Transition metal amino acid complexes

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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.

Contents

AAcomplexation.png

Binding modes

Three coordination modes for 2-aminocarboxylates and related ligands. MLn(aa).png
Three coordination modes for 2-aminocarboxylates and related ligands.

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+).

Stoichiometry and structure

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 [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.

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)

Structure of RuCl(gly)(CO)3, known as CORM-3, is a CO-releasing drug. RuCl(gly)(CO)3.png
Structure of RuCl(gly)(CO)3, known as CORM-3, is a CO-releasing drug.

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

Intramolecular route to Co glycinamide complex. GlycinamideFormn.svg
Intramolecular route to Co glycinamide complex.

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]

[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(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. [12]

Aminocarboxylate complexes

A complex of ethylenediamine-N,N'-disuccinic acid M(edds)revd.png
A complex of ethylenediamine-N,N'-disuccinic acid

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.

Related Research Articles

<span class="mw-page-title-main">Coordination complex</span> Molecule or ion containing ligands datively bonded to a central metallic atom

A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.

<span class="mw-page-title-main">Ligand</span> Ion or molecule that binds to a central metal atom to form a coordination complex

In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs, often through Lewis bases. The nature of metal–ligand bonding can range from covalent to ionic. Furthermore, the metal–ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligands".

Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate ligand and a single central metal atom. These ligands are called chelants, chelators, chelating agents, or sequestering agents. They are usually organic compounds, but this is not a necessity.

<span class="mw-page-title-main">Metal ammine complex</span>

In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way for historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost all metal ions bind ammonia as a ligand, but the most prevalent examples of ammine complexes are for Cr(III), Co(III), Ni(II), Cu(II) as well as several platinum group metals.

<span class="mw-page-title-main">Pentetic acid</span> DTPA: aminopolycarboxylic acid

Pentetic acid or diethylenetriaminepentaacetic acid (DTPA) is an aminopolycarboxylic acid consisting of a diethylenetriamine backbone with five carboxymethyl groups. The molecule can be viewed as an expanded version of EDTA and is used similarly. It is a white solid with limited solubility in water.

<span class="mw-page-title-main">Hydroxamic acid</span> Organic compounds of the form –C(=O)N(OH)–

In organic chemistry, hydroxamic acids are a class of organic compounds bearing the functional group R−C(=O)−N(OH)−R', with R and R' as organic residues. They are amides wherein the nitrogen center has a hydroxyl substituent. They are often used as metal chelators.

<span class="mw-page-title-main">EDDHA</span> Chemical compound

EDDHA or ethylenediamine-N,N′-bis(2-hydroxyphenylacetic acid) is a chelating agent. Like EDTA, it binds metal ions as a hexadentate ligand, using two amines, two phenolate centers, and two carboxylates as the six binding sites. The complexes are typically anionic. The ligand itself is a white, water-soluble powder. Both the free ligand and its tetraanionic chelating agent are abbreviated EDDHA. In contrast to EDDHA, most related aminopolycarboxylic acid chelating agents feature tertiary amines and few have phenolate groups.

<span class="mw-page-title-main">Tris(ethylenediamine)cobalt(III) chloride</span> Chemical compound

Tris(ethylenediamine)cobalt(III) chloride is an inorganic compound with the formula [Co(en)3]Cl3 (where "en" is the abbreviation for ethylenediamine). It is the chloride salt of the coordination complex [Co(en)3]3+. This trication was important in the history of coordination chemistry because of its stability and its stereochemistry. Many different salts have been described. The complex was first described by Alfred Werner who isolated this salt as yellow-gold needle-like crystals.

In coordination chemistry, a stability constant is an equilibrium constant for the formation of a complex in solution. It is a measure of the strength of the interaction between the reagents that come together to form the complex. There are two main kinds of complex: compounds formed by the interaction of a metal ion with a ligand and supramolecular complexes, such as host–guest complexes and complexes of anions. The stability constant(s) provide(s) the information required to calculate the concentration(s) of the complex(es) in solution. There are many areas of application in chemistry, biology and medicine.

<span class="mw-page-title-main">Iminodiacetic acid</span> Chemical compound

Iminodiacetic acid is the organic compound with the formula HN(CH2CO2H)2, often abbreviated to IDA. A white solid, the compound is a dicarboxylic acid amine (the nitrogen atom forms a secondary amino group, not an imino group as the name suggests). The iminodiacetate dianion is a tridentate ligand, forming metal complexes by forming two, fused, five membered chelate rings. The proton on the nitrogen atom can be replaced by a carbon atom of a polymer to create an ion-exchange resin, such as chelex 100. Complexes of IDA and EDTA were introduced in the early 1950's by Schwarzenbach.

<span class="mw-page-title-main">DOTA (chelator)</span> Chemical compound

DOTA (also known as tetraxetan) is an organic compound with the formula (CH2CH2NCH2CO2H)4. The molecule consists of a central 12-membered tetraaza (i.e., containing four nitrogen atoms) ring. DOTA is used as a complexing agent, especially for lanthanide ions. Its complexes have medical applications as contrast agents and cancer treatments.

Metal acetylacetonates are coordination complexes derived from the acetylacetonate anion (CH
3COCHCOCH−
3) and metal ions, usually transition metals. The bidentate ligand acetylacetonate is often abbreviated acac. Typically both oxygen atoms bind to the metal to form a six-membered chelate ring. The simplest complexes have the formula M(acac)3 and M(acac)2. Mixed-ligand complexes, e.g. VO(acac)2, are also numerous. Variations of acetylacetonate have also been developed with myriad substituents in place of methyl (RCOCHCOR′−). Many such complexes are soluble in organic solvents, in contrast to the related metal halides. Because of these properties, acac complexes are sometimes used as catalyst precursors and reagents. Applications include their use as NMR "shift reagents" and as catalysts for organic synthesis, and precursors to industrial hydroformylation catalysts. C
5H
7O−
2 in some cases also binds to metals through the central carbon atom; this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III).

In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [M(H2O)n]z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand, but of course many complexes are known to consist of a mix of aquo and other ligands.

<span class="mw-page-title-main">Transition metal carboxylate complex</span> Class of chemical compounds

Transition metal carboxylate complexes are coordination complexes with carboxylate (RCO2−) ligands. Reflecting the diversity of carboxylic acids, the inventory of metal carboxylates is large. Many are useful commercially, and many have attracted intense scholarly scrutiny. Carboxylates exhibit a variety of coordination modes, most common are κ1- (O-monodentate), κ2 (O,O-bidentate), and bridging.

<span class="mw-page-title-main">Transition metal dithiocarbamate complexes</span>

Transition metal dithiocarbamate complexes are coordination complexes containing one or more dithiocarbamate ligand, which are typically abbreviated R2dtc−. Many complexes are known. Several homoleptic derivatives have the formula M(R2dtc)n where n = 2 and 3.

<span class="mw-page-title-main">Transition metal nitrite complex</span> Chemical complexes containing one or more –NO₂ ligands

In organometallic chemistry, transition metal complexes of nitrite describes families of coordination complexes containing one or more nitrite ligands. Although the synthetic derivatives are only of scholarly interest, metal-nitrite complexes occur in several enzymes that participate in the nitrogen cycle.

<span class="mw-page-title-main">Transition metal imidazole complex</span>

A transition metal imidazole complex is a coordination complex that has one or more imidazole ligands. Complexes of imidazole itself are of little practical importance. In contrast, imidazole derivatives, especially histidine, are pervasive ligands in biology where they bind metal cofactors.

Cobalt compounds are chemical compounds formed by cobalt with other elements.

<span class="mw-page-title-main">Tris(glycinato)cobalt(III)</span> Chemical compound

Tris(glycinato)cobalt(III) describes coordination complexes with the formula Co(H2NCH2CO2)3. Several isomers exist of these octahedral complexes formed between low-spin d6 Co(III) and the conjugate base of the amino acid glycine.

<span class="mw-page-title-main">Transition metal phosphate complex</span> Coordination complexes with one or more phosphate ligands

Transition metal phosphate complexes are coordination complexes with one or more phosphate ligands. Phosphate binds to metals through one, two, three, or all four oxygen atoms. The bidentate coordination mode is common. The second and third pKa's of phosphoric acid, pKa2 and pKa3, are 7.2 and 12.37, respectively. It follows that HPO2−4 and PO3−4 are sufficiently basic to serve as ligands. The examples below confirm this expectation. Molecular metal phosphate complexes have no or few applications.

References

  1. ↑ Severin, K.; Bergs, R.; Beck, W. (1998). "Bioorganometallic Chemistry-Transition Metal Complexes with α-Amino Acids and Peptides". Angew. Chem. Int. Ed. 37 (12): 1635–1654. doi:10.1002/(SICI)1521-3773(19980703)37:12<1634::AID-ANIE1634>3.0.CO;2-C. PMID   29711516.
  2. ↑ K.-Q. Gu; Y.-X. Sun; R. Zhang; N.-W. Zhang; H.-W. Che (2007). "Tris(glycinato-κ2N,O)cobalt(III)". Acta Crystallogr. E63 (3): m740–m742. doi:10.1107/S1600536807005636.
  3. ↑ A. Abbasi, B. Safarkoopayeh, N. Khosravi, A. Shayesteh (217). "Structural Studies of Bis(histidinato)nickel(II): Combined Experimental and Computational Studies". Comptes Rendus Chimie. 20 (5): 467. doi:10.1016/j.crci.2016.12.006.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ↑ M. Scharwitz, T. van Almsick, W. S. Sheldrick (2007). "(S-Methylcysteinato)(η5-pentamethylcyclopentadienyl)iridium(III) Trifluoromethanesulfonate hemihydrate". Acta Crystallogr. E63: m230-m232. doi:10.1107/S1600536806053360.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ↑ Baidya, N.; Ndreu, D.; Olmstead, M. M.; Mascharak, P. K. (1991). "Synthesis, Structure, and Properties of Potassium bis(L-cysteinato-N,S)nickelate(II) sesquihydrate". Inorganic Chemistry. 30 (10): 2448–2451. doi:10.1021/ic00010a043.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. 1 2 Kauffman, George B.; Karbassi, Mohammad; Kyuno, Eishin (1989). Tris(glycinato)cobalt(III). Inorganic Syntheses. Vol. 25. pp. 135–139. doi:10.1002/9780470132562.ch32. ISBN   978-0-470-13256-2.
  7. ↑ Denning, R. G.; Piper, T. S. (1966). "Optical Activity, Absolute Configuration, and Rearrangement Reactions of Tris Amino Acid Complexes of Cobalt(III) with L-Alanine, L-Leucine, and L-Proline". Inorganic Chemistry. 5 (6): 1056–1065. doi:10.1021/ic50040a022.
  8. ↑ Arnold, Alan P.; Jackson, W. Gregory (1990). "Stereospecificity in the Synthesis of the Tris((R)-Cysteinato-N,S)- and Tris((R)-Cysteinesulfinato-N,S)cobaltate(III) Ions". Inorganic Chemistry. 29 (18): 3618–3620. doi:10.1021/ic00343a061.
  9. ↑ Motterlini R, Otterbein LE (September 2010). "The therapeutic potential of carbon monoxide". review article. Nature Reviews. Drug Discovery. 9 (9): 728–43. doi:10.1038/nrd3228. PMID   20811383. S2CID   205477130.
  10. ↑ Denning, R. G.; Piper, T. S. (1966). "Optical Activity, Absolute Configuration, and Rearrangement Reactions of Tris Amino Acid Complexes of Cobalt(III) with L-Alanine, L-Leucine, and L-Proline". Inorganic Chemistry. 5 (6): 1056–1065. doi:10.1021/ic50040a022.
  11. ↑ Iakovidis, A.; Hadjiliadis, N. (1994). "Complex Compounds of Platinum(II) and (IV) with Amino Acids, Peptides and Their Derivatives". Coordination Chemistry Reviews. 135–136: 17–63. doi:10.1016/0010-8545(94)80064-2.
  12. ↑ Wolfgang Beck (2009). "Metal Ions and Metal Complexes as Protective Groups of Amino Acids and Peptides – Reactions at Coordinated Amino Acids". Z. Naturforsch. 64b: 1221–1245. doi: 10.1515/znb-2009-11-1202 . S2CID   96555456.
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