2H NMR Studies on Two-Homopolypeptide Lyotropic Enantiodiscriminating Mesophases: Experimental Quantification of Solute-Fiber Affinities
Résumé
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--benzyl-L-glutamate (PBLG) and poly--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 (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.