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 ΔHMvs. 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.

► The series of d-erythro-sphingomyelins (SMs) with a sphingoshine chain of 18 carbons long was synthesized by varying the length of acyl chain from 16 to 24 carbons long. ► All the SMs were found to exist in an asymmetric chain-length lipid. ► A partial melting of a mismatched segment in the acyl chain was observed for the gel phase which appeared at temperatures just below the gel-to-liquid crystal phase ► transition. ► The transition enthalpy of the gel-to-liquid crystal phases decreased slightly with increasing the acyl chain-length, indicating no contribution from the mismatched acyl chain segment. ► Both a loose chain-packing arrangement and a highly curved surface of vesicular structure were observed to proceed more and more with an increase in the acyl chain length.