Methyl group substitution at C(1), C(2) or C(3) of the glycerol backbone of a diether phosphocholine: a comparative study of bilayer chain disorder in the gel and liquid-crystalline phases

Alterations in the inter- and intramolecular packing characteristics of aqueous dispersions of methyl derivatives of di-O-hexadecylglycerophosphocholine (DHPC), an ether lipid in which the methyl group is substituted at the 1, 2 or 3 position of the glycerol backbone, were monitored by changes in the vibrational frequencies and intensities of selected spectral features by Raman spectroscopy. Temperature profiles constructed from spectra reflecting intermolecular order/disorder rearrangements (C-H stretching mode region) and intramolecular order/disorder processes (C-C stretching mode region) provide insight into several important structural properties of diether lipid bilayers. The introduction of a methyl group into any position of the glycerol backbone alters both the characteristics of the DHPC pretransition and the temperature of the gel to liquid-crystalline phase transition. The main gel to liquid-crystalline phase transitions are 42.8 degrees C in the pure diether lipid, 41.6 degrees C for 3-Me-DHPC, 40.5 degrees C for 2-Me-DHPC and 38.1 degrees C for 1-Me-DHPC. Temperature profiles indicate that the degree of disordering for both the gel and liquid-crystalline states follows the sequence 2-Me-DHPC less than 3-Me-DHPC less than DHPC less than 1-Me-DHPC. Phase transition widths, delta T, determined from the spectroscopic temperature profiles, are discussed in terms of van't Hoff enthalpy functions involving both interchain and trans/gauche effects.