| Fundamental studies of metal-transport biochemistry are presented, with particular emphasis on two metal ions: uranium(VI), as the uranyl cation (UO 22+), and chromium(III). Solution thermodynamic interactions of uranyl peptide complexes and the kinetics of solvolysis of trinuclear chromium complexes were investigated.; Solution studies indicate peptide-carboxylato donors can coordinate to the uranyl biscarbonato complex [UO2(CO3)2] 2-. The conditional association constants for uranyl carbonate peptide complexes were determined by NMR titration as follows: log K = 3.1 +/- 0.4 for the [UO2(CO3)2DAHK]2- species, log K = 2.2 +/- 0.4 for the [UO2(CO3) 2GGH]- species. The biocoordination chemistry of uranyl cation was assessed utilizing X-ray structural data from the Protein Data Bank. Carboxylate donors from aspartate or glutamate in protein make up the majority of inner-sphere interactions with uranyl. The binding affinity between uranyl cation and histidine and N-acetyl histidine were determined by potentiometry as: log KML < 5.5, and log KML < 6.3, respectively. The interaction between uranyl cation and the peptides, DDD, DD, DA, AD, HD, and GGG, in aqueous solution was studied by potentiometry and NMR. Conditional binding constants for uranyl-peptide complexes were determined as follows: log KU-DDD = 5.11, log KU-DD = 3.96, log KU-DA = 3.21, log KU-AD = 3.22, log KU-HD = 2.10, log KU-GGG = 1.94. While these data support a non-specific mechanism of transport in the serum, this mechanism is still effective to eliminate soluble uranium from the body.; The 'basic chromium acetate' cluster, [Cr3O(OAc)6(H 2O)3]+, was used as a model of possible Cr(III) species in vivo. Solvolysis of the cluster was studied by visible spectrophotometry from pH 2 to pH 10 at varied temperature. At 50°C, the rate constant, kobs, was 3-4 x 10-5 s -1 under acidic conditions, while under basic conditions, kobs increased to 12-16 x 10-5 s-1. The presence of carbonate at basic pH did not catalyze the solvolysis. The rate increase in solvolysis is derived from a trans-effect in the conjugate base of the complex. Thermodynamic parameters suggest the solvolysis reaction proceeds through an associative interchange mechanism. |
