A Slow Exchanging Vanadium(V) Peptide Complex: Vanadium(V)-Glycine-Tyrosine
AuthorCrans, D. C.
Keramidas, Anastasios D.
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The solution structure and rate of formation for a vanadium(V)-peptide complex, vanadium(V)-glycine-tyrosine (V-Gly-Tyr), is examined. Multinuclear NMR spectroscopic studies suggest the amide nitrogen is deprotonated and chelated to the vanadium as observed in peptide complexes of Ni(II) and Cu(II). The carboxylate and the amine groups in the dipeptide also appear to be chelated to the vanadium atom. The rate law for V-Gly-Tyr complex formation suggests that the complex is generated according to two major pathways, both involving H2VO4 − and deprotonated glycine-tyrosine dipeptide (Gly-Tyr−), one with and one without acid catalysis. The rate constant for the first pathway was 1.0 (±0.1) M−2 min‒1 (0.017 (±0.002) M‒2 s‒1), and the rate constant for the second pathway was 1.5 (±0.1) M‒1 min−1 (0.025 (±0.003) M−1 s‒1) The kinetics of complex formation were found to be significantly different from reported vanadate complexes such as vanadate dimer, vanadium(V)- alizarin, vanadium(V)-N-[tris(hydroxymethyl)methyl]glycine, vanadium(V)-EDTA, and vanadium(V)-ATPase. All of the previously studied complexes formed with H3O+ independent rate constants of about 104 M−1 s−1. The slow rates of formation of V-Gly-Tyr imply that equilibrium in such solutions is only slowly achieved. Time dependence curves reveal that the other complexes in the reaction between vanadate and Gly-Tyr form even more slowlyhowever, such complexes are likely to be oxidation products since these complexes were not observed when oxygen was excluded from the solutions. Only low levels of vanadium(IV) complexes were observed in the absence or the presence of oxygen, despite significant concentrations of oxidation products. These observations are consistent with the recycling of the vanadium(IV) to vanadium(V) under the consumption of oxygen. Increasing temperatures also increase the V-Gly-Tyr concentration, however, since the vanadate monomer (V1) concentration increases, little variation in the formation constant is observed. The reaction of vanadate with Gly-Ser was also found to be significantly slower than the formation of vanadate esters and required incubation for more than an hour before equilibrium was achieved. It appears that complex formation between vanadate and ligands containing the peptide functionalities occurs slowly, analogous to formation of peptide complexes of Ni(II) and Cu(II). © 1995, American Chemical Society. All rights reserved.