Effect of counterion binding and alkyl chain length on the phase transition behaviour of DI‐n‐alkyl phosphate vesicles

Abstract

AbstractThis paper describes a fluorescence depolarization and 31P NMR spectroscopic study of the phase transition behaviour of a series of identical and mixed‐chain di‐n‐alkyl phosphate vesicles in the presence of different counterions (Na+,K+,Me4N+,Ca2+). Using trans, trans, trans‐1, 6‐diphenyl‐1, 3, 5‐hexatriene (DPH) as a fluorescent probe, the fluorescence polarization (P) was measured for the identical‐chain vesicles (Na+, K+, Me4N+) as a function of temperature. The temperature for the main phase transition (Tm) only responded to variation of the counterion in the case of the longer‐chain di‐n‐alkyl phosphates, with Tm decreasing in the sequence Na+ > K+ > Me4N+. This result is rationalized in terms of a decreasing counterion binding, which affects chain ordening in the core of the bilayer. Peak intensities and line widths of the 31P NMR resonances for the bilayer vesicles suggest a more complex phase behaviour, but the overall results are reconcilable with the picture emerging from the fluorescence depolarization experiments. Fluorescence depolarization measurements were also carried out with vesicles formed from the sodium di‐n‐alkyl phosphates and in the presence of various concentrations of Ca2+ (0‐6 mM) at temperatures above Tm. For both the identical‐chain and mixed‐chain di‐n‐alkyl phosphate vesicles, a steep increase in P was found between ca 1.0 and 1.4 mM Ca2+, indicative of a strong Ca2+‐induced ordening of the alkyl chains.