Carbon-13 and phosphorus-31 nuclear magnetic resonance studies on interaction of calcium with phosphatidylserine.

The interaction between Ca2+ and phosphatidylserine was studied by 13C and 31P NMR spectroscopy, by IR analysis, by binding constant measurements, and through use of space-filling molecular models. NMR measurements of various salt forms of the lipid were made in two types of organic solvents that allowed sufficient averaging of chemical shift anisotropy and dipolar couplings to yield high resolution spectra. 13C resonances of the polar head-group carbons were broadened relative to those of the acyl chains. This was especially true in samples prepared at neutral pH where ionic interactions appeared to restrict molecular motion. In CDCl3 the marked line broadening of the resonances of the polar head-group atoms in the Ca2+ form indicated the formation of large, slow tumbling micelles. In the amphipathic solvent the large reduction in line broadening indicated the presence of freely tumbling Ca-(phosphatidylserine)2 dimeric complexes. The 2:1 binding stoichiometry and the low chemical activity of the Ca-phosphatidylserine complex support this view. Analysis of the chemical shifts of the various lipid atoms under the differing ionic environments indicates that Ca2+ enhanced the deprotonation of both the carboxyl and amino groups and stabilized the entire polar head group against the effects of changing pH. The marked upfield shift of the 31P phosphate resonance in the Ca2+ form and its insensitivity to changing pH indicate strong coordination binding. IR data indicate direct involvement of the carboxyl group in Ca2+ binding, as evidenced by the appearance of a C=O stretching mode. Binding studies indicated that the phosphate group was the primary binding force but that the carboxyl group also contributes positively. The amino group appears to exert a repulsive effect, which is supported by the chemical shift data which indicate that Ca2+ enhances the deprotonation of the amino group. Molecular models indicate direct involvement of the carboxyl and phosphate oxygens and that the amino group must be deprotonated to participate.