Molecular conformations of cerebrosides in bilayers determined by Raman spectroscopy

Vibrational Raman spectra of the solid and gel phases of bovine brain cerebrosides and the component fractions, kerasin and phrenosin, provide conformational information for these glycosphingolipids in bilayer systems. The carbon-carbon stretching mode profiles (1,150-1,000 cm-1) indicate that at 22 degrees C the alkyl chains assume an almost all-trans arrangement. These spectral data, combined with those from the C-H stretching region (3,050-2,800 cm-1), show that phrenosin forms the most highly ordered polycrystalline solid and kerasin the most ordered gel phase. The conformation of the unsaturated, 24-carbon acyl chains is monitored independently by a skeletal stretching mode at 1,112 cm-1. The alkyl chains in the kerasin and phrenosin gels are sufficiently extended to allow interdigitation of the 24-carbon acyl chains across the midplane of the bilayer. The amide I vibrational mode occurs at a lower frequency in solid phrenosin than kerasin, a shift consistent with stronger hydrogen bounding. This band is broadened and shifted to higher frequencies, however, in the phrenosin gel phase. In both the solid and gel phases natural cerebroside exhibits a composite amide I mode. The disruptive effects on cerebroside chain packing and headgroup orientation arising from mixing with dimyristoyl phosphatidylcholine are examined. Vibrational data for cerebroside are also compared to those for ceramide, sphingosine, and distearoyl phosphatidylcholine structures. Spectral interpretations are discussed in terms of calorimetric and X-ray structural data.