A Fourier transform infrared spectroscopic study of the interaction of alkaline earth cations with the negatively charged phospholipid 1,2-dimyristoyl- sn-glycero-3-phosphoglycerol

Abstract

The interaction of aqueous phospholipid dispersions of negatively charged 1,2-dimyristoyl- sn-glycero-3-phosphoglycerol, sodium salt (DMPG) with the divalent cations Mg 2+, Ca 2+ and Sr 2+ at equimolar ratios in 100 mM NaCl at pH 7 was investigated by Fourier transform infrared spectroscopy. The binding of the three cations induces a crystalline-like gel phase with highly ordered and rigid all- trans acyl chains. These features are observed after storage below room temperature for 24 h. When the gel phase is heated after prolonged incubation at low temperature phase transitions into the liquid crystalline phase are observed at 58°C for the DMPG:Sr 2+, 65°C for the DMPG:Mg 2+, and 80°C for the DMPG:Ca 2+ complex. By subsequent cooling from temperatures above T m these complexes retain the features of a liquid crystalline phase with disordered acyl chains until a metastable gel phase is formed at temperatures between 38 and 32°C. This phase is characterized by predominantly all- trans acyl chains, arranged in a loosely packed hexagonal or distorted hexagonal subcell lattice. Reheating the DMPG:Sr 2+ samples after a storage time of 2 h at 4°C results in the transition of the metastable gel to the liquid crystalline phase at 35°C. This phase transition into the liquid crystalline state at 35°C is also observed for the Mg 2+ complex. However, for DMPG:Mg 2+ at higher temperatures, a partial recrystallization of the acyl chains occurs and the high temperature phase transition at 65°C is also detected. In contrast, DMPG:Ca 2+ exhibits only the phase transition at 80°C from the crystalline gel into the fluid state upon reheating. Below 20°C, the rate of conversion from the metastable gel to a thermodynamically stable, crystalline-like gel phase decreases in the order Ca 2+≫Mg 2+>Sr 2+. This conversion into the crystalline gel phase is accompanied by a complete dehydration of the phosphate groups in DMPG:Mg 2+ and by a reorientation of the polar lipid head groups in DMPG:Ca 2+ and in DMPG:Sr 2+. The primary binding sites of the cations are the PO 2 − groups of the phosphodiester moiety. Our infrared spectroscopic results suggest a deep penetration of the divalent cations into the polar head group region of DMPG bilayers, whereby the ester carbonyl groups, located in the interfacial region of the bilayers, are indirectly affected by strong hydrogen bonding of immobilized water molecules. In the liquid crystalline phase, the interaction of all three cations with DMPG is weak, but still observable in the infrared spectra of the DMPG:Ca 2+ complex by a slight ordering effect induced in the acyl chains, when compared to pure DMPG liposomes.