Hydrocarbon trans-gauche isomerization in phospholipid bilayer gel assemblies.

The vibrational Raman spectra of dimyristoyl (DMPC)-, dipalmitoyl (DPPC)-, and distearoylphosphatidycholine (DSPC)-water bilayer system were used to probe lipid hydrocarbon chain trans-gauch 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 three bilayer systems at approximately 1122 cm-1. This carbon-carbon stretching mode is associated with the formation of the gauche bond rotation of the terminal methyl group oriented toward the center of the bilayer. Estimates of the enthalpy differences (deltaH) 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 are interpreted to reflect approximately two gauche bonds per lipid molecule for the DMPC-H2O and the DPPC-H2O gels and six gauche bonds per molecule for the DSPC-H2O gels.