The lateral order of dipalmitoylphosphatidylcholine model membranes in the presence of N-alkyl-N,N,N-trimethylammonium ions as studied by Raman spectroscopy.

Effects of N-alkyl-N,N,N-trimethylammonium ions with different alkyl substituents (hexyl, nonyl, dodecyl, and octadecyl) on the lateral packing of lipids in dipalmitoylphosphatidylcholine (DPPC) dispersions in H2O was investigated by Raman spectroscopy in a spectral region of 2800--3100 cm-1 at temperatures between 22--70 degrees C. The lateral order parameter Slat calculated by empirical equation reveals that the addition of the ions decreases the lateral ordering of lipid hydrocarbon chains in the gel phase, while in the liquid crystalline state the lateral ordering is increased. In addition, this observation is supported by decomposition of the spectra into component bands using a computer fitting program. This enabled to follow changes in individual band parameters (position, amplitude, and height) in dependence on temperature and/or additives. The results suggest that N-alkyl-N,N,N-trimethylammonium ions have a condensing effect on DPPC bilayer in the liquid crystalline state, the effect increasing with the increasing length of the alkyl substituent.