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.{{cite journal|journal=Angew. Chem. Int. Ed. |author1=Severin, K. |author2=Bergs, R. |author3=Beck, W. |title=Bioorganometallic Chemistry-Transition Metal Complexes with α-Amino Acids and Peptides|year=1998|volume=37|issue=12 |pages=1635–1654|doi=10.1002/(SICI)1521-3773(19980703)37:12<1634::AID-ANIE1634>3.0.CO;2-C|pmid=29711516 }} 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.
Binding modes
Most commonly, amino acids coordinate to metal ions as N,O bidentate ligands, utilizing the amino group and the carboxylate. A five-membered chelate ring (NCCCOM) is formed. The chelate ring is only slightly ruffled at the sp3-hybridized carbon and nitrogen centers.
N,O bidentate amino carboxylates are "L-X" ligands in the Covalent bond classification method. With respect to HSAB theory, N,O bidentate amino carboxylate is a pair of hard ligands.
For those amino acids containing coordinating substituents, the resulting complexes are more structurally diverse since these substituents can coordinate. Histidine, aspartic acid, and methionine sometimes function as tridentate N,N,O, N,O,O, and S,N,O, ligands, respectively. Doubly deprotonated cysteine is often an N,S-bidentate ligand.
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+).
Stoichiometry and structure
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=Homoleptic complexes (only amino acid ligands)=
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{{cite book |doi=10.1002/9780470132562.ch32|title=Inorganic Syntheses |year=1989 |last1=Kauffman |first1=George B. |last2=Karbassi |first2=Mohammad |last3=Kyuno |first3=Eishin |chapter=Tris(Glycinato)Cobalt(III) |pages=135–139 |volume=25|isbn=978-0-470-13256-2 }} With L-alanine, L-leucine, and other amino acids, one obtains four stereoisomers.{{cite journal |doi=10.1021/ic50040a022|title=Optical Activity, Absolute Configuration, and Rearrangement Reactions of Tris Amino Acid Complexes of Cobalt(III) with L-Alanine, L-Leucine, and L-Proline |year=1966 |last1=Denning |first1=R. G. |last2=Piper |first2=T. S. |journal=Inorganic Chemistry |volume=5 |issue=6 |pages=1056–1065 }} With cysteine, the amino acid binds through N and thiolate.{{cite journal |doi=10.1021/ic00343a061|title=Stereospecificity in the Synthesis of the Tris((R)-Cysteinato-N,S)- and Tris((R)-Cysteinesulfinato-N,S)cobaltate(III) Ions |year=1990 |last1=Arnold |first1=Alan P. |last2=Jackson |first2=W. Gregory |journal=Inorganic Chemistry |volume=29 |issue=18 |pages=3618–3620 }}
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.
=Peptides and proteins=
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).
=Heteroleptic complexes (amino acids plus other ligands)=
File:RuCl(gly)(CO)3.png, known as CORM-3, is a CO-releasing drug.{{cite journal | vauthors = Motterlini R, Otterbein LE | title = The therapeutic potential of carbon monoxide | journal = Nature Reviews. Drug Discovery | volume = 9 | issue = 9 | pages = 728–43 | date = September 2010 | pmid = 20811383 | doi = 10.1038/nrd3228 | s2cid = 205477130 | department = review article }}]]
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).
Synthesis and reactions
A well studied complex is tris(glycinato)cobalt(III). It is produced by the reaction of glycine with sodium tris(carbonato)cobalt(III). Similar synthetic methods apply to the preparation of tris(chelates) of other amino acids.{{cite journal |doi=10.1021/ic50040a022|title=Optical Activity, Absolute Configuration, and Rearrangement Reactions of Tris Amino Acid Complexes of Cobalt(III) with L-Alanine, L-Leucine, and L-Proline |year=1966 |last1=Denning |first1=R. G. |last2=Piper |first2=T. S. |journal=Inorganic Chemistry |volume=5 |issue=6 |pages=1056–1065 }}
Commonly amino acid complexes are prepared by ligand displacement reactions of metal aquo complexes and the conjugate bases of amino acids:{{cite journal |doi=10.1016/0010-8545(94)80064-2|title=Complex Compounds of Platinum(II) and (IV) with Amino Acids, Peptides and Their Derivatives|year=1994|last1=Iakovidis|first1=A.|last2=Hadjiliadis|first2=N.|journal=Coordination Chemistry Reviews|volume=135-136|pages=17–63}}{{cite journal |doi=10.1016/S0010-8545(97)00047-7|title=Donor Atom Preferences in Complexes of Platinum and Palladium with Amino Acids and Related Molecules|year=1997|last1=Appleton|first1=Trevor G.|journal=Coordination Chemistry Reviews|volume=166|pages=313–359}}
:[PtCl4]2- + 2{{nbsp}}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(κ1N-H2NCH(R)CO2Et)]2+ → [(en)2CoOH(κ2NO-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.{{cite journal|title=Metal Ions and Metal Complexes as Protective Groups of Amino Acids and Peptides – Reactions at Coordinated Amino Acids
|author=Wolfgang Beck|journal=Z. Naturforsch.|year=2009|volume=64b|pages=1221–1245|doi=10.1515/znb-2009-11-1202|s2cid=96555456 |doi-access=free}}
