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

In this study stereochemically pure d-erythro-sphingomyelins (SMs) with eit her 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 mo re densely than the corresponding racemic sphingomyelin. Cholesterol desorp tion to beta-cyclodextrin was significantly slower from d-erythro-N-16:0-SM monolayers than from racemic N-16:0-SM monolayers. Significantly more cond ensed 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 fluorescen ce microscopy. With monolayers of N-18:1-SMs, both the lateral packing dens ities (sphingomyelin monolayers) and the rates of cholesterol desorption (m ixed cholesterol/sphingomyelin monolayers) was found to be similar for d-er ythro and racemic sphingomyelins. The phase transition temperature and enth alpy of d-erythro-N-16:0-SM in bilayer membranes were slightly higher compa red 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). Fin ally, d-erythro-sphingomyelins in monolayers (both N-16:0 and N-18:1 specie s) were not as easily degraded at 37 degrees C by sphingomyelinase (Staphyl ococcus aureus) as the corresponding racemic sphingomyelins. We conclude th at racemic sphingomyelins differ significantly in their biophysical propert ies from the physiologically relevant d-erythro sphingomyelins.