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-alpha- and beta-glycerophosphates (alpha- and beta-CaGs, respectively) were investigated by X-ray diffraction methods. After many attempts, relatively large single crystals of beta-CaG were prepared from the aqueous solution containing HCl, while crystals of CaHPO4.2H2O were obtained from alpha-CaG solution under the same crystallization conditions. The crystal structure of beta-CaG is orthorhombic with space group Pna2(1) and cell dimensions of a = 8.251(1), b = 13.038(3), c = 25.483 (10) A, V = 2741.5 (13) A3 and Z = 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 independent beta-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, respectively. 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 arranged face-to-face along the b-direction and were piled up in the a-direction, thus forming the stacked bilayer unit with the thickness of d002 = 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 structural parameters of the present beta-CaG crystal, a possible interaction model of phospholipid with calcium ions was proposed.