Investigation of Solute-Fiber Affinity and Orientational Ordering of Norbornadiene Interacting with Two-Polypeptide Chiral Liquid Crystalline Solvents by Natural Abundance Deuterium (NAD) NMR
Résumé
A prochiral bridged compound of C2v symmetry, the norbornadiene (NBD), oriented in a chiral liquid crystal composed of various mixtures of poly-γ-benzyl-L-glutamate (PBLG) and poly-ε-carboxy-L-lysine (PCBLL), two chiral homopolypeptides, is investigated using natural abundance deuterium 2D-NMR (NAD 2D-NMR) spectroscopy. In such chiral oriented solvents, enantiotopic directions are spectrally nonequivalent, and two distinct 2H quadrupolar doublets associated with enantioisotopomeric pairs of NBD are detected. As the two homopolypeptides have the same absolute configuration but distinct chemical functions in their side chains, the variation of residual quadrupolar couplings (RQC’s) allows the determination of the relative solute-fiber affinities toward the two polypeptides in these lyotropic bipolymeric systems. Besides the experimental measurement of RQC’s and the determination of their signs at each inequivalent 2H site, the elements of the second-rank order tensor, Sαβ, are calculated by assuming a modeled structure. The variations of RQC’s and diagonalized order parameters, Sα′α′, are followed versus the relative proportion of two polypeptides in the chiral oriented mixture. The influence of the solute mass fraction in the two-homopolypeptide oriented samples is also examined as well as the case of homogeneous and uniform achiral mesophases “PBG−PCBL” made of two pairs of mirror-image homopolypeptides (PBLG/PBDG and PCBLL/PCBDL). In the latter, the solute ordering is modulated by the proportion of each type of homopolypeptide (chemical nature and absolute configuration), leading to eliminate the enantiodiscrimination mechanisms on the average. In the frame of a model, new insights on the solute−homopolypeptide fiber interactions are discussed.