Electron paramagnetic resonance, kinetics of formation and decomposition studies of (bis(hydroxyethyl)amino-tris(hydroxymethyl)-methane)oxochromate(V): a model chromium(V) complex for DNA damage studies.

A new chromium complex, (bis(hydroxyethyl)amino-tris(hydroxymethyl)methane)oxochromate(V), has been characterized by epr spectroscopy. The chromium(V) complex was formed by the ligand displacement reaction of bis(2-ethyl-2-hydroxybutanato) oxochromate(V) with bis(hydroxyethyl)amino-tris(hydroxy-methyl)methane (BT). Both epr and kinetic data indicate that the reaction proceeds through a chromium(V) intermediate. Kinetics of formation of the intermediate exhibit a rate saturation at higher [BT] (> 30 mM) indicating a rate law constituting an equilibrium between the parent Cr(V) complex and the bis-tris ligand followed by a pure first order process. The g-value of the intermediate is consistent with a Cr(V) complex in which the BT is coordinated in a bidentate fashion replacing a coordinated hydroxy butanoic acid ligand, affording a mixed ligand complex. The equilibrium step (K = 36 M-1) consists of monodentate coordination by the BT ligand and the limiting first order rate constant (1.9 x 10(-2) s-1) manifests the rate of chelation by the polydentate ligand. The intermediate is converted to the product upon further chelation through the complete displacement of the remaining 2-ethyl-2-hydroxy butanoic acid by a first order process (k = 0.023 s-1). The epr data support a pair of products that are in rapid equilibrium. In these products, BT functions either as a tetra or a penta-dentate ligand coordinating through four or five alkoxy sites. The enthalpy and entropy of activations related to the two chelation steps were found to be 32 +/- 2 kJ/mol and -(1.7 +/- 0.2) x 10(2) J/mol K for the intermediate, and 36 +/- 1 kJ/mol and -(1.5 +/- 0.2) x 10(2) J/mol K for the product. Our data support an associative mechanism for the chelation steps. The Cr(V)-BT product is more stable than the parent complex. The second order disproportionation rate constant for the Cr(V)-BT complex was evaluated to be 0.1 M-1 s-1 compared to 8.0 M-1 s-1 for the parent complex. This is the first example of a chromium(V) complex with a non-macrocyclic ligand coordinating through oxygen donor atoms which is stable in aqueous solution at neutral pH over a long period of time.