Amino acids thioether ligands

In 1995, Bolm et al. reported that the vanadium complexes of the triden-tate and amino-acid derived ligands 15 catalyze the enantioselective sulfoxidation of prochiral thioethers with dihydrogen peroxide as the terminal oxidant (Scheme 8) (24). The ligands are synthesized by a one-step condensation of the readily available salicylic aldehydes 16 with amino-acid derived amino alcohols, such as 17a. 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

Ring-opened adducts of carboplatin can form not only from reactions with nucleotides, but also by the attack of sulfur amino acids [44]. Reactions with thioether ligands are much more rapid compared with thiols. Surprisingly very stable ring-opened species such as [Pt(CBDCA-0)(NH3)2(L-HMet-S)] are formed, which has a half-life for Met-S,N closure of 28 h at 310 K. Such an intermediate could also play a role in the biological activity of this drug. 

Eow-spin Fe porphyrins continue to be of major interest to many investigators, both because of the large number of different low-spin complexes that can be prepared, and because of the relevance of this spin state to a variety of heme proteins. Ligands that give rise to low-spin complexes of Fe porphyrins include two imidazoles, two imidazolates, two pyridines (for TPPs but not OEP and natural porphyrins for lower-basicity pyridines), " two thioethers, two thiolates, two isocyanides, two phosphines, two anunonia molecules, two primary amines (as amino acid esters), two cyanides, two nitrite ions, two methoxide ions, two alkyl... 

Most known multiheme cytochromes and enzymes belong to the family of cytochromes c (see Iron Heme Proteins Electron Transport), which contain Fe-protoporphyrin IX covalently attached to the polypeptide chain by two thioether bonds, formed by addition of two cysteinyl residues to the vinyl side-chains of the porphyrin ring. The two cysteines form a characteristic amino acid sequence motif CXXCH, usually indicative of heme c ligation, and where the histidine is the axial fifth ligand to the iron. For some cytochromes (see Section 2), the number of residues between the two cysteines can be three or four. The heme redox potentials in cytochromes c cover a wide range and are tuned by several factors, usually dominated by the type of axial ligation and the extent of solvent exposure of the heme. ... 

Although we have discussed the low oxidation state of the early atmosphere and its implications for the form of sulfur in organisms, today this chemistry is largely restricted to anoxic environments. Here sulfur is available as hydrogen sulhde and can be incorporated into amino acids such as cysteine and methionine and then into proteins. The thiolate group RS of cysteine of the thioether of methionine can act as bases or ligands for transition metals such as iron, zinc, molybdenum, and copper. 

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