Publications by authors named "Russell Hopson"

27 Publications

  • Page 1 of 1

Aggregation and Solvation of n-Butyllithium.

Org Lett 2017 08 27;19(15):3966-3969. Epub 2017 Jul 27.

Department of Chemistry, Brown University , Providence, Rhode Island 02912, United States.

Solution characterizations and ligand binding constants were determined for n-butyllithium in hydrocarbon and ethereal solvents using diffusion-ordered NMR. In hydrocarbon solvents, n-butyllithium exists primarily as an octamer at -40 °C and deaggregates to a hexamer when the temperature is increased. In the presence of THF or diethyl ether, n-butyllithium exists predominantly as a tetra-solvated tetramer and deaggregates to a tetra-solvated dimer in the presence of a large excess or neat THF. The ligand binding constants for the tetra-solvated tetramers were measured using H NMR/DOSY titration.
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http://dx.doi.org/10.1021/acs.orglett.7b01644DOI Listing
August 2017

Ligand Binding Constants to Lithium Hexamethyldisilazide Determined by Diffusion-Ordered NMR Spectroscopy.

J Org Chem 2017 06 8;82(12):6223-6231. Epub 2017 Jun 8.

Department of Chemistry, Brown University , Providence, Rhode Island 02912, United States.

We report the direct measurement of ligand-binding constants of organolithium complexes using a H NMR/diffusion-ordered NMR spectroscopy (DOSY) titration technique. Lithium hexamethyldisilazide complexes with ethereal and ester donor ligands (THF, diethyl ether, MTBE, THP, tert-butyl acetate) are characterized using H NMR and X-ray crystallography. Their aggregation and solvation states are confirmed using diffusion coefficient-formula weight correlation analysis, and the H NMR/DOSY titration technique is applied to obtain their binding constants. Our work suggests that steric hindrance of ethereal ligands plays an important role in the aggregation, solvation, and reactivity of these complexes. It is noteworthy that diffusion methodology is utilized to obtain binding constants.
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http://dx.doi.org/10.1021/acs.joc.7b00800DOI Listing
June 2017

Conformational Polymorphism of Lithium Pinacolone Enolate.

J Am Chem Soc 2016 11 11;138(46):15177-15188. Epub 2016 Nov 11.

Department of Chemistry, Brown University , Providence, Rhode Island 02912, United States.

A metastable, polymorphic hexameric crystal structure of lithium pinacolone enolate (LiOPin) is reported along with three preparation methods. NMR-based structural characterization implies that the lithium pinacolate hexamer deaggregates to a tetramer in toluene but retains mainly the hexameric structure in nonaromatic hydrocarbon solvents such as cyclohexane. Moreover, the presence of a small amount of lithium aldolate (LiOA) dramatically influences the aggregation state of LiOPin by forming a mixed aggregate with a 3:1 ratio (LiOPin·LiOA).
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http://dx.doi.org/10.1021/jacs.6b08177DOI Listing
November 2016

Perfluoroalkyl Grignard Reagents: NMR Study of 1-Heptafluoropropylmagnesium Chloride in Solution.

J Org Chem 2016 07 28;81(14):5922-8. Epub 2016 Jun 28.

Department of Applied Chemistry, Tokyo Institute of Technology , Tokyo 152-8552, Japan.

We report on the generation of a perfluoroalkyl Grignard reagent ((F)RMgX) by exchange reaction between a perfluoroalkyl iodide ((F)R-I) and a Grignard reagent (RMgX). (19)F NMR was applied to monitor the generation of n-C3F7MgCl. Additional NMR techniques, including (19)F COSY, NOESY, and pulsed gradient spin-echo (PGSE) diffusion NMR, were invoked to assign peaks observed in (19)F spectrum. Schlenk equilibrium was observed and was significantly influenced by solvent, diethyl ether, or THF.
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http://dx.doi.org/10.1021/acs.joc.6b00807DOI Listing
July 2016

Diffusion Coefficient-Formula Weight (D-FW) Analysis of (2)H Diffusion-Ordered NMR Spectroscopy (DOSY).

J Org Chem 2015 Sep 9;80(18):9102-7. Epub 2015 Sep 9.

Department of Chemistry, Brown University , Providence, Rhode Island 02912, United States.

We report extension of the D-FW analysis using referenced (2)H DOSY. This technique was developed in response to limitations due to peak overlay in (1)H DOSY spectra. We find a corresponding linear relationship (R(2) > 0.99) between log D and log FW as the basis of the D-FW analysis. The solution-state structure of THF solvated lithium diisopropyl amide (LDA) in hydrocarbon solvent was chosen to demonstrate the reliability of the methodology. We observe an equilibrium between monosolvated and disolvated dimeric LDA complexes at room temperature. Additionally we demonstrate the application of the (2)H D-FW analysis using a compound with an exchangeable proton that is readily labeled with (2)H. Hence, the (2)H DOSY D-FW analysis is shown to provide results consistent with the (1)H DOSY method, thereby greatly extending the applicability of the D-FW analysis.
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http://dx.doi.org/10.1021/acs.joc.5b01457DOI Listing
September 2015

Lithium pinacolone enolate solvated by hexamethylphosphoramide.

J Am Chem Soc 2015 Jun 15;137(23):7347-56. Epub 2015 May 15.

Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.

We report the crystal structure of a substoichiometric, HMPA-trisolvated lithium pinacolone enolate tetramer (LiOPin)4·HMPA3 abbreviated as T3. In this tetramer one HMPA binds to lithium more strongly than the other two causing a reduction in spatial symmetry with corresponding loss of C3 symmetry. A variety of NMR experiments, including HMPA titration, diffusion coefficient-formula weight (D-FW) analysis, and other multinuclear one- and two-dimensional NMR techniques reveal that T3 is the major species in hydrocarbon solution when more than 0.6 equiv of HMPA is present. Due to a small amount of moisture from HMPA or air leaking into the solution, a minor complex was identified and confirmed by X-ray diffraction analysis as a mixed aggregate containing enolate, lithium hydroxide, and HMPA in a 4:2:4 ratio, [(LiOPin)4·(LiOH)2·HMPA4], that we refer to as pseudo-T4. A tetra-HMPA-solvated lithium cyclopentanone enolate tetramer was also prepared and characterized by X-ray diffraction, leading to the conclusion that steric effects dominate the formation and solvation of the pinacolone aggregates. An unusual mixed aggregate consisting of pinacolone enolate, lithium diisopropyl amide, lithium oxide, and HMPA in the ratio 5:1:1:2 is also described.
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http://dx.doi.org/10.1021/jacs.5b01906DOI Listing
June 2015

Chiral lithium diamides derived from linked N-isopropyl valinol or alaninol.

J Am Chem Soc 2014 Aug 8;136(33):11735-47. Epub 2014 Aug 8.

Department of Chemistry, Brown University , Providence, Rhode Island 02912, United States.

Four different chiral diamino diethers synthesized from N-isopropyl valinol or N-isopropyl alaninol were lithiated with n-butyllithium in tetrahydrofuran or diethyl ether. Crystal structures of the dilithiated diamino diethers were determined by X-ray diffraction. Three dilithiated diamino diethers including (2S,2'S)-1,1'-(butane-1,4-diylbis(oxy))bis(N-isopropylpropan-2-amine) 7, (2S,2'S)-1,1'-(pentane-1,5-diylbis(oxy))bis(N-isopropylpropan-2-amine) 8, and (2S,2'S)-1,1'-(heptane-1,7-diylbis(oxy))bis(N-isopropyl-3-methylbutan-2-amine) 9 are dimers, whereas dilithiated (2S,2'S)-1,1'-(pentane-1,5-diylbis(oxy))bis(N-isopropyl-3-methylbutan-2-amine) 10 is a monomer. The lithium atoms in all crystal structures adopt a nonequivalent coordination protocol and exist in two different environments in which one of the lithium atoms is tetra-coordinated while the other one is tri-coordinated. The solution structures of the dilithiated diamino diethers are also characterized by a variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula (D-FW) weight correlation analyses and other one- and two-dimensional NMR techniques.
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http://dx.doi.org/10.1021/ja505179yDOI Listing
August 2014

Influence of steric factors on chiral lithium amide aggregates.

J Am Chem Soc 2014 Feb 14;136(8):3246-55. Epub 2014 Feb 14.

Department of Chemistry, Brown University , 324 Brook Street, Providence, Rhode Island 02912 United States.

The solution structures of three mixed aggregates dissolved in toluene-d8 consisting of the lithiated amides derived from (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amine, (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amine, or (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amine and n-butyllithium are characterized by various NMR experiments including diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses (D-FW) and other one- and two-dimensional NMR techniques. We report that steric hindrance of R1 and R2 groups of the chiral lithium amide controls the aggregation state of the mixed aggregates. With a less hindered R2 group, lithium (S)-N-isopropyl-1-((triisopropylsilyl)oxy)propan-2-amide forms mostly a 2:2 ladder-type mixed aggregate with n-butyllithium. Increase of steric hindrance of the R1 and R2 groups suppresses the formation of the 2:2 mixed aggregate and promotes formation of a 2:1 mixed aggregate. We observe that lithium (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amide forms both a 2:2 mixed aggregate and a 2:1 mixed trimer with n-butyllithium. Further increase in the steric hindrance of R1 and R2 groups results in the formation of only 2:1 mixed aggregate as observed with lithium (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amide.
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http://dx.doi.org/10.1021/ja4123957DOI Listing
February 2014

PT19c, Another Nonhypercalcemic Vitamin D2 Derivative, Demonstrates Antitumor Efficacy in Epithelial Ovarian and Endometrial Cancer Models.

Genes Cancer 2013 Nov;4(11-12):524-34

Molecular Therapeutics Laboratory, Program in Women's Oncology, Department of Obstetrics and Gynecology, Women and Infants' Hospital, Brown University, Providence, RI, USA.

Hypercalcemia remains a major impediment to the clinical use of vitamin D in cancer treatment. Approaches to remove hypercalcemia and development of nonhypercalcemic agents can lead to the development of vitamin D-based therapies for treatment of various cancers. In this report, in vitro and in vivo anticancer efficacy, safety, and details of vitamin D receptor (VDR) interactions of PT19c, a novel nonhypercalcemic vitamin D derived anticancer agent, are described. PT19c was synthesized by bromoacetylation of PTAD-ergocalciferol adduct. Broader growth inhibitory potential of PT19c was evaluated in a panel of chemoresistant breast, renal, ovarian, lung, colon, leukemia, prostate, melanoma, and central nervous system cancers cell line types of NCI60 cell line panel. Interactions of PT19c with VDR were determined by a VDR transactivation assay in a VDR overexpressing VDR-UAS-bla-HEK293 cells, in vitro VDR-coregulator binding, and molecular docking with VDR-ligand binding domain (VDR-LBD) in comparison with calcitriol. Acute toxicity of PT19c was determined in nontumored mice. In vivo antitumor efficacy of PT19c was determined via ovarian and endometrial cancer xenograft experiments. Effect of PT19c on actin filament organization and focal adhesion formation was examined by microscopy. PT19c treatment inhibited growth of chemoresistant NCI60 cell lines (log10GI50 ~ -4.05 to -6.73). PT19c (10 mg/kg, 35 days) reduced growth of ovarian and endometrial xenograft tumor without hypercalcemia. PT19c exerted no acute toxicity up to 400 mg/kg (QDx1) in animals. PT19c showed weak VDR antagonism, lack of VDR binding, and inverted spatial accommodation in VDR-LBD. PT19c caused actin filament dysfunction and inhibited focal adhesion in SKOV-3 cells. PT19c is a VDR independent nonhypercalcemic vitamin D-derived agent that showed noteworthy safety and efficacy in ovarian and endometrial cancer animal models and inhibited actin organization and focal adhesion in ovarian cancer cells.
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http://dx.doi.org/10.1177/1947601913507575DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3877664PMC
November 2013

Isotopically enriched 13C diffusion-ordered NMR spectroscopy: analysis of methyllithium.

J Org Chem 2013 Dec 13;78(23):11733-46. Epub 2013 Nov 13.

Department of Chemistry, Brown University , Providence, Rhode Island 02912, United States.

We report the development of isotopic-labeled (13)C diffusion-ordered NMR spectroscopy (DOSY) NMR with diffusion coefficient-formula weight (D-FW) analysis and its application in characterizing the aggregation state of methyllithium aggregates and complexes with several widely used diamines. Commercially available (13)C-labeled benzene and several easily synthesized (13)C-labeled compounds using (13)C-labeled iodomethane as the isotopic source are developed as internal references for diffusion-formula weight analysis (D-FW). The technique greatly expands the applicability of DOSY D-FW analysis to a much wider variety of compounds because of isotopic labeling. These results reveal that methyllithium exists as a tetrasolvated tetramer in diethyl ether and exclusively as bis-solvated dimers with chelating diamines.
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http://dx.doi.org/10.1021/jo401740gDOI Listing
December 2013

Mixed aggregates of an alkyl lithium reagent and a chiral lithium amide derived from N-ethyl-O-triisopropylsilyl valinol.

J Am Chem Soc 2013 Sep 17;135(38):14367-79. Epub 2013 Sep 17.

Department of Chemistry, Brown University , 324 Brook Street,Providence, Rhode Island 02912, United States.

The crystal structure of a mixed aggregate containing lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine derived from (S)-valinol and cyclopentyllithium is determined by X-ray diffraction. The mixed aggregate adopts a ladder structure in the solid state. The ladder-type mixed aggregate is also the major species in a toluene-d8 solution containing an approximately 1:1 molar ratio of the lithiated chiral amide to cyclopentyllithium. A variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula (D-FW) weight correlation analyses and other one- and two-dimensional NMR techniques allowed us to characterize the complex in solution. Solution state structures of the mixed aggregates of n-butyl, sec-butyllithium, isopropyllithium with lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine are also reported. Identical dimeric, ladder-type, mixed aggregates are the major species at a stoichiometric ratio of 1:1 lithium chiral amide to alkyllithium in toluene-d8 solution for all of the different alkyllithium reagents.
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http://dx.doi.org/10.1021/ja406912hDOI Listing
September 2013

Characterization of cyclopentyllithium and cyclopentyllithium tetrahydrofuran complex.

J Am Chem Soc 2013 Aug 12;135(33):12400-6. Epub 2013 Aug 12.

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

The solid-state structures of unsolvated, hexameric cyclopentyllithium and tetrameric cyclopentyllithium tetrahydrofuran solvate were determined by single-crystal X-ray diffraction. Cyclopentyllithium easily crystallized in hydrocarbon solvents. Solution-state structural analyses of cyclopentyllithium and cyclopentyllithium-tetrahydrofuran complexes in toluene-d8 were also carried out by diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques. The solution-state studies suggest that unsolvated cyclopentyllithium exists as hexamer and tetramer equilibrating with each other. Upon solvation with tetrahydrofuran, cyclopentyllithium exists mostly as a tetrahydrofuran tetrasolvated tetramer.
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http://dx.doi.org/10.1021/ja4059102DOI Listing
August 2013

Crystal structure and solution state characterization of lithium (S)-(1-(bis(2-methoxyethyl)amino)-3-methylbutan-2-yl)(methyl)amide.

J Org Chem 2013 Jul 5;78(14):7288-92. Epub 2013 Jul 5.

Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.

The solid state structure of lithiated (S)-N(1),N(1)-bis(2-methoxyethyl)-N(2),3-dimethylbutane-1,2-diamine, which is a chiral amide base synthesized from (S)-valine was determined by single-crystal X-ray diffraction. The complex in solution state is also characterized by a variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques by dissolving the crystal in toluene-d8. The crystallography and NMR results suggest that the chiral amide is dimeric in both solid and solution states.
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http://dx.doi.org/10.1021/jo400839qDOI Listing
July 2013

Bioinspired bioadhesive polymers: dopa-modified poly(acrylic acid) derivatives.

Macromol Biosci 2012 Nov 24;12(11):1555-65. Epub 2012 Sep 24.

Department of Molecular Pharmacology, Physiology, and Biotechnology, Brown University, Providence, RI 02912, USA.

The one-step synthesis and characterization of novel bioinspired bioadhesive polymers that contain Dopa, implicated in the extremely adhesive byssal fibers of certain gastropods, is reported. The novel polymers consist of combinations of either of two polyanhydride backbones and one of three amino acids, phenylalanine, tyrosine, or Dopa, grafted as side chains. Dopa-grafted hydrophobic backbone polymers exhibit as much as 2.5 × the fracture strength and 2.8 × the tensile work of bioadhesion of a commercially available poly(acrylic acid) derivative as tested on live, excised, rat intestinal tissue.
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http://dx.doi.org/10.1002/mabi.201200179DOI Listing
November 2012

Characterization of dimeric chiral lithium amide structures derived from N-isopropyl-O- triisopropylsilyl valinol.

J Am Chem Soc 2011 May 12;133(17):6596-602. Epub 2011 Apr 12.

Department of Chemistry, Brown University, 324 Brook Street, Providence, Rhode Island 02912, USA.

The dimeric structure is characterized for a chiral amide base complex consisting of an (S)-N-isopropyl-O-triisopropylsilyl valinol ligand and lithium. The complex is characterized by a variety of NMR techniques, including multinuclear one- and two-dimensional NMR experiments and diffusion-ordered NMR spectroscopy (DOSY) as well as diffusion coefficient-formula weight (D-fw) correlation analyses. Spartan calculations are presented which support the structural assignment. This structural characterization leads to an explanation of the behavior and the reactivity of these complexes in solution.
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http://dx.doi.org/10.1021/ja109041zDOI Listing
May 2011

Genome mining in Streptomyces clavuligerus: expression and biochemical characterization of two new cryptic sesquiterpene synthases.

Chem Biol 2011 Jan;18(1):32-7

Department of Chemistry, Brown University, Box H, Providence, RI 02912-9108, USA.

Two presumptive terpene synthases of unknown biochemical function encoded by the sscg_02150 and sscg_03688 genes of Streptomyces clavuligerus ATCC 27074 were individually expressed in Escherichia coli as N-terminal-His₆-tag proteins, using codon-optimized synthetic genes. Incubation of recombinant SSCG_02150 with farnesyl diphosphate (1, FPP) gave (-)-δ-cadinene (2) while recombinant SSCG_03688 converted FPP to (+)-T-muurolol (3). Individual incubations of (-)-δ-cadinene synthase with [1,1-²H₂]FPP (1a), (1S)-[1-²H]-FPP (1b), and (1R)-[1-²H]-FPP (1c) and NMR analysis of the resulting samples of deuterated (-)-δ-cadinene supported a cyclization mechanism involving the intermediacy of nerolidyl diphosphate (4) leading to a helminthogermacradienyl cation 5. Following a 1,3-hydride shift of the original H-1(si) of FPP, cyclization and deprotonation will give (-)-δ-cadinene. Similar incubations with recombinant SSCG_03688 supported an analogous mechanism for the formation of (+)-T-muurolol (3), also involving a 1,3-hydride shift of the original H-1(si) of FPP.
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http://dx.doi.org/10.1016/j.chembiol.2010.11.008DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3034312PMC
January 2011

Synthesis, characterization, and reaction of a ketone-derived 1,4-dienolate compound.

J Org Chem 2011 Jan 9;76(1):65-70. Epub 2010 Dec 9.

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

The tetrahydrofuran tetrasolvated dimeric lithium dienolate derived from 2,2,7,7-tetramethyloctan-3,6-dione is characterized in the solid state by X-ray diffraction analysis and in solution by diffusion NMR. This dienolate was reacted with tropanone to yield two new products that are also described.
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http://dx.doi.org/10.1021/jo1015163DOI Listing
January 2011

Physically separated references for diffusion coefficient-formula weight (D-FW) analysis of diffusion-ordered NMR spectroscopy (DOSY) in water.

Org Lett 2010 Jun;12(12):2698-701

Department of Chemistry, Brown University Providence, Rhode Island 02912, USA.

Development and application of physically separated references for aqueous (1)H DOSY diffusion coefficient-formula weight (D-FW) correlation analysis is reported. Commercially available biological buffers (Tris and HEPES) and a water-soluble alcohol (tert-butanol) were used as physically separated references for a Ru and a Mn complex in D(2)O. This extension of DOSY D-FW analysis expands its applicability to a wide variety of water-soluble molecules or metal complexes, with particular application to green chemistry.
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http://dx.doi.org/10.1021/ol100686eDOI Listing
June 2010

6Li diffusion-ordered NMR spectroscopy (DOSY) and applications to organometallic complexes.

Org Lett 2010 Feb;12(3):520-3

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

The development of (6)Li diffusion-ordered NMR spectroscopy (DOSY) is reported. This technique is applied to (6)Li organometallic complexes. (6)Li DOSY provides a facile means of identification of peaks in the (6)Li spectrum, as well as evidence of mixed aggregates based on relative diffusion coefficients. (6)Li data is correlated to (1)H diffusion experiments through (6)Li{(1)H} HOESY and/or (1)H{(6)Li} HMBC experiments to obtain formula weight information of Li aggregates.
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http://dx.doi.org/10.1021/ol902713hDOI Listing
February 2010

Internally referenced diffusion coefficient-formula weight (D-FW) analysis of 31P diffusion-ordered NMR spectroscopy (DOSY).

Org Lett 2009 Nov;11(21):4818-21

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

The development of (31)P DOSY NMR with diffusion coefficient-formula weight (D-FW) analysis is reported. Commercially available trialkyl phosphine internal references were used in a model system to establish the molecular weight of a phosphorous containing organolithium compound. The feasibility of (31)P DOSY D-FW studies is established. This extension of DOSY D-FW analysis expands its applicability to solution structure studies of a wide variety of compounds.
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http://dx.doi.org/10.1021/ol9019106DOI Listing
November 2009

Biosynthesis of the sesquiterpene botrydial in Botrytis cinerea. Mechanism and stereochemistry of the enzymatic formation of presilphiperfolan-8beta-ol.

J Am Chem Soc 2009 Jun;131(24):8360-1

Department of Chemistry, Box H, Brown University, Providence, Rhode Island 02912-9108, USA.

Presilphiperfolan-8beta-ol synthase, encoded by the BcBOT2 gene from the necrotrophic plant pathogen Botrytis cinerea, catalyzes the multistep cyclization of farnesyl diphosphate (2) to the tricyclic sesquiterpene alcohol presilphiperfolan-8beta-ol (3), the preursor of the phytotoxin botrydial, a strain-dependent fungal virulence factor. Incubation of (1R)-[1-(2)H]farnesyl diphosphate (2b) with recombinant presilphiperfolan-8beta-ol synthase gave exclusively (5R)-[5alpha-(2)H]-3b, while complementary incubation of (1S)-[1-(2)H]FPP (2c) gave (5S)-[5beta-(2)H]-3c. These results established that cyclization of farnesyl diphosphate involves displacement of the diphosphate group from C-1 with net inversion of configuration and ruled out the proposed intermediacy of the cisoid conformer of nerolidyl diphosphate (9) in the cyclization. While not a mandatory intermediate, (3R)-nerolidyl diphosphate was shown to act as a substrate surrogate. Cyclization of [13,13,13-(2)H(3)] farnesyl diphosphate (2d) gave [14,14,14-(2)H(3)]-3d, thereby establishing that electrophilic attack takes place exclusively on the si face of the 12,13-double bond of 2. The combined results provide a detailed picture of the conformation of enzyme-bound farnesyl diphosphate at the active site of presilphiperfolan-8beta-ol synthase.
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http://dx.doi.org/10.1021/ja9021649DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2702122PMC
June 2009

Formula weight prediction by internal reference diffusion-ordered NMR spectroscopy (DOSY).

J Am Chem Soc 2009 Apr;131(15):5627-34

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

Formula weight (FW) information is important to characterize the composition, aggregation number, and solvation state of reactive intermediates and organometallic complexes. We describe an internal reference correlated DOSY method for calculating the FW of unknown species in different solvents with different concentrations. Examples for both the small molecule (DIPA) and the organometallic complex (aggregate 1) yield excellent correlations. We also found the relative diffusion rate is inversely proportional to the viscosity change of the solution, which is consistent with the theoretical Stokes-Einstein equation. The accuracy of the least-squares linear prediction r(2) and the percentage difference of FW prediction are directly related to the density change; greater accuracy was observed with decreasing density. We also discuss the guidelines and other factors for successful application of this internal reference correlated DOSY method. This practical method can be conveniently modified and applied to the characterization of other unknown molecules or complexes.
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http://dx.doi.org/10.1021/ja810154uDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2888872PMC
April 2009

Characterization of reactive intermediates by multinuclear diffusion-ordered NMR spectroscopy (DOSY).

Acc Chem Res 2009 Feb;42(2):270-80

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

Nuclear magnetic resonance (NMR) is the most powerful and widely utilized technique for determining molecular structure. Although traditional NMR data analysis involves the correlation of chemical shift, coupling constant, and NOE interactions to specific structural features, a largely overlooked method introduced more than 40 years ago, pulsed gradient spin-echo (PGSE), measures diffusion coefficients of molecules in solution, thus providing their relative particle sizes. In the early 1990s, the PGSE sequence was incorporated into a two-dimensional experiment, dubbed diffusion-ordered NMR spectroscopy (DOSY), in which one dimension represents chemical shift data while the second dimension resolves species by their diffusion properties. This combination provides a powerful tool for identifying individual species in a multicomponent solution, earning the nickname "chromatography by NMR". In this Account, we describe our efforts to utilize DOSY techniques to characterize organometallic reactive intermediates in solution in order to correlate structural data to solid-state crystal structures determined by X-ray diffraction and to discover the role of aggregate formation and solvation states in reaction mechanisms. In 2000, we reported our initial efforts to employ DOSY techniques in the characterization of reactive intermediates such as organolithium aggregates. Since then, we have explored DOSY experiments with various nuclei beyond (1)H, including (6)Li, (7)Li, (11)B, (13)C, and (29)Si. Additionally, we proposed a diffusion coefficient-formula weight relationship to determine formula weight, aggregation number, and solvation state of reactive intermediates. We also introduced an internal reference system to correlate the diffusion properties of unknown reactive intermediates with known inert molecular standards, such as aromatic compounds, terminal olefins, cycloolefins, and tetraalkylsilanes. Furthermore, we utilized DOSY to interpret the role of aggregation number and solvation state of organometallic intermediates in the reactivity, kinetics, and mechanism of organic reactions. By utilizing multinuclear DOSY methodologies at various temperatures, we also correlated solid-state X-ray structures with those in solution and discovered new reactive complexes, including a monomeric boron enolate, a product-inhibition aggregate, and a series of intermediates in the vinyl lithiation of allyl amines. As highlighted by our efforts, DOSY techniques provide practical and feasible NMR procedures and hold the promise of even more powerful insights when extended to three-dimensional experiments.
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http://dx.doi.org/10.1021/ar800127eDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2666195PMC
February 2009

Microcoil NMR spectroscopy: a novel tool for biological high throughput NMR spectroscopy.

Methods Mol Biol 2008 ;426:447-58

Department of Chemistry, Brown University, Providence, RI, USA.

Microcoil NMR spectroscopy is based on the increase of coil sensitivity for smaller coil diameters (approximately 1/d). Microcoil NMR probes deliver a remarkable mass-based sensitivity increase (8- to 12-fold) when compared with commonly used 5-mm NMR probes. Although microcoil NMR probes are a well established analytical tool for small molecule liquid-state NMR spectroscopy, after spectroscopy only recently have microcoil NMR probes become available for biomolecular NMR spectroscopy. This chapter highlights differences between commercially available microcoil NMR probes suitable for biomolecular NMR spectroscopy. Furthermore, it provides practical guidance for the use of microcoil probes and shows direct applications for structural biology and structural genomics, such as optimal target screening and structure determination, among others.
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http://dx.doi.org/10.1007/978-1-60327-058-8_30DOI Listing
August 2008

Aggregation studies of complexes containing a chiral lithium amide and n-Butyllithium.

J Org Chem 2008 Mar 23;73(6):2373-81. Epub 2008 Feb 23.

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

A system consisting of a chiral lithium amide and n-BuLi in tol-d(8) solution was investigated with (1)H and (13)C INEPT DOSY, (6)Li and (15)N NMR, and other 2D NMR techniques. A mixed 2:1 trimeric complex was identified as the major species as the stoichiometry approached 1.5 equiv of n-BuLi to 1 equiv of amine compound. (1)H and (13)C INEPT DOSY spectra confirmed this lithium aggregate in the solution. The formula weight of the aggregate, correlated with diffusion coefficients of internal references, indicated the aggregation number of this complex. Plots of log D(rel) vs log FW are linear (r > 0.9900). (6)Li and (15)N NMR titration experiments also corroborated these results. These NMR experiments indicate that this mixed aggregate is the species that is responsible for asymmetric addition of n-BuLi to aldehydes.
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http://dx.doi.org/10.1021/jo702655mDOI Listing
March 2008

13C INEPT diffusion-ordered NMR spectroscopy (DOSY) with internal references.

Org Lett 2008 Mar 6;10(5):909-11. Epub 2008 Feb 6.

Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

13C INEPT Diffusion-ordered NMR spectroscopy (DOSY) with an internal reference system was developed to study the aggregation state of THF-solvated LDA dimeric complex. Six components are clearly identified in the diffusion dimension, and their DOSY-generated 13C INEPT spectrum slices agree extremely well with their respective INEPT spectra. The correlation between log D and log FW of the linear least-squares fit to reference points of all components is exceptionally high: (r = 0.9985).
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http://dx.doi.org/10.1021/ol703039vDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3220947PMC
March 2008

Genome mining in Streptomyces coelicolor: molecular cloning and characterization of a new sesquiterpene synthase.

J Am Chem Soc 2006 May;128(18):6022-3

Department of Chemistry, Brown University, Box H, Providence, Rhode Island 02912-9108, USA.

The terpene synthase encoded by the SCO5222 (SC7E4.19) gene of Streptomyces coelicolor was cloned by PCR and expressed in Escherichia coli as an N-terminal-His6-tag protein. Incubation of the recombinant protein, SCO5222p, with farnesyl diphosphate (1, FPP) in the presence of Mg(II) gave a new sesquiterpene, (+)-epi-isozizaene (2), whose structure and stereochemistry were determined by a combination of 1H, 13C, COSY, HMQC, HMBC, and NOESY NMR. The steady-state kinetic parameters were kcat 0.049 +/- 0.001 s-1 and a Km (FPP) of 147 +/- 14 nM. Individual incubations of recombinant epi-isozizaene synthase with [1,1-2H2]FPP (1a), (1R)-[1-2H]-FPP (1b), and (1S)-[1-2H]-FPP (1c) and NMR analysis of the resulting deuterated epi-isozizaenes supported an isomerization-cyclization-rearrangement mechanism involving the intermediacy of (3R)-nerolidyl diphosphate (3).
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http://dx.doi.org/10.1021/ja061292sDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2533732PMC
May 2006