Vibrational Raman and infrared spectroscopy are used to probe the dynamical, conformational and packing properties associated with lipid-lipid interactions in membrane assemblies. For example, the thermotropic behavior of a series of n-alkylphosphocholines dispersed in excess water has been examined by vibrational Raman spectroscopy in both the C-H stretching (2800-3100 cm-1) and the C-C stretching (1000-1200 cm-1) mode regions, spectral intervals characteristic of alkyl chain lateral interactions and intrachain disorder, respectively. The single chain, interdigitated systems, which include the odd number n-C(15), n-C(17), and n-C(19) phosphocholine homologs, exhibit sharp lamellar to micellar transitions at 13.8, 26.1 and 33.2 degrees C, respectively. Additional polymorphic forms, whose transition temperatures are 4 to 15 degrees C below the more stable structures, were identified. Raman spectra provided clear and definitive spectral patterns enabling the subcell chain packing characteristics of each polymorph to be characterized. For n-C(15) phosphocholine the higher melting interdigitated lamellae exhibit spectra characteristic of a hybrid lattice composed of both orthorhombic (or monoclinic) and hexagonally packed domains. The less stable polymorphic form assumes, however, a lattice whose alkyl chains pack in triclinic subcells. The alkyl chains of the less stable polymorphic forms of the n-C(17) and n-C(19) phosphocholines are hexagonally packed, while the higher melting, more stable lamellae exhibit spectra suggestive of the hybrid lattice structure observed for the n-C(15) system. Spectral splittings in the TM3040 cm-1 choline methyl asymmetric stretching mode region indicate an ordered, perhaps dehydrated, headgroup for the polymorphs in the hybrid, higher melting lamellae. In contrast to the n-C(17) and n-C(19) phosphocholine lamellae, the low temperature triclinic polymorph of the n-C(15) system displays a variable alkyl chain recognition at 6 degrees C.
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