Coordination of Biologically Important α-Amino Acids to Calcium(II) at High pH: Insights from Crystal Structures of Calcium α-Aminocarboxylates

A series of calcium α-aminocarboxylates was prepared by refluxing aqueous solutions/suspensions of calcium
hydroxide and the respective α-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 ≈ 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
α-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- = α-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.