Infrared and ³¹P-NMR spectra of solid samples of Full-size image 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS) and 1.2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) have been recorded. Comparison of the spectra of the Na⁺ salts of these phospholipids with those of complexes formed with Li⁺ and Ca²⁺ ions allows the characterization of conformational changes induced by complexation with Li⁺ and Ca²⁺. Ca²⁺ forms tight, crystalline complexes with these phosphatidylserines (PS), irrespective of the degree of unsaturation in the hydrocarbon chains. In these PS-Ca²⁺ complexes the torsion angles of the two P-O ester bonds exhibit the antiplanar-antiplanar conformation which is significantly different from the standard gauche-gauche conformation commonly found in phosphodiesters. In contrast, complexation with Li⁺ does not induce this conformational change in the phosphodiester group. It is shown that the degree of unsaturation in the hydrocarbon chains, and related to it, the cross-sectional area of the phospholipid or the surface charge density, determine the affinity of the phosphatidylserine for the metal ion. In general, the affinity of phosphatidylserines for both Li⁺ and Ca²⁺ decreases with increasing unsaturation in the hydrocarbon chains or decreasing surface charge density; it is in the order DMPS > POPS > DOPS.
Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism919, no. 3: 275–286.