Cysteine cobalt complexes

NOjQH, 4-Pyridinecarboxylic acid, cobalt complexes, 23 113 NOjSCjHj, L-Cysteine, gold complex, 21 31 N03CjH7, Serine, copper complex, 21 115 NO,H, Nitric acid, cerium complexes, 23 178 cobalt complexes, 23 171 N03SC7Hs, o-Benzosulfimide (saccharin), metal complexes, 23 47 N05P2WC4 Hj, Tungstate(l -), pentacarbon-ylhydrido-, p,-nitrido-bis(triphenylphos-phorus)(l + ), 22 182... 

NOCgHj, Benzoyl isocyanide, chromium complex, 26 32,34,35 N0C,3H,3, Formanide, N-(l-(l-naphthal-enyl)ethyll-, rhenium complex, 29 217 NOPCjsHjj, Benzamide. 2-(diphenylphos-phino)-N-phenyl-, 27 324 NOPCjjHjj, Benzamide, Af-[2-(diphenyl-phosphino)phenyl]-, 27 323 NO2CH3, Methane, nitro-, antimony complex, 29 113 cobalt complex, 29 114 NOjCjHj, 4-Pyridinecarboxylic acid, rhodium complex, 27 292 NO2P2C3JH33, Phosphorus(l-t-), p-nitrido-bis(triphenyl-, acetate, 27 296 NO2SC3H7, L-Cysteine, molybdenum complex, 29 255,258... 

The fact that proteins, in contrast to free cystine and cysteine, are capable of producing a catalytic wave even in solutions of complex trivalent cobalt ions may be explained only if we assume that cobalt is probably capable of entering with sufficient velocity into a complex with proteins, which in turn conditions the catalytic reaction, whereas with free cysteine cobalt reacts relatively more slowly. This explanation is supported by the fact that if the concentration of free cystine is substantially increased, a catalytic wave can also be produced in a solution containing Co (23). Thus the use of the latter solution in polarographic examinations of proteins and high-molecular polypeptides helps eliminate the undesirable influence of free cystine, cysteine, and their low-molecular polypeptides. 

Several synthetic Co(III) and Fe(III) complexes have been generated as models for the active-site metal center in nitrile hydratases [8, 10]. However, few have been examined in terms of water coordination and acidity. Cobalt complexes supported by Ns-type donor ligands, with two carboxamido nitrogen donors, exhibit a pfCa near 7 (Fig. 8.14a and b) [59, 60]. Introduction of two thiolate sulfur donors into the Co(III) coordination sphere increases the pK of the bound water to 8.3 (Fig. 8.14c) [61]. Interestingly, oxidation of one of the sulfur donors to a S-bound sulfmate (Fig. 8.14d) reduces the pK by around 1 unit [62]. As the active-site metal center in nitrile hydratases contain oxidatively modified cysteine residues coordinated to the metal center [8], it has been suggested that the oxidized sulfur donors play a role in modulating the acidity of the metal-bound water molecule. 

For tridentate amino acids with three non-equivalent donor atoms such as L-aspartic acid or L-cysteine, the isomers possible are illustrated below (252-254). There have been a number of reports of the preparation of L-aspartic acid complexes.1180,1181,1182. In the earlier work the isomers were not identified, however in the later study, the complexes were tentatively identified by comparison of their spectroscopic properties with those of the corresponding cobalt(III) complexes.1183 The order of elution of the complexes on HPLC was also similar to that observed for the corresponding cobalt(III) complexes. Mixed complexes containing l- or D-aspartate and L-histidine were also prepared.1182 A crystal structure of one salt obtained from this kind of system, bis(L-histidinato-0,Ar,Ar )chromium(III) nitrate, has been determined.1184... 

For catalytic waves of hydrogen evolution in ammoniacal cobalt solutions, it has been observed (132) that ery/Aro-phenylcysteine gives a higher catalytic wave than the threo form (Fig. 28). These differences can be explained partly by differences in acid dissociation constants, and partly by variations in the stability constants of the cobalt-phenyl-cysteine complexes. 

The crystal structures of dGTP-free AdoCbl-dependent RNR in the apo-form complexed with Co -aderunylpentyl-cobalamin (a structural analog of AdoCbl (3), see e.g. ) confirmed the base-on nature of the bound corrinoid, deduced from ESR data earlier. In crystalline RNR, the cracial cysteine is at a distance of about 10 A from the cobalt(II) center of the bound corrinoid, and in a region of space that is also well conserved in the three classes of RNRs. ... 

Fe- and Co-complexes of pyridines and triazacyclononane as synthetic analogs of cysteinate-ligated non-heme iron and non-corrinoid cobalt enzymes 04CRV825. 

Early work on the catalytic autoxidation of carboxythiols confirmed the effectiveness of manganese, iron, cobalt, copper, and arsenic, but the first major assault on the mechanism of the reaction was due to Michaelis and Barron [123,124]. The oxidation of cysteine at pH 7—8 was found to be zero order in cysteine and to involve metal—cysteine complexes as active intermediates. Several studies of metal—thiol complexes have been... 

---