CALCIUM-ION COORDINATION - A COMPARISON WITH THAT OF BERYLLIUM, MAGNESIUM, AND ZINC

The coordination geometry of divalent calcium ions has been investigat ed by analyses of the crystal structures of small molecules containing this cation that are found in the Cambridge Structural Database, prot ein crystal structures in the Protein Databank, and by ab initio molec ular orbital calculations on hydrated structures of the form Ca[H2O](2 +)(n). mH(2)O, in which there are n water molecules in the first coord ination shell and m water molecules in the second coordination shell ( hydrogen bonded to water molecules in the first shell). Calcium ions i n crystal structures generally bind to oxygen atoms in ligands (rather than any other element), and their preferred coordination numbers ran ge from 6 to 8. In protein crystal structures the tendency of calcium to bind water molecules is less than for magnesium (1.5 versus 2.2 wat er molecules on the average per metal ion site, respectively). The rat io of bidentate to monodentate binding of calcium ions to carboxylate groups is similar for small molecules and protein structures in that n o bidentate binding occurs if the coordination number of Ca2+ is 6, bu t its occurrence rises to near 20% for coordination numbers 7 and 8. C omplexes of the form Ca[H2O](2+)(5). H2O and Ca[H2O](2+)(4). 2H(2)O we re found (by ab initio molecular orbital calculations in vacuo) to be significantly higher in energy than Ca[H2O](2+)(6) (by 8.2 and 15.0 kc al/mol, respectively). For Ca2+ surrounded by seven or eight water mol ecules, the differences in energy between Ca[H2O](2+)(6). H2O and Ca[H 2O](2+)(7) and among Ca[H2O](2+)(6). 2H(2)O, Ca[H2O](2+)(7). H2O, and Ca[H2O](2+)(8) are extremely small when diffuse functions are included in the basis set. Thus, the net energy penalty for changing the numbe r of water molecules in the first coordination shell between 6 and 8 i s small. Molecular orbital calculations also indicate that the effect of a calcium ion on the H-O-H angle to bound water is less (at normal coordination numbers) than that of magnesium, zinc, or beryllium.