Phase transitions of highly asymmetric chain-length N-lignocerylsphingomyelin (C24:0-SM) bilayer

d-erythro (2S, 3R) sphingomyelin (C24:0-SM) whose acyl chain was 24 carbons long was synthesized by the acylation of d-erythro-sphingosylphosphorylcholine. Reversible and reproducible transitions of the SM was attained for the first time by thermal annealing adopted for its dispersion at a temperature of the main gel-to-liquid crystal transition up to periods as long as 5 h. This fact indicated that the main transition of the SM involved another component proceeding at an extremely slow rate, in addition to the well-known fast rate chain-melting component. A schematic diagram of relative enthalpy versus temperature curves for the gel (I), gel (II), and liquid crystal phases was constructed from a repeated cycle DSC and the thermodynamic stability of the three phases was evaluated for varying annealing periods. Electron microscopic experiments showed lamellae stacks with planer surfaces for all the three phases of the highest stability, which contrasted with small size, unilamellar vesicles observed for all the phases of the lowest stability. Such a markedly different structure for the highly asymmetry SM suggested that a change in chain packing arrangement, presumably from a full to a partial interdigitation, occurred at the main transition and proceeded gradually at the transition temperature over a period up to 5 h.