Interactions of Ca2+ with sphingomyelin and dihydrosphingomyelin

The changes induced by Ca2+ on human lens sphingolipids, sphingomyelin (SM), and dihydrosphingomyelin were investigated by infrared spectroscopy. Ca2+-concentration-dependent studies of the head group region revealed that, for both sphingolipids, Ca2+ partially dehydrates some of the phosphate groups and binds to others. Ca2+ affects the interface of each sphingolipid differently. In SM, Ca2+ shifts the amide I' band to frequencies lower than those in dehydrated samples of SM alone. This could be attributed to the direct binding of Ca2+ to carbonyl groups and/or strong tightening of interlipid H-bonds to levels beyond those in dehydrated samples of SM only. In contrast, Ca2+ induces relatively minor dehydration around the amide groups of dihydrosphingomyelin and a slight enhancement of direct lipid-lipid interactions. Temperature-dependent studies reveal that 0.2 M Ca2+ increases the transition temperature T-m from 31.6 +/- 1.0degreesC to 35.7 +/- 1.1degreesC for SM and from 45.5 +/- 1.1degreesC to 48.2 +/- 1.0degreesC for dihydrosphingomyelin. Binding of Ca2+ to some phosphate groups remains above T-m. The strength of the interaction is, however, weaker. This allows for the partial rehydration of these moieties. Similarly, above T-m, Ca2+-lipid and/or direct inter-lipid interactions are weakened and lead to the rehydration of amide groups.