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Kinetic waves in systems containing cobalt(II) and cysteine-like compounds

1968; Elsevier BV; Volume: 18; Issue: 3 Linguagem: Inglês

10.1016/s0022-0728(68)80262-1

2590-2954

I. M. Kolthoff, P. Mäder, S. E. Khalafalla,

Protein Interaction Studies and Fluorescence Analysis

Well-defined prewaves of cobalt are observed in its complex with borate in the presence of cysteine. The characteristics of the prewaves have been studied in the pH range 6.1–9.2. Below about pH 8, the prewave is caused by a complex of cobalt involving both the carboxylate and thiolate groups of cysteine. At higher pH, the complex with thiolate and uncharged amine becomes effective. No indication has been obtained that the complex is adsorbed at the potentials of the prewaves. The equilibria in systems containing cysteine, cobalt and borate are complex and a rate constant cannot be calculated. The situation is simpler with the ethyl ester of cysteine, in which the carboxylate is blocked. A rate constant, k, for the sequence: Co(II)−Et−ester→+2eCo(O)+Et−ester→k+Co(II)→frombulkCo(II)−Et−ester has been calculated and a value of 4.0·103 mole−1I sec−1 has been found at 25°. Cysteamine, like the ethyl ester, also forms a cobalt complex only with -S and NH2-groups, and gives prewaves. On the other hand, β-mercaptopronic and thioglycolic acid, which give prewaves of cobalt of complex appearance, can only form complexes with carboxylate and thiolate groups. Cobalt(III) hexammine chloride behaves like cobalt(II), taking into account the reduction of Co(III) to Co(II). The overvoltage of cobalt complexed to oxalate is very large and is greatly decreased by cysteine. However, no well-developed prewaves are observed.