CHARACTERIZATION OF THE INTERACTION OF CA2-HYDROXY FATTY-ACID SPECIESOF CEREBROSIDE SULFATE BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY ANDMOLECULAR MODELING( WITH HYDROXY AND NON)

Ca2+-mediated interactions between the carbohydrate groups of glycolip ids, including that of cerebroside sulfate (galactosylceramide I-3-sul fate), have recently been implicated as a basis of cell recognition an d adhesion. Hydroxylation of the fatty acid of this lipid has an effec t on these interactions, Therefore, FT-IR spectroscopy was used to stu dy the interaction of Ca2+ with semisynthetic hydroxy (HFA) and non-hy droxy fatty acid (NFA) species of cerebroside sulfate (CBS). Ca2+ caus ed partial dehydration of the sulfate group and reduced hydrogen bondi ng of the sugar hydroxyls of both species. The amide I and II bands of the lipids in the absence of Ca2+ (NH4+ salt forms) suggested that th e N-H of the HFA species is involved in a bent intramolecular hydrogen bond, probably with the fatty acid hydroxyl group and the glycosidic oxygen, while that of the NFA species is involved in a linear intermol ecular hydrogen bond with the C=O and/or other oxygens, Ca2+ caused a rearrangement of the hydrogen-bonding network in the interfacial regio n of the HFA species involving the amide group. The results suggested increased hydrogen bonding of the C=O and a shift in hydrogen bonding of the N-H of the Ca2+ salt form of the HFA species from a bent intram olecular hydrogen bond to a linear intermolecular hydrogen bond, proba bly with the C=O of neighboring molecules, similar to the NFA species, The involvement of the fatty acid alpha-hydroxyl group in the rearran ged network was indicated by a reduction in mobility of the alpha-CH g roup of the HFA species, in contrast to that of the NFA species. Parti cipation of the alpha-OH group in hydrogen-bonding networks in the int erfacial region of both the NH4+ and Ca2+ salt forms caused a signific ant increase in the interchain packing, as evident from correlation fi eld splitting of the HFA-CBS methylene scissoring mode, while this did not occur for the NFA species. The absence of intramolecular hydrogen bonding of the N-H with the glycosidic oxygen for both salt forms of the NFA species and for the Ca2+ salt form of the HFA species may dest abilize the ''bent shovel'', bilayer planar conformation of the sugar and cause it to be in the extended, bilayer perpendicular conformation . Calculations of the three-dimensional interaction energy of Ca2+ wit h CBS showed strong binding around the sulfate and the surface of gala ctose facing the bilayer in the bent shovel conformation. Ca2+ binding at this surface would disrupt intra- and intermolecular hydrogen-bond ing interactions of the head group, thus accounting for its effect in inducing a transition to the extended conformation.