Thermal and Chemical Perturbations of Molecular Order at the Interface and in the Alkyl Chain Region of a Lyotropic Liquid Crystal
Magnetic field-orientable lyotropic mesophases are useful both as ordering solvents in NMR spectroscopy and as models for membranes and membrane processes. In NMR studies of the latter category, detailed information concerning the behavior of the alkyl chain of the amphiphilic component of the system is obtainable from the quadrupole splittings of the specifically deuterated chain positions (1). Although this is a highly useful and widely applicable approach it is unable to provide the signs of the order parameters in the general case (unless |s|>.5) where the quadrupole splitting is much greater than the direct and indirect couplings (2). We have now measured the 13C shift anisotropies for positions in the alkyl chain for the laurate amphiphile over a range of temperatures. Not only does this eliminate sign ambiguities in the order parameters for the alkyl chain, it also provides a measure of order and its sign for the carboxyl group. The latter information, which is not otherwise available, makes it possible to characterize the situation at the lipid-aqueous interface, an important consideration for model membrane studies. Conventional high-resolution 13 C NMR techniques are not ordinarily suitable for shift anisotropy determination, in ordered systems but the solid-state proton-enhanced cross-polarization methods (3) have been successfully applied to thermotropic systems and lecithin dispersions (4–7).
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