Hydrocarbon chain disorder in lipid bilayers. Temperature dependent Raman spectra of 1,2-diacyl phosphatidylcholine-water gels.

Abstract Vibrational Raman spectra of multilayers of 1,2-dimyristoyl, 1,2-dipalmitoyl and 1,2-distearoyl phosphatidylcholine were recorded as a function of temperature for the gel phase from −180° C to values slightly below the pretransition temperature. Temperature profiles for band intensity ratios involving the acyl chain C-C stretching modes define a characteristic temperature Tg which denotes the onset of hydrocarbon chain trans-gauche isomerization in the gel. Tg for the dimyristoyl (C14 chain lengths), dipalmitoyl (C16 chain lengths) and distearoyl (C18 chain lengths) systems are approximately −40, −40 and 5°C, respectively. The higher Tg value for distearoyl phosphatidylcholine is associated with increasing interactions between the terminal chain areas of the individual monolayers forming the bilayer unit. This increased interaction between bilayer halves is presumably a consequence of the larger angle through which the hydrocarbon chains are tilted in the C18 phospholipid gel. Temperature-dependent peak height intensities for the 2800–3000 C-H stretching region provide convenient probes for quantitatively describing the increase in lateral chain packing disorder as the gel assemblies are warmed. The temperature profiles for various methylene mode peak height ratios indicate that both the inter- (lateral packing) and intramolecular (trans-gauche) disordering processes are related and are initiated at nearly the same temperatures for each lipid system. Increases in the van 't Hoff enthalpies which result from the disordering of the lipid gel matrix by a lattice expansion were estimated for the C14, C16 and C18 phospholipids to be approximately 1.5, 1.2 and 3.2 kcal/ mol, respectively.