Transition metal amino acid complexes

Transition metal amino acid complexes are a large family of coordination complexes containing the conjugate bases of the amino acids, the 2-aminocarboxylates. Amino acids are prevalent in nature, and all of them function as ligands toward the transition metals.^[1] Not included in this article are complexes of the amides (including peptide) and ester derivatives of amino acids. Also excluded are the polyamino acids including the chelating agents EDTA and NTA.

Most commonly, amino acids coordinate to metal ions as N,O bidentate ligands, utilizing the amino group and the carboxylate. They are "L-X" ligands. A five-membered chelate ring is formed. The chelate ring is only slightly ruffled at the sp³-hybridized carbon and nitrogen centers.

For those amino acids containing coordinating substituents, the resulting complexes are more structurally diverse since these substituents can coordinate. Histidine, aspartic acid, methionine, and cysteine sometimes form tridentate N,N,O, N,O,O, S,N,O, and S,N,O complexes, respectively.

Using kinetically inert metal ions, complexes containing monodentate amino acids have been characterized. These complexes exist in either the N or the O linkage isomers. It can be assumed that such monodentate complexes exist transiently for many kinetically labile metal ions (e.g. Zn²⁺).

Mixing simple metal salts with solutions of amino acids near neutral or elevated pH often affords bis- or tris complexes. For metal ions that prefer octahedral coordination, these complexes often adopt the stoichiometry M(aa)₃ (aa = amino carboxylate, such as glycinate, H₂NCH₂CO₂⁻).

Complexes of the 3:1 stoichiometry have the formula is [M(O₂CC(R)HNH₂)₃]^z. Such complexes adopt octahedral coordination geometry. These complexes can exist in facial and meridional isomers, both of which are chiral. The stereochemical possibilities increase when the amino acid ligands are not homochiral. Both the violet meridional and red-pink facial isomers of tris(glycinato)cobalt(III) have been characterized^[6] With L-alanine, L-leucine, and other amino acids, one obtains four stereoisomers.^[7] With cysteine, the amino acid binds through N and thiolate.^[8]

Complexes with the 2:1 stoichiometry are illustrated by copper(II) glycinate [Cu(O₂CC(R)HNH₂)₂], which exists both in anhydrous and pentacoordinate geometries. When the metal is square planar, these complexes can exist as cis and trans isomers. The stereochemical possibilities increase when the amino acid ligands are not homochiral. Homoleptic complexes are also known where the amino carboxylate is tridentate amino acids. One such complex is Ni(κ³-histidinate)₂.

In addition to the amino acids, peptides and proteins bind metal cofactors through their side chains. For the most part, the α-amino and carboxylate groups are unavailable for binding as they are otherwise engaged in the peptide bond. The situation is more complicated for the N-terminal and O-terminal residues where the α-amino and carboxylate groups are unavailable, respectively. Especially important in this regard are histidine (imidazole), cysteine (thiolate), methionine (thioether).

Mixed ligand complexes are common for amino acids. Well known examples include [Co(en)₂(glycinate)]²⁺, where en (ethylenediamine) is a spectator ligand. In the area of organometallic complexes, one example of Cp*Ir(κ³-methionine).

A well studied complex is tris(glycinato)cobalt(III). It is produced by the reaction of glycine with sodium tris(carbonato)cobalt(III).^[6] Similar synthetic methods apply to the preparation of tris(chelates) of other amino acids.^[10]

Commonly amino acid complexes are prepared by ligand displacement reactions of metal aquo complexes and the conjugate bases of amino acids:^[11][12]

[PtCl₄]^2- + 2 H₂NCH(R)CO₂⁻ → [Pt(H₂NCH(R)CO₂)₂] + 4 Cl⁻

Relevant to bioinorganic chemistry, amino acid complexes can be generated by the hydrolysis of amino acid esters and amides (en = ethylenediamine):

[(en)₂CoOH(κ^1N-H₂NCH(R)CO₂Et)]²⁺ → [(en)₂CoOH(κ^2NO-H₂NCH(R)CO₂)]²⁺ + EtOH

Because their 5-membered MNC₂O chelate ring is rather stable, amino acid complexes represent protecting groups for amino acids, allowing diverse reactions of the side chains.^[13]

Organic compounds featuring two or more 2- and 3-aminocarboxylate groups are ligands of extensive use in nature, industry, and research. Famous examples include EDTA and NTA.