Infrared and 31P-NMR spectra of solid samples of 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 greater than POPS greater than DOPS.