Publications by authors named "Biswajit Sadhu"

17 Publications

  • Page 1 of 1

Periodic trends and complexation chemistry of tetravalent actinide ions with a potential actinide decorporation agent 5-LIO(Me-3,2-HOPO): A relativistic density functional theory exploration.

J Comput Chem 2020 Jun 3;41(15):1427-1435. Epub 2020 Mar 3.

Health Physics Division, Health Safety and Environment Group, Bhabha Atomic Research Centre, Mumbai, India.

A relativistic density functional theory (DFT) study is reported which aims to understand the complexation chemistry of An ions (An = Th, U, Np, and Pu) with a potential decorporation agent, 5-LIO(Me-3,2-HOPO). The calculations show that the periodic change of the metal binding free energy has an excellent correlation with the ionic radii and such change of ionic radii also leads to the structural modulation of actinide-ligand complexes. The calculated structural and binding parameters agree well with the available experimental data. Atomic charges derived from quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) analysis shows the major role of ligand-to-metal charge transfer in the stability of the complexes. Energy decomposition analysis, QTAIM, and electron localization function (ELF) predict that the actinide-ligand bond is dominantly ionic, but the contribution of orbital interaction is considerable and increases from Th to Pu . A decomposition of orbital contributions applying the extended transition state-natural orbital chemical valence method points out the significant π-donation from the oxygen donor centers to the electron-poor actinide ion. Molecular orbital analysis suggests an increasing trend of orbital mixing in the context of 5f orbital participation across the tetravalent An series (Th-Pu). However, the corresponding overlap integral is found to be smaller than in the case of 6d orbital participation. An analysis of the results from the aforementioned electronic structure methods indicates that such orbital participation possibly arises due to the energy matching of ligand and metal orbitals and carries the signature of near-degeneracy driven covalency.
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http://dx.doi.org/10.1002/jcc.26186DOI Listing
June 2020

Enhancing Actinide(III) over Lanthanide(III) Selectivity through Hard-by-Soft Donor Substitution: Exploitation and Implication of Near-Degeneracy-Driven Covalency.

Inorg Chem 2019 Aug 25;58(15):9738-9748. Epub 2019 Jul 25.

Institute for Theoretical Chemistry , University of Cologne , Greinstrasse 4 , 50939 Cologne , Germany.

Soft donor ligands often provide higher selectivity for actinides(III) over chemically similar lanthanides(III), e.g., in the Am-Eu pair. Frequently, the origin of such selectivity is associated with an increased covalency in actinide-ligand bonding. However, the relationship between the degree of covalency and ion selectivity has yet to reach general consensus. Further, it is unclear whether the enhanced covalency leads to a thermodynamic stabilization of the complex or not. Using relativistic density functional theory, we have addressed these outstanding issues by analyzing the subtle change of metal-ligand interactions from a hard donor ligand to a mixed soft-hard one. The present comparative study on the structure of and binding in Am and Eu complexes with 3,4,3-LI(1,2-HOPO) (L) and its mixed-donor variant (LS) shows that the introduction of sulfur as a soft donor atom into the metal coordination sphere indeed infuses an Am selectivity into the otherwise nonselective ligand L but also leads to a significant reduction of the metal-binding Gibbs free energies. Natural population analysis, charge decomposition analysis, and its extended version point to the critical role of ligand-to-metal charge transfer in the overall thermodynamic stability of the complexes. A detailed energy decomposition analysis combining the extended transition state with the natural orbitals chemical valence method reveals an enhancement of the covalency upon switching to the soft-hard donor ligand because of the different nature of the metal-ligand interaction. The ligand L predominantly binds the metal via π donation, whereas the ligand LS prefers σ donation. Molecular orbital and quantum theory of atoms in molecules analyses as well as a comparison to a simple model system show that the covalency occurs as a result of orbital mixing and is near-degeneracy-driven in nature. This enhanced covalency, however, fails to thermodynamically compensate for the loss of strong electrostatic interaction and thus does not lead to an additional stabilization of the metal-LS complexes.
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http://dx.doi.org/10.1021/acs.inorgchem.9b00705DOI Listing
August 2019

Thorium decorporation efficacy of rationally-selected biocompatible compounds with relevance to human application.

J Hazard Mater 2019 03 13;365:952-961. Epub 2018 Nov 13.

Radiation Biology & Health Sciences Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400 085, India. Electronic address:

During civil, nuclear or defense activities, internal contamination of actinides in humans and mitigation of their toxic impacts are of serious concern. Considering the health hazards of thorium (Th) internalization, an attempt was made to examine the potential of ten rationally-selected compounds/formulations to decorporate Th ions from physiological systems. The Th-induced hemolysis assay with human erythrocytes revealed good potential of tiron, silibin (SLB), phytic acid (PA) and Liv.52 (L52) for Th decorporation, in comparison to diethylenetriaminepentaacetic acid, an FDA-approved decorporation drug. This was further validated by decorporation experiments with relevant human cell models (erythrocytes and liver cells) and biological fluid (blood) under pre-/post-treatment conditions, using inductively coupled plasma mass spectrometry (ICP-MS) and transmission electron microscopy (TEM). Furthermore, density functional theory-based calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy confirmed the formation of Th complex by these agents. Amongst the chosen biocompatible agents, tiron, SLB, PA and L52 hold promise to enhance Th decorporation for human application.
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http://dx.doi.org/10.1016/j.jhazmat.2018.11.038DOI Listing
March 2019

The coordination chemistry of lanthanide and actinide metal ions with hydroxypyridinone-based decorporation agents: orbital and density based analyses.

Dalton Trans 2018 Nov;47(46):16603-16615

Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India.

In the context of the mitigation of the biological effects of internal radionuclide contamination and for efficient decorporation, the design and development of efficient chelators for lanthanide and actinide metal ions has become a central issue. The pioneering work of Raymond and coworkers (Chem. Rev., 2003, 103, 4207-4282) led to the development of siderophore-related hydroxypyridinonate ligands for possible treatment of internalized radionuclides. However, the structure-function relationship of Ln/An bound to these ligands, particularly the bonding and coordination aspects are not clearly understood at the atomic level. Here, we have investigated the structure, binding and energetics of trivalent and tetravalent Ln/An (Sm3+, Eu3+, Am3+, Cm3+, Th4+, Pu4+) ions with spermine-based octadentate hydroxypyridinonate chelators, namely 3,4,3-LI(1,2-HOPO) and its 3,3,3 variant, using relativistic density functional theory (DFT). Furthermore, we have performed orbital and density based analyses to elucidate the nature of bonding in these complexes. In accordance with the experimental stability constant, we found the maximum binding free energy for An4+ (Pu4+, Th4+) as compared to trivalent metal ions. CDA and ECDA analyses along with orbital-based population analyses confirmed the higher ligand to metal charge transfer for An4+ than for trivalent metal ions. Furthermore, the aromaticity index analysis suggested the presence of crucial chelatoaromatic stabilization for all these metal ions with the maximum for An4+. QTAIM descriptors indicated that the binding of An/Ln with the hard oxygen donor of the ligands is of the donor-acceptor type but a higher degree of covalency exists for actinides as compared to lanthanides. Furthermore, QTAIM and molecular orbital analysis confirmed that such covalency is of the energy-driven type and strictly originates from the orbital mixing event of An-5f orbitals with the ligand orbitals.
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http://dx.doi.org/10.1039/c8dt03262aDOI Listing
November 2018

Unusual intramolecular CHO hydrogen bonding interaction between a sterically bulky amide and uranyl oxygen.

Dalton Trans 2017 Dec;46(48):16939-16946

Theoretical Chemistry Section, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India.

The selective separation of toxic heavy metals such as uranyl can be accomplished using ligands with stereognostic hydrogen bonding interactions to the uranyl oxo group, as proposed by Raymond and co-workers (T. S. Franczyk, K. R. Czerwinski and K. N. Raymond, J. Am. Chem. Soc., 1992, 114, 8138-8146). Recently, several ligands possessing this weak interaction have been proposed involving the hydrogen bonding of NH and OH based moieties with uranyl oxygen. We herein report the structurally and spectroscopically characterized CHO hydrogen bonding using a sterically bulky amide based ligand. In conjunction with experiments, electronic structure calculations are carried out to understand the structure, binding and the strength of the CHO hydrogen bonding interactions. This weak interaction is mainly due to the steric effect caused by a bulky substituent around the donor group which has direct relevance in designing novel ligands in nuclear waste management processes. Although the kinetics are very slow, the ligand is also highly selective to uranyl in the presence of other interfering ions such as lanthanides.
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http://dx.doi.org/10.1039/c7dt02760eDOI Listing
December 2017

Divalent ions are potential permeating blockers of the non-selective NaK ion channel: combined QM and MD based investigations.

Phys Chem Chem Phys 2017 Oct;19(40):27611-27622

Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.

The bacterial NaK ion channel is distinctly different from other known ion channels due to its inherent non-selective feature. One of the unexplored and rather interesting features is its ability to permeate divalent metal ions (such as Ca and Ba) and not monovalent alkali metal ions. Several intriguing questions about the energetics and structural aspects still remain unanswered. For instance, what causes Ca to permeate as well as block the selectivity filter (SF) of the NaK ion channel and act as a "permeating blocker"? How and at what energetic cost does another chemical congener, Sr, as well as Ba, a potent blocker of the K ion channel, permeate through the SF of the NaK ion channel? Finally, how do their translocation energetics differ from those of monovalent ions such as K? Here, in an attempt to address these outstanding issues, we elucidate the structure, binding and selectivity of divalent ions (Ca, Sr and Ba) as they permeate through the SF of the NaK ion channel using all-atom molecular dynamics simulations and density functional theory based calculations. We unveil mechanistic insight into this translocation event using well-tempered metadynamics simulations in a polarizable environment using the mean-field model of water and incorporating electronic continuum corrections for ions via charge rescaling. The results show that, akin to K coordination, Sr and Ba bind at the SF in a very similar fashion and remain octa-coordinated at all sites. Interestingly, differing from its local hydration structure, Ca interacts with eight carbonyls to remain at the middle of the S3 site. Furthermore, the binding of divalent metals at SF binding sites is more favorable than the binding of K. However, their permeation through the extracellular entrance faces a considerably higher energetic barrier compared to that for K, which eventually manifests their inherent blocking feature.
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http://dx.doi.org/10.1039/c7cp05586bDOI Listing
October 2017

Speciation of uranium-mandelic acid complexes using electrospray ionization mass spectrometry and density functional theory.

Rapid Commun Mass Spectrom 2017 Mar;31(6):561-571

Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai, 400085, India.

Rationale: Mandelic acid is a complexing agent employed for the liquid chromatographic separation of actinides. However, the types of species and the structural details of the uranyl-mandelate complexes are still unknown. Understanding the nature of these complex species would provide better insight into the mechanism of their separation in liquid chromatography.

Methods: Formation of different species of the uranyl ion (UO ) with mandelic acid was studied using electrospray ionization mass spectrometry (ESI-MS) with a quadrupole time-of-flight analyzer. The different species of uranyl nitrate with mandelic acid (MA) at ligand (L) to metal ratios in the range 1-10 were examined in both positive and negative ion modes. The stability of different species with the possible pathways of formation was scrutinized using density functional theory (DFT) calculations.

Results: In negative ion mode, nitrate-containing UO (MA) , UO (MA) and UO (MA) species were found in good abundance. In positive ion mode, under-coordinated uranyl-mandelate species, and solvated (S) species of types UO (MA) (S), UO (MA) (S) and UO (MA) (S), were observed whereas nitrate-containing species were absent. Interestingly, doubly and singly charged dimeric species were also identified in positive ion mode. The theoretically computed energetics of the various species are in close agreement with their experimentally observed intensities in ESI-MS.

Conclusions: The most intense peak observed in ESI-MS, UO (MA) , was found to be the energetically most favorable amongst different UO (MA) type species. Metal-ligand equilibria studied in the two modes yielded similar results. The combined experimental and quantum chemical investigations predict that T-shape complexes may be formed even in the gas phase. Copyright © 2016 John Wiley & Sons, Ltd.
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http://dx.doi.org/10.1002/rcm.7817DOI Listing
March 2017

Evaluation of an Ultrafast Molecular Rotor, Auramine O, as a Fluorescent Amyloid Marker.

J Phys Chem B 2016 10 30;120(40):10496-10507. Epub 2016 Sep 30.

Radiation & Photochemistry Division, ‡Radiation Safety Systems Division, Bhabha Atomic Research Centre , Mumbai 400085, India.

Recently, Auramine O (AuO) has been projected as a fluorescent fibril sensor, and it has been claimed that AuO has an advantage over the most extensively utilized fibril marker, Thioflavin-T (ThT), owing to the presence of an additional large red-shifted emission band for AuO, which was observed exclusively for AuO in the presence of fibrillar media and not in protein or buffer media. As fibrils are very rich in β-sheet structure, a fibril sensor should be more specific toward the β-sheet structure so as to produce a large contrast between the fibril form and native protein form, for efficient detection and in vitro mechanistic studies of fibrillation. However, in this report, we show that AuO interacts significantly with the native form of bovine serum albumin (BSA), which is an all-α-helical protein and lacks the β-sheet structure, which are the hallmarks of a fibrillar structure. This strong interaction of AuO with the native form of BSA leads to a large emission enhancement of AuO for the native protein itself, and leads to a low contrast between the BSA protein and its fibrils. More importantly, the large red-shifted emission band of AuO, reported in the presence of human insulin fibrils, and which was projected as its major advantage over ThT, is not observed in the presence of BSA fibrils as well as fibrils from other proteins, such as lysozyme, human serum albumin, and β-lactoglobulin. Thus, our results provide information on the universal applicability of the distinctive and claimed-to-be-advantageous photophysical features reported for AuO in human insulin fibrils towards fibrils from other proteins. Time-resolved fluorescence measurements also support the proposition of a strong interaction of AuO with native BSA. Additionally, tryptophan emission of the protein has been explored to further elucidate the binding mechanism of AuO with native BSA. Evaluation of thermodynamic parameters revealed that the binding of AuO with native BSA involved positive enthalpy and entropy changes, suggesting dominant contributions from hydrophobic and electrostatic interactions toward the association of AuO with native BSA. Molecular docking calculations have been performed to identify the principal binding location of AuO in native BSA.
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http://dx.doi.org/10.1021/acs.jpcb.6b07807DOI Listing
October 2016

Asn47 and Phe114 modulate the inner sphere reorganization energies of type zero copper proteins.

Phys Chem Chem Phys 2016 Jun;18(25):16748-56

Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai - 400 094, India.

The geometric structures and electron transfer properties of type 1 Cu proteins are reasonably understood at the molecular level (E. I. Solomon and R. G. Hadt, Coord. Chem. Rev., 2011, 255, 774-789, J. J. Warren, K. M. Lancaster, J. H. Richards and H. B. Gray, J. Inorg. Biochem., 2012, 115, 119-126). Much understanding of type 1 copper electron transfer reactivity has come from site directed mutagenesis studies. For example, artificial "type zero" Cu-centres constructed in cupredoxin-azurin have showcased the capacity of outer-sphere hydrogen bonding networks to enhance Cu II/I electron transfer reactivity. In this paper, we have elaborated on earlier kinetics and electronic structural studies of type zero Cu by calculating the inner sphere reorganization energies of type 1, type 2, and type zero Cu proteins using density functional theory (DFT). Although the choice of density functionals for copper systems is not straightforward, we have benchmarked the density functionals against the recently reported ESI-PES data for two synthetic copper models (S. Niu, D.-L. Huang, P. D. Dau, H.-T. Liu, L.-S. Wang and T. J. Ichiye, Chem. Theory Comput., 2014, 10, 1283). For the Cu proteins, our calculations predict that changes in the coordination number upon metal reduction lead to large inner sphere reorganization energies for type 2 Cu sites, whereas retention in the coordination number is observed for type zero Cu sites. These variations in the coordination number are modulated by the outer-sphere coordinating residues Asn47 and Phe114, which are involved in hydrogen bonding with the Asp112 side chain.
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http://dx.doi.org/10.1039/c6cp00747cDOI Listing
June 2016

Selective recognition of uranyl ions from bulk of thorium(iv) and lanthanide(iii) ions by tetraalkyl urea: a combined experimental and quantum chemical study.

Dalton Trans 2016 Jun;45(25):10319-25

Solid state physics division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India.

The selective separation of uranyl ions from an aqueous solution is one of the most important criteria for sustainable nuclear energy production. We report herein a known, but unexplored extractant, tetraalkyl urea, which shows supreme selectivity for uranium in the presence of interfering thorium and other lanthanide ions from a nitric acid medium. The structural characterization of the uranyl complex (UO2X2·2L, where X = NO3(-), Cl(-) and Br(-)) by IR, NMR and single crystal X-ray diffraction provides insight into the strong interaction between the uranyl ion and the ligand. The origin of this supreme selectivity for uranyl ions is further supported by electronic structure calculations. Uranyl binding with the extractant is thermodynamically more favourable when compared to thorium and the selectivity is achieved through a combination of electronic and steric effects.
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http://dx.doi.org/10.1039/c6dt01191hDOI Listing
June 2016

Efficient Separation of Europium Over Americium Using Cucurbit-[5]-uril Supramolecule: A Relativistic DFT Based Investigation.

Inorg Chem 2016 Jan 7;55(2):598-609. Epub 2016 Jan 7.

Radiation Safety Systems Division and ‡Theoretical Chemistry Section, Bhabha Atomic Research Centre , Mumbai-400 085, India.

Achieving an efficient separation of chemically similar Am(3+)/Eu(3+) pair in high level liquid waste treatment is crucial for managing the long-term nuclear waste disposal issues. The use of sophisticated supramolecules in a rigid framework could be the next step toward solving the long-standing problem. Here, we have investigated the possibility of separating Am(3+)/Eu(3+) pair with cucurbit-[5]-uril (CB[5]), a macrocycle from the cucurbit-[n]-uril family, using relativistic density functional theory (DFT) based calculations. We have explored the structures, binding, and energetics of metal-CB[5] complexation processes with and without the presence of counterions. Our study reveals an excellent selectivity of Eu(3+) over Am(3+) with CB[5] (ion exchange free energy, ΔΔGAm/Eu > 10 kcal mol(-1)). Both metals bind with the carbonyl portals via μ(5) coordination arrangement with the further involvement of three external water molecules. The presence of counterions, particularly nitrate, inside the hydrophobic cavity of CB[5], induces a cooperative cation-anion binding, resulting in enhancement of metal binding at the host. The overall binding process is found to be entropy driven resembling the recent experimental observations (Rawat et al. Dalton Trans. 2015, 44, 4246-4258). The optimized structural parameters for Eu(3+)-CB[5] complexes are found to be in excellent agreement with the available experimental information. To rationalize the computed selectivity trend, electronic structures are further scrutinized using energy decomposition analysis (EDA), quantum theory of atom in molecules (QTAIM), Mülliken population analysis (MPA), Nalewajski-Mrojek (NM) bond order, and molecular orbital analyses. Strong electrostatic ion-dipole interaction along with efficient charge transfer between CB[5] and Eu(3+) outweighs the better degree of covalency between CB[5] and Am(3+) leading to superior selectivity of Eu(3+) over Am(3+).
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http://dx.doi.org/10.1021/acs.inorgchem.5b01627DOI Listing
January 2016

Selectivity of a Singly Permeating Ion in Nonselective NaK Channel: Combined QM and MD Based Investigations.

J Phys Chem B 2015 Oct 30;119(40):12783-97. Epub 2015 Sep 30.

Radiation Safety Systems Division and ‡Theoretical Chemistry Section, Bhabha Atomic Research Centre , Mumbai 400 085, India.

Ion channels, such as potassium channels are known to discriminate ions to achieve remarkable selective transportation of K(+) over Na(+) through the membrane. The recently reported NaK ion channel, on the contrary, seems to be an exception, as it is observed to permeate most of the group IA alkali metal cations and hence is suggested to be nonselective in nature. However, does that correspond to a complete annihilation of selectivity inside the selectivity filter (SF) of the channel? What is the origin of such nonselectivity/selectivity, if any? The present computational study is an extensive multiscale modeling approach to find the probable answers to these intriguing questions. Here, we have used density functional theory (DFT) based calculations using a realistic truncated model of SF from the crystal structures of the NaK ion channel to evaluate the binding of various alkali metal ions (Na(+), K(+) and Cs(+)), free from "contamination" due to the absence any other "rivalry" cations, in its different binding sites. Among all of the possible binding sites, a vestibule is noticed to be nonselective and seen to act as a probable binding site only in the presence of multiple ions. Binding sites S3 and S4 are found to be selective for K(+) and Na(+), respectively. As an important observation, we find that calculations on oversimplified models using an isolated ion binding site may lead to an erroneous selectivity trend as it neglects the synergetics of consecutive binding sites on the final outcome. Energy decomposition analysis revealed ion-dipole electrostatics as the major contributing interaction in metal-bound binding sites. Our investigations find that although NaK is permeable to monovalent alkali metal ions, strongly "site specific" selectivity does exist at the three well-defined noncontiguous binding sites of the SF. Different important physicomechanical parameters (such as ligating environment, synergistic influence of binding sites, and topological constraints) are found to be the determining factor to induce the "site specific" selectivity of ions during translocation. Wherever possible, our computed results are compared with the available experimental findings. We finally conduct a detailed umbrella sampling-corrected metadynamics simulation in order to obtain an ion permeation free energy landscape within the SF that corroborates well with the "site specific" selectivity trend.
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http://dx.doi.org/10.1021/acs.jpcb.5b05996DOI Listing
October 2015

Elucidating the structures and cooperative binding mechanism of cesium salts to the multitopic ion-pair receptor through density functional theory calculations.

Dalton Trans 2015 Sep;44(35):15450-62

Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India.

Designing new and innovative receptors for the selective binding of radionuclides is central to nuclear waste management processes. Recently, a new multi-topic ion-pair receptor was reported which binds a variety of cesium salts. Due to the large size of the receptor, quantum chemical calculations on the full ion-pair receptors are restricted, thus the binding mechanisms are not well understood at the molecular level. We have assessed the binding strengths of various cesium salts to the recently synthesized multi-topic ion-pair receptor molecule using density functional theory based calculations. Our calculations predict that the binding of cesium salts to the receptor predominantly occurs via the cooperative binding mechanism. Cesium and the anion synergistically assist each other to bind favorably inside the receptor. Energy decomposition analysis on the ion-pair complexes shows that the Cs salts are bound to the receptor mainly through electrostatic interactions with small contribution from covalent interactions for large ionic radius anions. Further, QTAIM analysis characterizes the importance of different inter-molecular interactions between the ions and the receptor inside the ion-pair complexes. The role of the crystallographic solvent molecule contributes significantly by ~10 kcal mol(-1) to the overall binding affinities which is quite significant. Further, unlike the recent molecular mechanics (MM) calculations, our calculated binding affinity trends for various Cs ion-pair complexes (CsF, CsCl and CsNO3) are now in excellent agreement with the experimental binding affinity trends.
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http://dx.doi.org/10.1039/c5dt01095kDOI Listing
September 2015

Water-Mediated Differential Binding of Strontium and Cesium Cations in Fulvic Acid.

J Phys Chem B 2015 Aug 1;119(34):10989-97. Epub 2015 Apr 1.

Theoretical Chemistry Section, Bhabha Atomic Research Centre , Mumbai 400 085, India.

The migration of potentially harmful radionuclides, such as cesium ((137)Cs) and strontium ((90)Sr), in soil is governed by the chemical and biological reactivity of soil components. Soil organic matter (SOM) that can be modeled through fulvic acid (FA) is known to alter the mobility of radionuclide cations, Cs(+) and Sr(2+). Shedding light on the possible interaction mechanisms at the atomic level of these two ions with FA is thus vital to explain their transport behavior and for the design of new ligands for the efficient extraction of radionuclides. Here we have performed molecular dynamics, metadynamics simulations, and density-functional-theory-based calculations to understand the binding mechanism of Sr(2+) and Cs(+) cations with FA. Our studies predict that interaction of Cs(+) to FA is very weak as compared with Sr(2+). While the water-FA interaction is largely responsible for the weak binding of Cs(+) to FA, leading to the outer sphere complexation of the ion with FA, the interaction between Sr(2+) and FA is stronger and thus can surpass the existing secondary nonbonding interaction between coordinated waters and FA, leading to inner sphere complexation of the ion with FA. We also find that entropy plays a dominant role for Cs(+) binding to FA, whereas Sr(2+) binding is an enthalpy-driven process. Our predicted results are found to be in excellent agreement with the available experimental data on complexation of Cs(+) and Sr(2+) with SOM.
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http://dx.doi.org/10.1021/acs.jpcb.5b01659DOI Listing
August 2015

Effect of successive alkylation of N,N-dialkyl amides on the complexation behavior of uranium and thorium: solvent extraction, small angle neutron scattering, and computational studies.

J Phys Chem B 2014 Dec 25;118(49):14388-96. Epub 2014 Nov 25.

Radiochemistry Division, ‡Radiation Safety Systems Division, §Theoretical Chemistry Section, and ∥Solid State Physics Division, Bhabha Atomic Research Centre , Trombay, Mumbai 400085, India.

The effect of successive alkylation of the Cα atom adjacent to the carbonyl group in N,N-dialkyl amides (i.e., di(2-ethylhexyl)acetamide (D2EHAA), di(2-ethylhexyl)propionamide (D2EHPRA), di(2-ethylhexyl)isobutyramide (D2EHIBA), and di(2-ethylhexyl)pivalamide (D2EHPVA)) on the extraction behavior of hexavalent uranium (U(VI)) and tetravalent thorium (Th(IV)) ions has been investigated. These studies show that the extraction of Th(IV) is significantly suppressed compared to that of U(VI) with increased branching at the Cα atom adjacent to the carbonyl group. Small angle neutron scattering (SANS) studies showed an increased aggregation tendency in the presence of nitric acid and metal ions. D2EHAA showed more aggregation compared to its branched homologues, which explains its capacity for higher extraction of metal ions. These experimental observations were further supported by density function theory calculations, which provided structural evidence of differential binding affinities of these extractants for uranyl cations. The complexation process is primarily controlled by steric and electronic effects. Quantum chemical calculations showed that local hardness and polarizability can be extremely useful inputs for designing novel extractants relevant to a nuclear fuel cycle.
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http://dx.doi.org/10.1021/jp5074285DOI Listing
December 2014

Investigations on preferential Pu(IV) extraction over U(VI) by N,N-dihexyloctanamide versus tri-n-butyl phosphate: evidence through small angle neutron scattering and DFT studies.

J Phys Chem A 2014 Jun 21;118(22):3996-4004. Epub 2014 May 21.

Radiochemistry Division, ‡Radiation Safety Systems Division, §Theoretical Chemistry Section, and ∥Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India.

Straight chain amide N,N-dihexyloctanamide (DHOA) has been found to be a promising alternative extractant to tri-n-butyl phosphate (TBP) for the reprocessing of irradiated uranium- and thorium-based fuels. Unlike TBP, DHOA displays preferential extraction of Pu(IV) over U(VI) at higher acidities (≥3 M HNO3) and poor extraction at lower acidities. Density functional theory (DFT) based calculations have been carried out on the structures and relative binding energies of U(VI) and Pu(IV) with the extractant molecules. These calculations suggest that the differential hardness of the two extractants is responsible for the preferential binding/complexation of TBP to uranyl, whereas the softer DHOA and the bulky nature of the extractant lead to stronger binding/complexation of DHOA to Pu(IV). In conjunction with quantum chemical calculations, small angle neutron scattering (SANS) measurements have also been performed for understanding the stoichiometry of the complex formed that leads to relatively lower extraction of Th(IV) (a model for Pu(IV)) as compared to U(VI) using DHOA and TBP as the extractants. The combined experimental and theoretical studies helped us to understand the superior complexation/extraction behavior of Pu(IV) over U(VI) with DHOA.
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http://dx.doi.org/10.1021/jp503037qDOI Listing
June 2014

Modulation of protonation-deprotonation processes of 2-(4'-pyridyl)benzimidazole in its inclusion complexes with cyclodextrins.

J Phys Chem B 2013 Jul 5;117(28):8603-10. Epub 2013 Jul 5.

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.

2-(4'-Pyridyl)benzimidazole (4PBI) can exist in several states of protonation, having three basic nitrogen atoms. The equilibria involving these states, in ground as well as in excited states, are found to be affected significantly by cyclodextrins (CDs). The formation of inclusion complexes of this compound with all three varieties of cyclodextrins is observed to be more favorable at pH 9 than at pH 4, due to the predominance of the neutral form of dye at pH 9. The binding affinity of 4PBI to CDs is found to be governed by two factors: (i) the size of the host and (ii) the mode of insertion of 4PBI. We find that, for the host with a smaller cavity (α-CD), insertion of the dye with a pyridyl face is favored, whereas, for γ-CD, the preference is shifted toward the benzimidazole face of the dye. For β-CD, the binding affinity of the dye is maximum due to perfect cavity matching with the guest. A combination of steric factor and hydrogen bonding interaction is found to be responsible for modulation of the protonation-deprotonation equilibria of the guest molecule in the inclusion complex. Surprisingly, a protonated form is found to be promoted upon inclusion in cyclodextrins, under certain conditions. This is an unusual behavior and has been rationalized by prototropism involving the hydroxyl protons of cyclodextrin molecules.
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http://dx.doi.org/10.1021/jp403476nDOI Listing
July 2013