Vibrational Raman and infrared spectroscopy are used to probe the dynamical, conformational and packing properties associated with lipid-lipid and lipid-protein interactions in membrane assemblies. For example, among the various choline containing phospholipids present in eukaryotic biological membranes, sphingomyelin, or N-acyl sphingosylphosphorylcholine, is a major component of most plasma membranes. We have performed vibrational Raman spectroscopic experiments as a function of temperature on aqueous dispersions of synthetic D,L-erythro-N-lignoceroyl-sphingosylphosphorylcholine, C(24):SPM), a racemic mixture of two highly asymmetric hydrocarbon chain-length sphingomyelins. Raman spectral peak-height intensity ratios of vibrational transitions in the C-H stretching mode region show that the (C(24):SPM-H20 system undergoes two thermal phase transitions centered at 48.5 and 54.5 degrees C. Vibrational data for fully hydrated C(24):SPM are compared to those of highly asymmetric phosphatidylcholine dispersions. The Raman data suggest that the lower temperature transition can be ascribed to the conversion of a mixed interdigitated gel state (gel II) to a partially interdigitated gel state (gel I) and that the higher temperature transition corresponds to a gel - greater than liquid crystalline phase transition. The observation of a mixed interdigitated gel state at temperatures below 48.5 degrees C implies that biological membranes may have lipid domains in which some of the lipid hydrocarbon chains penetrate completely across the entire hydrocarbon width of the lipid bilayer. This mixed interdigitated state for the lipid hydrocarbon chains spanning the width of the bilayer may afford a transmembrane linkage which could play important roles in the transmission of information across intact biological membranes.
