Calcium binding to phospholipid: structural study of calcium glycerophosphate.

To consider possible interaction of the phospholipid membrane with calcium ions, crystal structures of calcium dl-a- and /3-glycerophosphates (a- and p-CaGs, respectively) were in- vestigated by X-ray diffraction methods. After many attempts, relatively large single crystals of @-CaG were prepared from the aqueous solution containing HCl, while crystals of CaHPOt 2HtO were obtained from a-CaG solution under the same crystallization conditions. The crystal structure of p- CaG is orthorhombic with space group Pna2, and cell dime?- sions of a = 8.?51(1), b = 13.038(3), c = 25.483 (10) A, V = 2741.5 (13) A3 and 2 = 16 (four molecules (A to D) in an asymmetric unit). Molecules of A to D took, as a whole, similar extended conformations, although A and B were different from C and D in the orientation about a glycerol C-C bond. Four in- dependent /3-glycerophosphates commonly act as two types of bidentate ligands, where one is the coordination to the calcium ion by the glycerol O(1) and phosphate O(22) atoms, and the other by the phosphate O(22) and O(23) atoms, thus forming the calcium coordination of a distorted square plane, respec- tively. Each of four independent calcium ions forms the same coordination geometry of a distorted pentagonal bipyramid. Infinite double layers consisting of alternate A/B molecules and of alternative C/D ones and sandwiching calcium ions were ar- ranged face-to-face along the &direction and were piled up in the a-direction, thus formjng the stacked bilayer unit with the thickness of dOo2 = 12.75 A. The elaborate networks of calcium coordinations and hydrogen bondings were formed among the layers and stabilized the crystal structure. Based on the struc- tural parameters of the present P-CaG crystal, a possible inter- action model of phospholipid with calcium ions was pro- posed. - Inoue, M., Y. In, and T. Ishida. Calcium binding to phospholipid: structural study of calcium glycerophosphate. J. Lipid Res. 1992. 33: 985-994.