Raman spectroscopic study of semisynthetic species of cerebroside sulfate: two types of hydrocarbon chain interdigitation

Raman spectroscopy was used to study the phase behavior of several semisynthetic species of the acidic glycosphingolipid cerebroside sulfate (CBS) which occur in myelin. The C-H stretching mode region at 2800-3100 cm-1 of C18:0-CBS, C24:0-CBS, and C26:0-CBS, and the alpha-hydroxy fatty acid species C18:0h-CBS, was studied in the presence of 2 M Li+ and 2 M K+. Earlier studies have shown that K+ shields the negative charge on the sulfate more effectively than Li+, thus promoting intermolecular hydrogen-bonding interactions between the lipid molecules. Indeed, a novel broad background feature was present in the Raman spectra from 2900 to 3200 cm-1, which was attributed to O-H stretch associated with intermolecular hydrogen bonding between lipid hydroxyl groups. After subtraction of this broad feature, the intensities of the lipid C-H stretching vibrational transitions could be determined. These indicated that in K+, the degree of order (intrachain conformation and lateral chain-chain interactions) of C18:0-CBS, whose hydrocarbon region is fairly symmetrical in chain length, is similar to that of the symmetric chain length glycerolipid dipalmitoylphosphatidylcholine, while the degree of order is lower in Li+, as a result of the increased lateral charge repulsion of the head groups in Li+. Two phase transitions were observed for the highly asymmetric species C24:0-CBS and C26:0-CBS in K+ but only one transition in Li+.(ABSTRACT TRUNCATED AT 250 WORDS)