The effects of divalent cations on the dynamics of sulfogalactosylceramide and sulfogalactosylacylalkylglycerol multilayers: A study using the technique of high pressure Fourier-transform infrared spectroscopy.

Abstract

Sulfogalactosylacylalkylglycerol (SGG) and sulfogalactosylceramide (SGC) are structurally related sulfatides that are found ubiquitously in the plasma membrane of male germ cells. Since Ca$\sp{2+}$ is a cation that plays important roles in many fertilization related events (Yanagimachi, 1988), we have been interested in determining whether Ca$\sp{2+}$ interacts with the negatively charged SGG/SGC and whether this interaction results in changes to the lipid dynamics. In this study, we used high pressure Fourier-transform infrared spectroscopy, a powerful technique that reveals information on different structural regions of the lipid molecule based on their specific infrared vibrational absorption modes. The study was performed using multilayer suspensions of the sulfatides. Our results showed that all three divalent cations electrostatically interacted with the sulfate moiety of hydrated SGG, although with varying degrees of strength. Similarly Ca$\sp{2+}$ bound to the sulfate moiety of solid SGC. It was found that hydrated SGG-Na$\sp+$ was interdigitated whereas hydrated SGC-Na$\sp+$ was not. The divalent cations presumably crosslinked the sulfate groups of neighbouring lipid molecules. In the case of SGG, this crosslinking removed the interdigitation. With SGC, the crosslinking by Ca$\sp{2+}$ weakened the hydrogen bonding of the interfacial region and the hydrocarbon chains became more disordered. Similar lipid dynamic effects of divalent cations were found in the SGG studies, with Mg$\sp{2+}$ inducing the greatest chain disorder followed by Ca$\sp{2+}$ then Sr$\sp{2+}$. An increase in chain disorder would increase the bilayer fluidity. Such a phenomenon may prove relevant to the changes of the sperm plasma membrane during fertilization-related event. (Abstract shortened by UMI.)