Amino acids tridentate chelation

Because of its biological and therapeutic importance L-Dopa and its coordination properties have been extensively studied. The weight of potentiometric and spectroscopic evidence points to the ligand chelating either as a normal bidentate amino acid (N,0) or through the two phenolate oxygens (0,0) the ligand is therefore classed as ambidentate (a similar situation occurs for tyrosine in which it is sterically impossible to achieve tridentate behaviour). 

Schiff base complexes formed from the tridentate amino acids are ambidentate in nature. Thus, with histidine, only pyruvate of those mentioned above leads to the His binding through the N(amino) and N-3(imidazole) with the others glycine-like chelation occurs. These different modes can be readily distinguished by their CD spectra. There appears to have been only a limited number of studies on the S-containing amino acids.45 7... 

Other representatives of the N,0,0-chelates are diorganotin-N-aryI idcne-ce-amino acid complexes (418)859,860, including 418a859 and 418b860, for which five-coordinate monomeric structures were established by X-ray studies. The tridentate ligands in these compounds are near-planar. The tin centers have a distorted TBP geometry with the two... 

The fundamental prerequisite for the applicability of this technique is the necessity of (usually bidentate or tridentate) chelation properties of both the chiral SO and the SA. This requirement is fulfilled by compounds having two polar functional groups in a favourable arrangement to each other, like a/p-amino acids, amino alcohols, a-hydroxy... 

Fig. 2.2.1. Various ligand bifunctional, tridentate chelating systems designed for the coupling to biomolecules and subsequent radiolabeling with the [M(OH2)3(CO)3] (coordinative atoms in boldface). Single amino acid chelates (SAAC) R" CO2H, R " NH2...

From potentiometric and spectroscopic studies it is concluded that the main species at neutral pH is a 1 1 tridentate chelate (11) with a log stability constant of ca. — 4. The claim that the stability of this species is comparable to that of the Cu(albumin) complex is rather surprising, since for this to occur the involvement of a histidine in the third amino acid position is normally required, and furthermore others have concluded that in blood plasma at least the tripeptide is unlikely to compete against other ligands for the available Cu". To illustrate the point that such conclusions from blood plasma simulations are only applicable to that medium, Pickart and Thaler have shown that in a cell culture medium the tripeptide considerably enhanced Cu uptake into cells and that this was not affected by a 300-fold molar excess of amino acids, including histidine. 

Lanthanide, alkaline earth metals, and alkaline metals coordination to peptides and amino acids generally involves the carboxylate terminal. Various coordination modes are identified in the literatnre. Monodentate modes are generally associated with alkaline metal-carboxylate interactions. Bidentate carboxylate coordination networks in the typical syn-syn bridging, chelate bidentate and tridentate modes... 

The copper complex of glycine has been used for the synthesis of j3-hydroxy-ot-amino acids. Under alkaline conditions the carbanion (1) formed from the complex adds to aliphatic and aromatic aldehydes, as in the formation of threonine (2) [18]. Further work on this reaction has shown that the protons on nitrogen in (1) also dissociate under the basic conditions and this has led to an improved procedure which employs the tridentate complex (3), having the double protection of Schiff s base and chelate formation [19]. 

Adolfsson et al. have used related amino acid-derived mOTio-l,4-disubstituted-1,2,3-triazole containing tridentate chelators, 86, as ligands in ATH reactions (Fig. 19) [190]. The authors screened a family of ligands with different steric and electronic properties in the Rh(I)-catalyzed ATH of acetophenone to 1-phenyethanol and showed that the use of these click catalysts led to good conversion (62-79%) and high ee (71-93%). 

Fig. 19 Amino acid-derived tridentate click chelators, 86, used as ligands in asymmetric transfer hydrogenation (ATH) reactions...

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