Mixed micelles of dioctanoyl-L-.alpha.-lecithin and hydrocarbon amphiphiles. Aspects of fluidization of the micellar interior

Abstract

CTAB/TTAB mixed micelles, studied by Lindman et al.,lS correspond nicely with those of the 1:l mixed micelles of decanoate and dodecanoate of this study. In retrospect, the CTAB/TTAB mixed micelles form a special case of the more general situation as presented.J. Phys. Chem. 1982Chem. , 86, 2528Chem. -2533 Acknowledgment. This investigation has been supported by the Netherlands Foundation for Chemical Research (SON) 13C NMR measurements of dioctanoyl-L-a-lecithin micellar solutions detect an almost complete visibility of the intrinsic magnetic nonequivalent behavior of the two lipid acyl chains. This is an extension of recently published observations. According to the latter, the chemical shift data are interpreted in terms of effectively different lengths of the sn-1 and sn-2 acyl chains due to a bending near the C2 carbon atom of the sn-2 chain. Mixed micellar systems of DOPC and several n-alkyltrimethylammonium bromides show a difference in effective chain lengths of the constituent detergent types. 13C NMR shieldings are observed for n-alkyl detergent fragments which are longer than both acyl chains of the lipid molecules. At an effective chain length difference of seven carbon atoms a sizeable contribution of extra gauche conformers with respect to their single micelles occur for these n-alkyl surfactants. For smaller differences decreasing van der Waals interactions (i.e., decreasing molecular packing) participate almost exclusively leading to chain separation. The deshieldings observed for the n-alkyl segments situated directly between neighboring lecithin chains indicate conformational changes toward more extended forms, as compared with their single micellar solutions, rather than increasing van der Waals interactions. The lipid molecules do not undergo measurable conformational changes upon mixed micelle formation but are only subject to increased molecular packing. This may indicate that conformational changes are of minor importance for solubilizing micelle bound hydrocarbon-like compounds.