Structural role of mismatched C–C bonds in a series of d-erythro-sphingomyelins as studied by DSC and electron microscopy

A series of d-erythro (2S, 3R) sphingomyelins (SMs) whose acyl chain was 16, 18, 20, 22, and 24 carbons long, respectively, was synthesized by the acylation of d-erythro-sphingosylphosphorylcholine. For all the SM dispersions, reversible and reproducible thermal behavior was observed to show the gel-to-gel and the main gel-to-liquid crystal phase transition in heating scan. The main transition enthalpy (ΔHM) decreased linearly with increasing acyl chain length. The vesicular structures were observed for all the gel phases at temperatures just below the main transition, but the mean diameter of these vesicles changed markedly from ∼1.5 to 100 nm with increasing acyl chain length. On this basis, the decrease in ΔHM with increasing acyl chain length was discussed from the viewpoint of the effect of the mismatched C–C bonds in the acyl chain on the van der Waals attractive force between the matched acyl chain segment and the sphingoshine chain of the gel phase at temperatures just below the main transition.