Fourier transform infrared spectroscopic studies of the effect of calcium ions on phosphatidylserine

Fourier transform infrared (FT-IR) spectroscopy is used to investigate the complex conformational changes that occur as phosphatidylserine (PS) binds to calcium. The spectra confirm the isothermal crystallization of the hydrocarbon chains in the PS-Ca²⁺ complex. However, in contrast to differential scanning calorimetry, which detects no phase transitions under 100ºC in PS-Ca²⁺ complexes, several FT-IR parameters detect structural changes at 30-40ºC in these complexes analogous to those observed in solid-solid phase transitions of alkanes. Site symmetry splitting observed in the PO₂⁻ bands suggests that Ca²⁺ binds to the PS phosphate as a bidentate ligand; in addition, Ca²⁺ causes a dehydration of the phosphate ester. No evidence is found for the specific chelation of Ca²⁺ by the ionized carboxylate group or the dehydration of this group; instead, the carboxylate exists in an immobilized conformation in the presence of Ca²⁺. Splitting of the degenerate vibrations of the carbonyl group at the interfacial region suggests different rotational chain isomers in the Ca²⁺ complex and the possibility of hydrogen bonding with trapped interstitial water.