Effects of single-stranded n-alkyl amphiphiles on the conformational and dynamic behavior of lecithin sonicated bilayers and micelles studied by carbon-13 NMR. A measure of lipid resistance against disruption of the bilayer orientation

Abstract

The triplet fine structures in the I3C NMR spectra of carbons in the a-position to nitrogen in several n-alkyltrimethylammonium bromides (TAB's), DMPC, and DPPC have been studied in different aggregational states under conditions of enhanced proton noise decoupling. Under the same conditions, the signals of the hydrophobic tails of the lecithins could also be studied better than hitherto, mainly by virtue of relatively narrow signals. It is shown that relatively small changes in packing and lateral motions can be detected. Intercalation of several n-alkyltrimethylammonium bromides in lecithin vesicles causes no decrease of the lipid -Nf(CH3)3 mobility around the CH2-CH2 head-group linkage nor a decrease in acyl chain mobility. Moreover, no changes in acyl chain kinking are detected. On the other hand, the incorporated TAB molecules are forced by the lecithin molecules toward severely restricted head-group and tail mobilities. For those combinations of PC and TAB's where the TAB I3C NMR signals are detectable, a change in the conformational equilibrium toward more extension is found. A packing model for the incorporation of TAB's in PC vesicles is presented which probably has a rather general validity. The behavior of mixed micelles of Pc's and TAB's, originating from enhancing the latter's concentration, is also described. In these systems, mobilities and chain kinking are increased with respect to the vesicular state.