MG(II) ADSORPTION TO A PHOSPHATIDYLGLYCEROL MODEL MEMBRANE STUDIED BYATOMIC-ABSORPTION AND FT-IR SPECTROSCOPY

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 spe cies present in the thylakoid membrane of plant chloroplasts. The numb er of Mg(II) binding sites (no) in PG bilayer vesicles (PGV) was deter mined 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 framewor k of the lattice created by intermingling PG molecules. The FT-IR stud y 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 C =O's, as was also observed in bilayer membranes constituted of digalac tolipids (Fragata, M.; Menikh, A.; Robert, S. J. Phys. Chem. 1993, 97, 13920). A major finding is that n(0) = 8.1, meaning that Mg(II) binds or coordinates to about eight PG molecules. This result is particular ly interesting, since it is directly related to the coordination numbe r (CN) 8 of the Mg ion in a crystal lattice. CN = 8 is thus a clear in dication that the metal ion-lipid array adopts a Mg(II)-8PG lattice or molecular arrangement. An important question in this respect is the d etermination of the lattice energy per PG mole, U-0/PG, and the Born's energy of charging a Mg ion, Delta mu, that is the change in free ene rgy on transferring Mg(II) from a medium of low dielectric constant (e psilon), i.e., the H2O-PG interface (epsilon approximate to 25-32), in to one of high dielectric constant, i.e., the bulk aqueous solvent (ep silon approximate to 78). The calculations show that Delta mu is betwe en -27 and -18 kJ mol(-1) and U-0/PG 129 kJ mol(-1). That is, Delta mu is considerably smaller than U-0/PG. A straightforward conclusion is that the diffusion of the Mg ions from the H2O-PG interface into the b ulk aqueous phase is energetically favored but might not occur. This t herefore means that the calculations are consistent with the experimen tal observation that extensive dialysis of the PGV membranes cannot ex trude the bound Mg ions out of the PG head group. In conclusion, the M g(II)-8PG lattice concept developed in the present work is a new molec ular 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 (se e, for example, Tanford, C. The Hydrophobic Effect: Formation of Micel les and Biological Membranes, 1973).