Chiral discrimination of L- and D-N-acyl-1-phenyl-D-5-2-aminopropanes in a cesium N-dodecanoyl-L-threoninate cholesteric nematic lyomesophase

Abstract

Molecular recognition based on chirality has fundamental importance in many biological processes. Deuterium quadrupole splittings from the aromatic ring and mesophase components of two series of optical isomers, L-and D-N-acyl-1-phenyl-d(5)-2-aminopropanes, dissolved in anionic nematic cholesteric lyotropic liquid crystals of cesium N-dodecanoyl-L-threoninate, were measured using H-2-NMR. The length of the acyl chain was 1, 2, 3, 4, 5, 7 and 10 carbon atoms. The two order parameters that fully characterize the average alignment of the aromatic ring were calculated. Both the L- and D-isomers are strongly attached to the aggregate. L-C-1, D-C-1, L-C-2 and D-C-2 derivatives have the same order parameters which suggests that are located in the same region of the interface, possibly H-bonded to the interstitial water molecules with the NH and/or CO groups. Increasing the hydrophobic chain length by one carbon atom decreases the overall alignment of the ring and differences between L-C-3 and D-C-3 were observed. Molecules with longer acyl chain progressively increase their quadrupole splittings, suggesting an increase in alignment. Increasing differences in the order parameter of the symmetry axis of the aromatic rings in both isomers were observed from C-3 to C-5, and almost no differentiation is detected between L-C-7 and D-C-7. However, differentiation appears again for C-10, and is attributable to interactions with a second chiral center of the head group.