Mg(II) Adsorption to a Phosphatidylglycerol Model Membrane Studied by Atomic Absorption and FT-IR Spectroscopy

Abstract

A study was undertaken of the interaction of the Mg ion, i.e., Mg(II), with the anionic phosphatidylglycerol (PG), one of the five lipid species present in the thylakoid membrane of plant chloroplasts. The number of Mg(II) binding sites (n0) in PG bilayer vesicles (PGV) was determined by equilibrium dialysis and atomic absorption spectroscopy, and the Mg(II) binding sites were identified by Fourier transform infrared (FT-IR) spectroscopy. The coordination interactions of the Mg ion in the phosphorylglyceryl moiety of PG were then examined in the framework of the lattice created by intermingling PG molecules. The FT-IR study shows that the sites of Mg(II) coordination are the negative charge in PO2 - , the C-O-P-O-C and C-O-C residues, and the sn1 and sn2 ester CdO’s, as was also observed in bilayer membranes constituted of digalactolipids (Fragata, M.; Menikh, A.; Robert, S. J. Phys. Chem. 1993, 97, 13920). A major finding is that n0 ) 8.1, meaning that Mg(II) binds or coordinates to about eight PG molecules. This result is particularly interesting, since it is directly related to the coordination number (CN) 8 of the Mg ion in a crystal lattice. CN ) 8 is thus a clear indication that the metal ion-lipid array adopts a Mg(II)-8PG lattice or molecular arrangement. An important question in this respect is the determination of the lattice energy per PG mole, U0/PG, and the Born’s energy of charging a Mg ion, ¢I, that is the change in free energy on transferring Mg(II) from a medium of low dielectric constant (), i.e., the H 2O-PG interface ( 25-32), into one of high dielectric constant, i.e., the bulk aqueous solvent ( 78). The calculations show that ¢I is between -27 and -18 kJ mol -1 and U0/PG ) 129 kJ mol -1 . That is, ¢I is considerably smaller than U0/PG. A straightforward conclusion is that the diffusion of the Mg ions from the H 2O-PG interface into the bulk aqueous phase is energetically favored but might not occur. This therefore means that the calculations are consistent with the experimental observation that extensive dialysis of the PGV membranes cannot extrude the bound Mg ions out of the PG head group. In conclusion, the Mg(II)-8PG lattice concept developed in the present work is a new molecular or fractal set, e.g., a Mandelbrot set, that will be instrumental in modeling the structures and geometries that minimize the opposing forces responsible for the stability of the lipid bilayer membrane (see, for example, Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 1973).