Influence of gangliosides GM 1 and GD 1a on structural and thermotropic properties of sonicated small 1,2-dipalmitoyl- l-α-phosphatidylcholine vesicles

Abstract

The effect of the gangliosides GM 1 and GD 1a on the structure- and temperature-induced transition behavior of sonicated, small 1,2-dipalmitoylphosphatidylcholine (DPPC) vesicles has been studied by 1H- and 13C-NMR, laser light scattering and differential scanning microcalorimetry. It became apparent from comparison of the results obtained by the various methods that great care must be taken when the occurrence of phase transitions is postulated on the basis of methods other than high-sensitivity microcalorimetry. Detection of a temperature-dependent decrease in the half-width of methylene signals, which was reminiscent of a phase transition, was not paralleled by characteristic changes in heat capacity in the microcalorimetric experiments. Probing of the local mobility of the choline head groups in vesicles of mixed composition by 1H-NMR and 13C-NMR spin-lattice relaxation time ( T 1 ) measurements suggested a decrease of mobility of the hydrophilic DPPC head groups in the presence of either ganglioside. The decreases in T 1 were identical for both gangliosides irrespective of the differences in the structure of the sugar moieties of GM 1 and GD 1a, whereas the heat capacity vs. temperature curves were rather different. Since also the average dimensions of pure DPPC vesicles and vesicles containing GM 1 or GD 1a were the same, the differences in temperature dependence of heat capacity in the presence of the gangliosides were attributed to differences in interaction of the hydrophobic parts of the molecules. The higher content of C 20 sphingosines in GD 1a as compared to GM 1 was envisaged as a possible source for these differences. The presence of a physiological CaCl 2 concentration (2 mM) did not affect the transition behavior of GD 1a-containing vesicles to any detectable level, as monitored by calorimetry.