CHAIN-LENGTH MISMATCH AND PACKING CONSTRAINTS OF FREE FATTY-ACIDS IN A HEXAGONAL DETERGENT HOST PHASE - A WIDE LINE DEUTERIUM NMR-STUDY

The conformational and dynamical constraints of a series of deuterated free fatty acids with increasing chain length were studied in a hexagonal (H-I) host phase of the nonionic detergent octaethyleneglycolmono-n-dodecylether (C(12)E(8)) by deuterium nuclear magnetic resonance spectroscopy (H-2-NMR). The nonionic detergent and free fatty acids ranging from decanoic to octadecanoic acid were chosen as a paradigm of the capacity of detergent micelles to solubilize lipids within a broad range of molecular weights. Segmental order parameters were obtained from the H-2-quadrupolar splittings. The parallel increase of the order parameter profiles in the plateau region indicates that slow, whole body motion of the fatty acid molecules decreases with increasing chain length. The unusual shape of the ordering profiles and the decreasing ordering of the terminal segments as the chain length increases suggest that mismatch between the average radius of the host phase and the length of the fatty acids results in chain backbending and interpenetration. Longitudinal and transverse relaxation rates R(1z) and R(2), respectively, of individual chain segments were determined in order to correlate the segmental ordering with chain dynamics. The relation between R(1z), and the H-2 order parameters indicates that theories of molecular motion that require time-scale separation between fast segmental and slow whole body motions are not applicable in the highly curved hexagonal phase structure. The relaxation rate R(2) is related to diffusional motion of the fatty acids about the long axis of the cylindrical aggregate. Motions other than axial diffusion however, significantly contribute to R(2).