Olivier Lafon

2H NMR Studies on Two-Homopolypeptide Lyotropic Enantiodiscriminating Mesophases: Experimental Quantification of Solute–Fiber Affinities

The analytical potential and enantioselective properties of lyotropic mesophases made by mixing two chemically different chiral polypeptides are described. Here we examine the case of a mixture of poly-gamma-benzyl-L-glutamate (PBLG) and poly-epsilon-carbobenzyloxy-L-lysine (PCBLL). We demonstrate that 2H NMR spectroscopy on these chiral oriented mixtures can discriminate both enantiomers and enantiotopic directions in prochiral molecules. Moreover, in such systems, degree of enantiodiscrimination, resolution, and sensitivity can be conjointly optimized by changing the relative proportion of the two polypeptides. Therefore, these new enantiodiscriminating media provide a favorable alternative to single-polypeptide mesophases with respect to stereochemical applications. At a more fundamental level, the present work points out that solute distribution in the vicinity of each polypeptide partly governs the degree of enantiodiscrimination and NMR relaxation rates. To this end, the experimental trends of solute NMR observables (Delta nu Q, T1) versus the fraction of peptide units of each polymer were analyzed by using a mean-field model derived from that proposed for mixtures of thermotropic nematic solvents, and based on the separation of intermolecular interactions between the solute and both polypeptides. This approach allows the relative solute-fiber affinities in these lyotropic systems to be determined. To identify the factors controlling solute-polypeptide affinities, we investigated various solutes (polar/apolar, rigid/flexible, achiral/prochiral/chiral molecules) using 2H NMR at natural abundance or on isotopically enriched solutes.

Study of molecular rotational isomerism using deuterium NMR in chiral oriented solvents

Deuterium NMR in polypeptide chiral liquid crystals is used for the first time to investigate the internal rotational isomerism phenomenon, illustrated by the interconversion between conformers of 1-(2,6-dideutero-4-methylphenyl)naphthalene

Use of two-dimensional correlation between 2H quadrupolar splittings and 13C CSA's for assignment of NMR spectra in chiral nematics

The possibility of correlating visible spectral enantiodiscriminations in a single 2D map using simultaneously both quadrupolar and dipolar nuclei is presented and experimentally demonstrated. The method uses deuterium–carbon-13 heteronuclear 2D experiments with 13C in natural abundance applied to deuterated compounds oriented in chiral, nematic liquid crystals (PBLG/CHCl3). Illustrative examples involving chiral and prochiral solute molecules are analysed and discussed. We show that this approach provides an efficient and elegant way to assign deuterium resonances for each enantiomer or enantiotopic directions in prochiral molecules.