Hydrocarbon chain trans-gauche isomerization in phospholipid bilayer gel assemblies

Abstract

The vibrational Raman spectra of dimyristoyl (DMPC)-, dipalmitoyl (DPPC)- and distearoylphosphatidylcholine (DSPC)-H2O bilayer systems were used to probe lipid hydrocarbon chain trans-gauche isomerization dynamics below the gel-liquid crystalline phase transition temperature. In addition to the 1090-1085 cm-1 vibrational transitions, which appear with increasing temperatures and are characteristic of gauche conformers within the acyl chains, a new feature arises in all 3 bilayer systems at .apprx. 1122 cm-1. This C-C stretching mode is associated with the formation of a gauche bond rotation of the terminal methyl group oriented toward the center of the bilayer. Estimates of the enthalpy differences (.DELTA.H) between hydrocarbon chains in an all-trans conformation and chain configurations containing gauche forms may be made from peak height intensities of vibrational features associated with the appropriate rotational isomers. For the DMPC-H2O, DPPC-H2O and DSPC-H2O assemblies, the Raman data yield enthalpy differences of 2.9 .+-. 0.6, 3.4 .+-. 0.5 and 9.9 .+-. 1.2 kcal/mol, respectively. These values probably reflect approximately 2 gauche bonds per lipid molecule for the DMPC-H2O and the DPPC-H2O gels and 6 gauche bonds/molecule for the DSPC-H2O gels.