Solubilization of DMPC-d54 and DMPG-d54 vesicles with octylglucoside and sodium dodecyl sulfate studied by FT-IR spectroscopy

The solubilization of deuterated dimyristoylphosphatidylcholine (DMPC-d54) and dimyristoylphosphatidylglycerol (DMPG-d54) vesicles by the surfactants sodium dodecyl sulfate (SDS) and octylglucoside (OG), respectively, was studied by Fourier transform infrared spectroscopy (FT-IR) at 30 °C. In order to obtain separate information about the structural changes of both, the lipid and the surfactant component during the solubilization process, isotopically labelled lipids (DMPC-d54 and DMPG-d54) were used. With increasing surfactant concentration, the shift of the CD2 stretching mode towards higher wavenumber indicates an increasing disorder of the lipid alkyl chains during the incorporation of the surfactants into the lipid bilayer. At the same time the frequency of the antisymmetric CH2 stretching mode of the surfactant alkyl chains shifts towards lower wavenumber with increasing amount of surfactant. This occurs due to the decreasing proportion of surfactant monomers until the saturation limit is reached. In the coexistence region the frequency of the CH2 vibration stays constant, which indicates no change in the monomer concentration of the surfactant. Only the proportion of micelles increases at the expense of the vesicles. Above the coexistence region no further frequency shift is observed except for the DMPC/SDS system, which show an additional frequency decrease of the CH2 stretching mode. This is due to the decreasing amount of SDS monomers according to the decrease of the CMC for charged systems. In DMPG/SDS mixtures, prepared in 0.1 mM NaCl, the surfactant charge is screened by the salt and no frequency shift is observed. The carbonyl vibrations show a pronounced frequency shift towards lower wavenumber exclusively for the DMPG/SDS system with high net charge. This indicates an increasing degree of hydration of the lipid head-group with increasing amount of surfactant due to the effort to separate the negative charge of the lipid head-group from the negatively charged surfactant head-group.