Coordination of biologically important alpha-amino acids to calcium(II) at high pH: insights from crystal structures of calcium alpha-aminocarboxylates.

A series of calcium alpha-aminocarboxylates was prepared by refluxing aqueous solutions/suspensions of calcium hydroxide and the respective alpha-amino acid. The colorless, crystalline hydrates Ca(gly)2.H2O (1), Ca(ala)2.3H2O (2), Ca(val)2.H2O (3), Ca(leu)2.3H2O (4), Ca(met)2.nH2O (5, n approximately 2), and Ca(pro)2.H2O (6) have been isolated in yields between 29 and 67% (gly- = glycinate, ala- = rac-alaninate, val- = rac-valinate, leu- = rac-leucinate, met- = rac-methioninate, pro- = rac-prolinate). The compounds 1-6 are readily soluble in water. The 0.10 M solutions have ca. pH 10-11 which is consistent with a noticeable degree of dissociation. The 13C NMR spectra of 1-6 in D2O were measured, and their comparison with those of the corresponding tetramethylammonium alpha-aminocarboxylates point to carboxylate coordination in solution, but no indication of nitrogen coordination was found. Infrared spectra of 1-6 gave similar results for the solid state. Complete single-crystal X-ray structure analyses of 1-4 and preliminary ones of 5 and 6, however, revealed that all aminocarboxylate ligands are N,O-chelating. Crystals of 2 consist of mononuclear complexes, while the other five compounds form three different types of one-dimensional coordination polymers. Structural diversity is also observed with the binding modes of the aminocarboxylate ligands and the calcium environment. Besides terminal aminocarboxylate coordination, there are three different types of aminocarboxylate bridges. The calcium ions are seven- or eight-coordinate in N2O5 and N2O6 coordination environments, respectively; one or three water molecules are part of the first ligand sphere of each metal ion. The crystal structures support conjectures about the existence of the yet undetected solution species [Cax(aa)2x(H2O)n] (aa- = alpha-aminocarboxylate). For example, x = 1 is realized in crystalline [Ca(ala)2(H2O)3] (2), and in 4 [Ca2(leu)4(H2O)4] complexes (x = 2) are linked to infinite chains by bridging aqua ligands.