Comparison of the biophysical properties of racemic and d-erythro-N-acyl sphingomyelins

In this study stereochemically pure d-erythro-sphingomyelins (SMs) with either 16:0 or 18:1(cis Delta 9) as the N-linked acyl-chain were synthesized. Our purpose was to examine the properties of these sphingomyelins and acyl-chain matched racemic (d-erythro/l-threo) sphingomyelins in model membranes. Liquid-expanded d-erythro-N-16:0-SM in monolayers was observed to pack more densely than the corresponding racemic sphingomyelin. Cholesterol desorption to beta-cyclodextrin was significantly slower from d-erythro-N-16:0-SM monolayers than from racemic N-16:0-SM monolayers. Significantly more condensed domains were seen in cholesterol/d-erythro-N-16:0-SM monolayers than in the corresponding racemic mixed monolayers, when [7-nitrobenz-2-oxa-1,3-diazol-4-yl]phosphatidylcholine was used as a probe in monolayer fluorescence microscopy. With monolayers of N-18:1-SMs, both the lateral packing densities (sphingomyelin monolayers) and the rates of cholesterol desorption (mixed cholesterol/sphingomyelin monolayers) was found to be similar for d-erythro and racemic sphingomyelins. The phase transition temperature and enthalpy of d-erythro-N-16:0-SM in bilayer membranes were slightly higher compared with the corresponding racemic sphingomyelin (41.1 degrees C and 8.4 +/- 0.4 kJ/mol, and 39.9 degrees C and 7.2 +/- 0.2 kJ/mol, respectively). Finally, d-erythro-sphingomyelins in monolayers (both N-16:0 and N-18:1 species) were not as easily degraded at 37 degrees C by sphingomyelinase (Staphylococcus aureus) as the corresponding racemic sphingomyelins. We conclude that racemic sphingomyelins differ significantly in their biophysical properties from the physiologically relevant d-erythro sphingomyelins.