition, the new azobenzene analogues provide enhanced thermal stability, high photoconversion, and responsiveness to visible light. These small-molecule photoswitches can reversibly photocontrol protein function with excellent spatiotemporal resolution, and preferred sites for incorporation can be computationally determined, thus providing a new tool for investigating biological processes.
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http://dx.doi.org/10.1002/cbic.201800226DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6540996PMC
October 2018

Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer.

Inorg Chem 2018 Oct 12;57(19):11995-12009. Epub 2018 Sep 12.

Department of Inorganic Chemistry , Indian Association for the Cultivation of Science , Jadavpur, Kolkata 700032 , India.

In this work, the effect of the electronically different ancillary ligands on the overall properties of the RuL moiety (L = 2,6-bis(phenylazo)pyridine) in heteroleptic complexes of general formula [RuLQCl] was investigated. Four different ancillary ligands (Q) with different electronic effects were used to prepare the heteroleptic compounds from the precursor complex, [RuL(CHCN)Cl] (1); Q = pcp: 2-(4-chloro-phenylazo)pyridine (strong π-acceptor), [2]; bpy: 2,2'-bipyridyl (moderate π-acceptor), [3]; acac: acetylacetonate (strong σ-donor), 4; and DTBCat: 3,5-di- tert-butyl catecholate (strong π-donor), 5. The complexes [2], [3], 4, and 5 were fully characterized and structurally identified. The electronic structures of these complexes along with their redox partners were elucidated by using a host of physical measurements: nuclear magnetic resonance, cyclic voltammetry, electronic paramagnetic resonance, UV-vis-NIR spectroscopy, and density functional theory. The studies revealed significant effects of the coligands on azo bond lengths of the RuL moiety and their redox behavior. Aerobic alcohol oxidation reactions using these Ru complexes as catalysts were scrutinized. It was found that the catalytic efficiency is primarily controlled by the electronic effect of the coligand. Accordingly, the complex [2] (containing a strong π-acceptor coligand, pcp) brings about oxidation efficiently, producing 86% of benzaldehyde. In comparison, however, the complexes 4 and 5 (containing electron donating coligand) furnished only 15-20% of benzaldehyde under identical reaction conditions. Investigations of the reaction mechanism suggest that an unstable Ru-H species is formed, which is transformed to a Ru-hydrazo intermediate by H-walking as reported by Hall et al. ( J. Am. Chem. Soc., 2015, 137, 12330). Aerial O regenerates the catalyst via oxidation of the hydrazo intermediate.
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http://dx.doi.org/10.1021/acs.inorgchem.8b01558DOI Listing
October 2018

Genetic Code Expansion in Zebrafish Embryos and Its Application to Optical Control of Cell Signaling.

J Am Chem Soc 2017 07 28;139(27):9100-9103. Epub 2017 Jun 28.

Department of Chemistry, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.

Site-specific incorporation of unnatural amino acids into proteins provides a powerful tool to study protein function. Here we report genetic code expansion in zebrafish embryos and its application to the optogenetic control of cell signaling. We genetically encoded four unnatural amino acids with a diverse set of functional groups, which included a photocaged lysine that was applied to the light-activation of luciferase and kinase activity. This approach enables versatile manipulation of protein function in live zebrafish embryos, a transparent and commonly used model organism to study embryonic development.
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http://dx.doi.org/10.1021/jacs.7b02145DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6022368PMC
July 2017

Role of Mediator and Effects of Temperature on ortho-C-N Bond Fusion Reactions of Aniline Using Ruthenium Templates: Isolation and Characterization of New Ruthenium Complexes of the in-Situ-Generated Ligands.

Inorg Chem 2017 May 20;56(9):4966-4977. Epub 2017 Apr 20.

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science , Kolkata 700 032, India.

In this work, ortho-C-N bond fusion reactions of aniline are followed by the use of two different ruthenium mediators. Reaction of aniline with [Ru(terpy)Cl] (terpy = 2,2':6',2″-terpyridine) resulted in a trans bis-aniline ruthenium(II) complex [1] which upon oxidation with HO produced compound [2] of a bidentate ligand, N-phenyl-1,2-benzoquinonediimine, due to an oxidative ortho-C-N bond fusion reaction. Complex [1] and aniline (neat) at 185 °C produced a bis-chelated ruthenium complex (3). A previously reported complex [Ru(N-phenyl-1,2-benzoquinonediimine)(aniline)(Cl)] (5) undergoes similar oxidation by air at 185 °C to produce complex [3]. A separate chemical reaction between aniline and strongly oxidizing tetra-n-propylammonium perruthenate [(n-pr)N][RuO] in air produced a ruthenium complex [4] of a N-tetraamidophenylmacrocycle ligand via multiple ortho-C-N bond fusion reaction. Notably, the yield of this product is low (5%) at 100 °C but increases to 25% in refluxing aniline. All these complexes are characterized fully by their physicochemical characterizations and X-ray structure determination. From their structural parameters and other spectroscopic studies, complex [2] is assigned as [Ru(terpy)(N-phenyl-1,2-benzoquinonediimine)(Cl)] whereas complex [4] is described as a ruthenium(VI) complex comprised of a reduced deprotonated N-phenyl-1,2-diamidobenzene and N-tetraamidophenylmacrocyclic ligand. Complex [2] exhibits one reversible oxidation at 1.32 V and one reversible reduction at -0.75 V vs Ag/AgCl reference electrode. EPR of the electrogenerated complexes has revealed that the oxidized complex is a ruthenium(III) complex with an axial EPR spectrum at g= 2.06. The reduced complex [2], on the other hand, shows a single-line EPR signal at g= 1.998. In contrast, complex [4] shows two successive one-electron oxidation waves at 0.5 and 0.8 V and an irreversible reduction wave at -0.9 V. EPR studies of the oxidized complexes [4] and [4] reveal that oxidations are ligand centered. DFT calculations were employed to elucidate the electronic structures as well as the redox processes associated with the above complexes. Aerial ortho-C-N bond fusion reactions of aniline using two different mediators, viz. [Ru(terpy)Cl] and [(n-pr)N][RuO], have been followed. It is found that in the case of oxidizable Ru(III) mediator complex, C-N bond fusion is limited only to dimerization reaction whereas the high-valent Ru(VII) salt mediates multiple C-N bond fusion reactions leading to the formation of a novel tetradentate N-tetraamidophenylmacrocyclic ligand. Valence ambiguity in the complexes of the resultant redox-active ligands is scrutinized.
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http://dx.doi.org/10.1021/acs.inorgchem.6b03163DOI Listing
May 2017

Optical Control of DNA Helicase Function through Genetic Code Expansion.

Chembiochem 2017 03 25;18(5):466-469. Epub 2017 Jan 25.

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania, 15260, USA.

Nucleotide excision repair (NER) is a general DNA repair mechanism that is capable of removing a wide variety of DNA lesions induced by physical or chemical insults. UvrD, a member of the helicase SF1 superfamily, plays an essential role in bacterial NER by unwinding the duplex DNA in the 3' to 5' direction to displace the lesion-containing strand. In order to achieve conditional control over NER, we generated a light-activated DNA helicase. This was achieved through a site-specific incorporation of a genetically encoded hydroxycoumarin lysine at a crucial position in the ATP-binding pocket of UvrD. The resulting caged enzyme was completely inactive in several functional assays. Moreover, enzymatic activity of the optically triggered UvrD was comparable to that of the wild-type protein, thus demonstrating excellent OFF to ON switching of the helicase. The developed approach provides optical control of NER, thereby laying a foundation for the regulation of ATP-dependent helicase functions in higher organisms. In addition, this methodology is applicable to the light-activation of a wide range of ATPases.
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http://dx.doi.org/10.1002/cbic.201600624DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5516474PMC
March 2017

Regioselective ortho Amination of Coordinated 2-(Arylazo)pyridine. Isolation of Monoradical Palladium Complexes of a New Series of Azo-Aromatic Pincer Ligands.

Inorg Chem 2015 Dec 12;54(23):11465-76. Epub 2015 Nov 12.

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science , Jadavpur, Kolkata 700 032, India.

In an unusual reaction of [Pd(L(1))Cl2] (L(1) = 2-(arylazo)pyridine) with amines, a new series of palladium complexes [Pd(L(2•-))Cl] (L(2) = 2-((2-amino)arylazo)pyridine) (1a-1h) were isolated. The complexes were formed via N-H and N-C bond cleavage reactions of 1°/2° and 3° amines, respectively, followed by regioselective aromatic ortho-C-N bond formation reaction and are associated with ortho-C-H/ortho-C-Cl bond activation. A large variety of amines including both aromatic and aliphatic were found to be effective in producing air-stable complexes. Identity of the resultant complexes was confirmed by their X-ray structure determination. Efforts were also made to understand the mechanism of the reaction. A series of experiments were performed, which point toward initial ligand reduction followed by intraligand electron transfer. Examination of the structural parameters of these complexes (1) indicates that the in situ generated ligand coordinated to the Pd(II) center serves as the backbone of these air-stable monoradical complexes. Molecular and electronic structures of the isolated complexes were further scrutinized by various spectroscopic techniques including cyclic voltammetry, variable temperature magnetic susceptibility measurements, electron paramagnetic resonance, and UV-vis spectroscopy. Finally the electronic structure was confirmed by density functional theory calculations. The isolated monoradical complexes adopt an unusual π-stacked array, which leads to a relatively strong antiferromagnetic interaction (J = -40 cm(-1) for the representative complex 1c).
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http://dx.doi.org/10.1021/acs.inorgchem.5b02110DOI Listing
December 2015

Red-Shifting Azobenzene Photoswitches for in Vivo Use.

Acc Chem Res 2015 Oct 28;48(10):2662-70. Epub 2015 Sep 28.

Department of Chemistry, University of Toronto , 80 St. George St., Toronto, ON M5S 3H6, Canada.

Recently, there has been a great deal of interest in using the photoisomerization of azobenzene compounds to control specific biological targets in vivo. These azo compounds can be used as research tools or, in principle, could act as optically controlled drugs. Such "photopharmaceuticals" offer the prospect of targeted drug action and an unprecedented degree of temporal control. A key feature of azo compounds designed to photoswitch in vivo is the wavelength of light required to cause the photoisomerization. To pass through tissue such as the human hand, wavelengths in the red, far-red, or ideally near infrared region are required. This Account describes our attempts to produce such azo compounds. Introducing electron-donating or push/pull substituents at the para positions delocalizes the azobenzene chromophore and leads to long wavelength absorption but usually also lowers the thermal barrier to interconversion of the isomers. Fast thermal relaxation means it is difficult to produce a large steady state fraction of the cis isomer. Thus, specifically activating or inhibiting a biological process with the cis isomer would require an impractically bright light source. We have found that introducing substituents at all four ortho positions leads to azo compounds with a number of unusual properties that are useful for in vivo photoswitching. When the para substituents are amide groups, these tetra-ortho substituted azo compounds show unusually slow thermal relaxation rates and enhanced separation of n-π* transitions of cis and trans isomers compared to analogues without ortho substituents. When para positions are substituted with amino groups, ortho methoxy groups greatly stabilize the azonium form of the compounds, in which the azo group is protonated. Azonium ions absorb strongly in the red region of the spectrum and can reach into the near-IR. These azonium ions can exhibit robust cis-trans isomerization in aqueous solutions at neutral pH. By varying the nature of ortho substituents, together with the number and nature of meta and para substituents, long wavelength switching, stability to photobleaching, stability to hydrolysis, and stability to reduction by thiols can all be crafted into a photoswitch. Some of these newly developed photoswitches can be used in whole blood and show promise for effective use in vivo. It is hoped they can be combined with appropriate bioactive targets to realize the potential of photopharmacology.
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http://dx.doi.org/10.1021/acs.accounts.5b00270DOI Listing
October 2015

Conditional control of alternative splicing through light-triggered splice-switching oligonucleotides.

J Am Chem Soc 2015 Mar 3;137(10):3656-62. Epub 2015 Mar 3.

†Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States.

The spliceosome machinery is composed of several proteins and multiple small RNA molecules that are involved in gene regulation through the removal of introns from pre-mRNAs in order to assemble exon-based mRNA containing protein-coding sequences. Splice-switching oligonucleotides (SSOs) are genetic control elements that can be used to specifically control the expression of genes through correction of aberrant splicing pathways. A current limitation with SSO methodologies is the inability to achieve conditional control of their function paired with high spatial and temporal resolution. We addressed this limitation through site-specific installation of light-removable nucleobase-caging groups as well as photocleavable backbone linkers into synthetic SSOs. This enables optochemical OFF → ON and ON → OFF switching of their activity and thus precise control of alternative splicing. The use of light as a regulatory element allows for tight spatial and temporal control of splice switching in mammalian cells and animals.
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http://dx.doi.org/10.1021/jacs.5b00580DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5545098PMC
March 2015

Light-cleavable rapamycin dimer as an optical trigger for protein dimerization.

Chem Commun (Camb) 2015 Apr;51(26):5702-5

Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA.

Rapamycin-induced protein heterodimerization of FKBP12 and FRB is one of the most commonly employed switches to conditionally control biological processes. We developed an optically activated rapamycin dimer that does not induce FKBP12-FRB dimerization until exposed to light, and applied it to control kinase, protease, and recombinase function.
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http://dx.doi.org/10.1039/c4cc09442eDOI Listing
April 2015

A two-in-one pincer ligand and its diiron(II) complex showing spin state switching in solution through reversible ligand exchange.

Angew Chem Int Ed Engl 2015 Jan 20;54(2):583-7. Epub 2014 Nov 20.

Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstrasse 4, 37077 Göttingen (Germany) http://www.meyer.chemie.uni-goettingen.de.

A novel pyrazolate-bridged ligand providing two {PNN} pincer-type compartments has been synthesized. Its diiron(II) complex LFe2(OTf)3(CH3CN) (1; Tf = triflate) features, in solid state, two bridging triflate ligands, with a terminal triflate and a MeCN ligand completing the octahedral coordination spheres of the two high-spin metal ions. In MeCN solution, 1 is shown to undergo a sequential, reversible, and complete spin transition to the low-spin state upon cooling. Detailed UV/Vis and (19)F NMR spectroscopic studies as well as magnetic measurements have unraveled that spin state switching correlates with a rapid multistep triflate/MeCN ligand exchange equilibrium. The spin transition temperature can be continuously tuned by varying the triflate concentration in solution.
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http://dx.doi.org/10.1002/anie.201408966DOI Listing
January 2015

Excited-state properties of fluorenones: influence of substituents, solvent and macrocyclic encapsulation.

Phys Chem Chem Phys 2014 Aug;16(31):16436-45

Jacobs University Bremen, Campus Ring 1, D-28759 Bremen, Germany.

A comprehensive investigation of the photophysics of a broad set of fluorenones substituted with methoxy groups at different positions brings out the importance of the location of substituents on the fluorenone core in modulating fluorescence and radiationless deactivation by way of modification of the singlet-excited state energy and its character. While the substituents at para positions are found to affect neither the fluorescence quantum yield nor the lifetime, those at meta positions are found to significantly modify the latter. A cumulative effect is observed for the substituents in that the nonradiative decay (knr) becomes progressively dominant with an increasing number of meta-methoxy substituents. For example, the trimethoxy substitution in 2,4,7-trimethoxyfluorenone (8) is found to increase knr by ca. 30 fold relative to that of the parent fluorenone (1) in a polar aprotic solvent such as acetonitrile. The predominance of nonradiative decay (knr) is rationalized from stabilization of the singlet-excited state via partial charge transfer from meta-methoxy substituents to the carbonyl group. Accordingly, a nice correlation is observed for the nonradiative (knr) rate constants versus singlet-excitation energies derived from fluorescence emission maxima of all fluorenones in acetonitrile. The macrocyclic host cucurbit[7]uril (CB7) is found to not only enhance the fluorescence of the parent fluorenone (1) significantly, but also increase the singlet lifetime considerably. Based on the changes observed in the absorption spectra and the lifetimes determined, a 1:1 host–guest complex has been proposed with CB7. The fluorescence lifetime observed in the presence of CB7 suggests that the hydrophobic fluorenone (1) can be employed as a probe to report on a polar microenvironment shielded from hydrogen bonding interactions in a polar protic solvent.
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http://dx.doi.org/10.1039/c4cp01724bDOI Listing
August 2014

Genetic encoding of caged cysteine and caged homocysteine in bacterial and mammalian cells.

Chembiochem 2014 Aug 27;15(12):1793-9. Epub 2014 Jun 27.

Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, NC 27695 (USA).

We report the genetic incorporation of caged cysteine and caged homocysteine into proteins in bacterial and mammalian cells. The genetic code of these cells was expanded with an engineered pyrrolysine tRNA/tRNA synthetase pair that accepts both light-activatable amino acids as substrates. Incorporation was validated by reporter assays, western blots, and mass spectrometry, and differences in incorporation efficiency were explained by molecular modeling of synthetase-amino acid interactions. As a proof-of-principle application, the genetic replacement of an active-site cysteine residue with a caged cysteine residue in Renilla luciferase led to a complete loss of enzyme activity; however, upon brief exposure to UV light, a >150-fold increase in enzymatic activity was observed, thus showcasing the applicability of the caged cysteine in live human cells. A simultaneously conducted genetic replacement with homocysteine yielded an enzyme with greatly reduced activity, thereby demonstrating the precise probing of a protein active site. These discoveries provide a new tool for the optochemical control of protein function in mammalian cells and expand the set of genetically encoded unnatural amino acids.
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http://dx.doi.org/10.1002/cbic.201400073DOI Listing
August 2014

Introducing a new azoaromatic pincer ligand. Isolation and characterization of redox events in its ferrous complexes.

Inorg Chem 2014 May 17;53(9):4678-86. Epub 2014 Apr 17.

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science , Jadavpur, Kolkata 700032, India.

The isolation and complete characterization of a new bis-azoaromatic ligand, 2,6-bis(phenylazo)pyridine (L), are described, and its coordination to iron(II) is reported. A pseudo-trigonal-bipyramidal mixed-ligand complex of iron(II), FeLCl2 (1), and a homoleptic octahedral iron complex, mer-[Fe(L)2]ClO4 [2]ClO4, have been synthesized from L and FeCl2 or hydrated Fe(ClO4)2, respectively, in boiling methanol. Determination of the X-ray crystallographic structure together with magnetic data (≈ 5.06 μB) and Mössbauer analysis of 1 established a high-spin Fe(II) complex ligated by one neutral 2,6-bis(phenylazo)pyridine ligand. The X-ray crystallographic structure (showing dN-N > 1.30 Å), Mössbauer data, and magnetic susceptibility measurements (≈ 1.65 μB) as well as a nearly isotropic EPR signal with only a small metal contribution at g = 1.968, on the other hand, suggest a low-spin Fe(II) complex with a one-electron-reduced radical ligand coordination in [2]ClO4. The ligand and the metal complexes have well-behaved redox properties, with the ligand(s) functioning as the redox-active site(s) responsible for redox events. The uncoordinated ligand, L, displays a reversible one-electron wave at -1.07 V and a quasi-reversible wave at -1.39 V. The partially reduced ligand L(•-) shows a single-line EPR spectrum at g = 2.001, signifying that L(•-) is a free radical. While complex 1 shows a reversible reduction at -0.08 V and an irreversible cathodic response at -0.98 V, the bis-chelate [2]ClO4 undergoes a reversible one-electron oxidation at 0.54 V and three successive reversible one-electron reductions at -0.18, -0.88, and -1.2 V, all occurring at the ligands without affecting the metal ion oxidation state. The electronic structures of the parent monocationic complex [2](+) and its oxidized and reduced forms, generated by exhaustive electrolyses, have been characterized by using a host of spectroscopic techniques and density functional theory (DFT). It is found that the 2,6-bis(phenylazo)pyridine ligand (L) is truly redox noninnocent and is capable of coordinating transition-metal centers in its neutral ([L](0)), monoanionic monoradical ([L(•)](-)), and dianionic diradical ([L(••)](2-)) forms.
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http://dx.doi.org/10.1021/ic500355fDOI Listing
May 2014

Double C(arom)-H activation associated with etheral oxygen insertion to phenazine architecture in oxidisable ruthenium(III) complexes: a mechanistic insight.

Chemistry 2014 Mar 30;20(10):2712-7. Epub 2014 Jan 30.

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032 (India).

Three examples of unusual double aromatic CH bond activation associated with insertion of etheral oxygen atom to phenazine architecture in Ru(III) complexes are reported. The chemical transformations have led to the formation of new Ru(IV) complexes with angular pentacyclo heterocyclic ligands. A mechanistic investigation indicates that the overall process is a combination of successive steps involving air (O2 ) and H2 O.
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http://dx.doi.org/10.1002/chem.201303932DOI Listing
March 2014

Redox noninnocence in coordinated 2-(arylazo)pyridines: steric control of ligand-based redox processes in cobalt complexes.

Inorg Chem 2013 Dec 21;52(24):14040-9. Epub 2013 Nov 21.

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science , Kolkata 700 032, India.

A series of cobalt complexes of ligands based on the 2-(arylazo)pyridine architecture have been synthesized, and the precise structure and stoichiometry of the complexes depend critically on the identity of substituents in the 2, 4, and 6 positions of the phenyl ring. The 2-(arylazo)pyridine motif can support either Co(II) complexes with neutral ligands, Co(II)Cl2(L(a))2 (1), Co(II)Cl2(L(c))2 (3), [Co(II)Cl(L(b))2]2(PF6)2 (5[PF6]2), or Co(III) complexes of reduced 2-(arylazo)pyridine ligand radical anions, L(•-), Co(III)Cl(L(b•-))2 (2), Co(III)Cl(L(c•-))2 (4), and Co(III)Me(L(b•-))2 (6). All three members of the latter class are based on approximately trigonal-bipyramidal CoX(L(•-))2 architectures [L = 2-(arylazo)pyridine] with two azo nitrogen atoms and the X ligand (X = Cl or Me) in the equatorial plane and two pyridine nitrogen atoms occupying axial positions. Density functional theory suggests that the electronic structure of the Co(III) complexes is also dependent on the identity of X: the strong σ-donor methyl gives a low-spin (S = 0) configuration, while the σ/π-donor chloro gives an intermediate-spin (S = 1) local configuration. In certain cases, one-electron reduction of the Co(II)X2L2 complex leads to the formation of Co(III)X(L(•-))2; i.e., reduction of one ligand induces a further one-electron oxidation of the metal center with concomitant reduction of the second ligand.
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http://dx.doi.org/10.1021/ic4018079DOI Listing
December 2013

Photoswitching of ortho-substituted azonium ions by red light in whole blood.

Angew Chem Int Ed Engl 2013 Dec 8;52(52):14127-30. Epub 2013 Nov 8.

Departement of Chemistry, University of Toronto, 80 St. George St., Toronto, ON M5S 3H6 (Canada).

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http://dx.doi.org/10.1002/anie.201306352DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3883038PMC
December 2013

Robust visible light photoswitching with ortho-thiol substituted azobenzenes.

Chem Commun (Camb) 2013 Nov;49(87):10314-6

Department of Chemistry, University of Toronto, 80 St. George St. Toronto, ON M5S 3H6, Canada.

Introduction of S-ethyl groups in all four ortho positions of azobenzene prevents reduction of the azo group by intracellular glutathione, while enhancing the absorptivity to ~10,000 M(-1) cm(-1) in the blue and green regions of the visible spectrum. cis-to-trans isomerization occurs thermally on the minutes timescale. Further, this substitution pattern permits switching with red light, a color that is more penetrating through biological tissues than other parts of the visible spectrum.
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http://dx.doi.org/10.1039/c3cc46045bDOI Listing
November 2013

Photoswitching azo compounds in vivo with red light.

J Am Chem Soc 2013 Jul 21;135(26):9777-84. Epub 2013 Jun 21.

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.

The photoisomerization of azobenzenes provides a general means for the photocontrol of molecular structure and function. For applications in vivo, however, the wavelength of irradiation required for trans-to-cis isomerization of azobenzenes is critical since UV and most visible wavelengths are strongly scattered by cells and tissues. We report here that azobenzene compounds in which all four positions ortho to the azo group are substituted with bulky electron-rich substituents can be effectively isomerized with red light (630-660 nm), a wavelength range that is orders of magnitude more penetrating through tissue than other parts of the visible spectrum. When the ortho substituent is chloro, the compounds also exhibit stability to reduction by glutathione, enabling their use in intracellular environments in vivo.
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http://dx.doi.org/10.1021/ja402220tDOI Listing
July 2013

Helicity as a steric force: stabilization and helicity-dependent reversion of colored o-quinonoid intermediates of helical chromenes.

J Am Chem Soc 2013 May 23;135(18):6872-84. Epub 2013 Apr 23.

Department of Chemistry, Indian Institute of Technology, Kanpur, India.

Photolysis of regioisomeric helical chromenes 1 and 2 leads to colored reactive intermediates. While the latter generally decay quite rapidly, they are found to be longer lived in 1 and highly persistent in 2. The remarkable stability of the otherwise fleeting transient in 2 allowed isolation and structural characterization by X-ray crystallography. The structural analyses revealed that steric force inherent to the helical scaffold is the origin of stability as well as differentiation in the persistence of the intermediates of 1 and 2 (1Q and 2Q). The structure further shows that diphenylvinyl moiety in the TT isomer of 2Q gets splayed over the helical scaffold such that it is fraught with a huge steric strain to undergo required bond rotations to regenerate the precursor chromene. Otherwise, reversion of 2Q was found to occur at higher temperatures. Aazahelical chromenes 3 and 4 with varying magnitudes of helicity were designed in pursuit of o-quinonoid intermediates with graded activation barriers. Their photogenerated intermediates 3Q and 4Q were also isolated and structurally characterized. The activation barriers for thermal reversion of 2Q-4Q, as determined from Arrhenius and Eyring plots, are found to correlate nicely with the helical turn, which decisively determines the steric force. The exploitation of helicity is thus demonstrated to develop a novel set of photoresponsive helicenes 2-4 that lead to colored intermediates exhibiting graded stability. It is further shown that the photochromism of 2-4 in conjunction with response of 2Q-4Q to external stimuli (acid, heat, and visible radiation) permits development of molecular logic gates with INHIBIT function.
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http://dx.doi.org/10.1021/ja312027cDOI Listing
May 2013

A bisazobenzene crosslinker that isomerizes with visible light.

Beilstein J Org Chem 2012 14;8:2184-90. Epub 2012 Dec 14.

Department of Chemistry, University of Toronto, 80 St. George St., Toronto, ON, M5S 3H6, Canada; telephone: (416) 978-0675, fax: (416) 978-8775.

Background: Large conformational and functional changes of azobenzene-modified biomolecules require longer azobenzene derivatives that undergo large end-to-end distance changes upon photoisomerization. In addition, isomerization that occurs with visible rather than UV irradiation is preferred for biological applications.

Results: We report the synthesis and characterization of a new crosslinker in which a central piperazine unit links two azobenzene chromophores. Molecular modeling indicates that this crosslinker can undergo a large change in end-to-end distance upon trans,trans to cis,cis isomerization. Photochemical characterization indicates that it does isomerize with visible light (violet to blue wavelengths). However, the thermal relaxation rate of this crosslinker is rather high (τ(½) ~ 1 s in aqueous buffer at neutral pH) so that it is difficult to produce large fractions of the cis,cis-species without very bright light sources.

Conclusion: While cis-lifetimes may be longer when the crosslinker is attached to a biomolecule, it appears the para-piperazine unit may be best suited for applications where rapid thermal relaxation is required.
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http://dx.doi.org/10.3762/bjoc.8.246DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3554325PMC
January 2013

Aerial oxidation of protonated aromatic amines. Isolation, X-ray structure, and redox and spectral characteristics of N-containing dyes.

J Org Chem 2012 Nov 30;77(22):10249-59. Epub 2012 Oct 30.

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India.

This work reports the results of our investigation on the aerial oxidation of aromatic amines that are promoted by protic acid. While primary aromatic amines produce substituted phenazines as major products, N-phenyl-o-phenylenediamine produces polycyclic aromatic heterocycles like azaacene and secondary and tertiary amines give exclusively the dyes containing a triphenylmethane moiety. Isolation of the compounds and the effects of substitutions on the aromatic rings have been investigated. In this context, plausible reaction steps that are involved have been discussed. Single-crystal X-ray structure analyses of the representative compounds are solved to authenticate their formation. In almost every case, a high degree of delocalization of electron was noted. The compounds have been characterized thoroughly and show rich spectral properties. For example, the phenazine molecules exhibited absorption peaks between 475 and 605 nm because of the charge-transfer transition from the amine and tricyclopyrazine moiety. Their acidochromic and solvatochromic behaviors, which are supported by theoretical calculations, are investigated. The polycyclic azacene molecule exhibits strong absorption in the visible region and fluoresces with high quantum yield. The phenazine dyes undergo a quasi-reversible reduction at a low cathodic potential that varies linearly as a function of Hammett's constant.
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http://dx.doi.org/10.1021/jo3019126DOI Listing
November 2012

Quantitative analysis of the effects of photoswitchable distance constraints on the structure of a globular protein.

Biochemistry 2012 Aug 3;51(32):6421-31. Epub 2012 Aug 3.

Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada.

Photoswitchable distance constraints in the form of photoisomerizable chemical cross-links offer a general approach to the design of reversibly photocontrolled proteins. To apply these effectively, however, one must have guidelines for the choice of cross-linker structure and cross-linker attachment sites. Here we investigate the effects of varying cross-linker structure on the photocontrol of folding of the Fyn SH3 domain, a well-studied model protein. We develop a theoretical framework based on an explicit-chain model of protein folding, modified to include detailed model linkers, that allows prediction of the effect of a given linker on the free energy of folding of a protein. Using this framework, we were able to quantitatively explain the experimental result that a longer, but somewhat flexible, cross-linker is less destabilizing to the folded state than a shorter more rigid cross-linker. The models also suggest how misfolded states may be generated by cross-linking, providing a rationale for altered dynamics seen in nuclear magnetic resonance analyses of these proteins. The theoretical framework is readily portable to any protein of known folded state structure and thus can be used to guide the design of photoswitchable proteins generally.
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http://dx.doi.org/10.1021/bi300685aDOI Listing
August 2012

Bidirectional photocontrol of peptide conformation with a bridged azobenzene derivative.

Angew Chem Int Ed Engl 2012 Jun 29;51(26):6452-5. Epub 2012 May 29.

Dept. of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada.

It goes both ways: A thiol-reactive cross-linker based on a bridged azobenzene derivative permits photoreversible control of peptide conformation on irradiation with violet (407 nm) and green (500-550 nm) light (see picture) through isomerization of the cross-linker. The large separation of the absorbance bands of the cis (yellow) and trans (red) isomers enables complete bidirectional photoswitching.
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http://dx.doi.org/10.1002/anie.201202383DOI Listing
June 2012

Bis(acetonitrile)bis(acetylacetonato)ruthenium(III) mediated chemical transformations of coordinated 2-methylthioanilide.

Dalton Trans 2012 Jun 3;41(23):7057-66. Epub 2012 May 3.

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India.

Chemical reaction of [Ru(III)(acac)(2)(CH(3)CN)(2)]ClO(4) (1) with 2-methylthioaniline, HL(1) in ethanol under basic conditions yielded three new complexes Ru(II)(acac)(2)(L(1b)) (1b), (L(1b) = 4-imino-3-(methylsulfanyl)cyclohexa-2,5-dien-1-one), Ru(III)(acac)(2)(L(1c)) (1c), (HL(1c) = N-(2-methylthiophenyl)formamide) and (acac)(2)Ru(II)(μ-L(1d))Ru(II)(acac)(2) (1d), (L(1d) = 1,4-bis(2-methylthioaniline)-1,4-diazabutadiene) via the intermediate Ru(III)(acac)(2)(L(1a)) (1a, L(1a) = (L(1))(-) = 2-methylthioanilide). The reaction proceeded through temperature induced valence tautomerisation between the Ru(III) center and its 2-methylthioanilide counterpart in 1a with concomitant reduction of ruthenium from +III to +II oxidation state and oxidation of ligand L(1a), resulting in aromatic ring hydroxylation, N-formylation and C-C bond formation reactions. All the complexes have been characterised by their single-crystal X-ray structure determination and various spectroscopic and electrochemical techniques. The identity of complex 1a has been confirmed by X-ray crystal structure determination of complex 2, a phenyl analogue of complex 1a. The complexes (1a-d) showed intense charge transfer (MLCT/LMCT) transition in the long wavelength region. The paramagnetic compounds, 1a and 1c, along with the diamagnetic compound 1b showed two one-electron responses in the ranges, -0.4 to -1.0 V and 0.3 to 1.1 V. The diamagnetic complex 1d displayed two successive one-electron reversible reductions (-1.31 and -1.55 V) and two one-electron reversible oxidation processes (-0.036 and 0.51 V). The redox processes are characterized by EPR spectroscopy and spectroelectrochemistry. The compound 1c has been found to exhibit solvatochromism and concentration dependent aggregation in solution.
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http://dx.doi.org/10.1039/c2dt30344bDOI Listing
June 2012

Reversible photocontrol of peptide conformation with a rhodopsin-like photoswitch.

J Am Chem Soc 2012 Apr 16;134(16):6960-3. Epub 2012 Apr 16.

Departamento de Química, Unidad Asociada al C.S.I.C., Universidad de La Rioja, Madre de Dios 51, E-26006 Logroño, La Rioja, Spain.

Reversible photocontrol of biomolecules requires chromophores that can efficiently undergo large conformational changes upon exposure to wavelengths of light that are compatible with living systems. We designed a benzylidene-pyrroline chromophore that mimics the Schiff base of rhodopsin and can be used to introduce light-switchable intramolecular cross-links in peptides and proteins. This new class of photoswitch undergoes an ~10 Å change in end-to-end distance upon isomerization and can be used to control the conformation of a target peptide efficiently and reversibly using, alternately, violet (400 nm) and blue (446 nm) light.
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http://dx.doi.org/10.1021/ja301868pDOI Listing
April 2012
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