The optical activity of d-erythro-sphingomyelin and its contribution to the circular dichroism of sphingomyelin-containing systems

Circular dichroism studies on bovine brain sphingomyelin show the presence of a strong negative cotton effect below 200 nm, the position and magnitude of which depend on the physical state of the lipid. This cotton effect is thought to arise from the π-π¹ transition of the amide group in the sphingomyelin backbone. The sphingomyelin contribution to the observed ellipticity of membranes and lipoprotein complexes depends on the mol fraction of amide groups present as sphingomyelin: this contribution is calculated to be less than 2% in the case of serum high density lipoprotein and the order of 20% below 200 nm in the case of the erythrocyte ghost membrane. Due to the similarity of the CD spectrum of sphingomyelin to that of a random coil polypeptide, use of uncorrected ellipticity data is expected to lead to an overestimate of the random coil content of proteins in systems containing a high sphingomyelin content.