The main transition enthalpy of the gel-to-liquid crystal phases for a series of asymmetric chain length d-erythro (2S, 3R) sphingomyelins

A series of d-erythro (2S, 3R) sphingomyelins (SMs) whose acyl chain was 16, 18, 20, 22, and 24 carbons long was synthesized by the acylation of d-erythro-sphingosyl-phosphorylcholine of 18 carbons long. All the SMs were found to exist as asymmetric chain-length lipids. DSC heating scans of the respective SM dispersions showed two transitions that were reversible and reproducible upon reheating. However, the enthalpy change (ΔHM) of the main gel-to-liquid crystal phase transition exhibited a slight decrease with an increase in the effective chain length (N), which contrasted with the generally accepted phenomenon for lipid-water systems. The behavior of ΔHM vs. N curve was discussed from the viewpoint of the effect of a mismatched chain-length segment in the acyl chain on the chain–chain van der Waals interaction energy that accounts for 2/3 of ΔHM, on the basis of a partial interdigitation for the chain packing arrangement of asymmetric chain length lipids.