STRUCTURAL AND KINETIC CHARACTERIZATION OF SIMPLE COMPLEXES AS MODELS FOR VANADATE PROTEIN INTERACTIONS

The structural and kinetic properties of the complexes that form between vanadate and diethanolamine (DEA) and vanadate and N-[tris(hydroxymethyl)methyl]glycine (Tricine) were characterized by use of potentiometry and V-51, H-1, and C-13 NMR spectroscopy. Formation constants were determined in 0.6 M Na(Cl) at 25-degrees-C. The pK(a) values were determined for DEA (pK(HDEA+) = 9.072 +/- 0.003) and Tricine (pK(HTricine+) = 2.020 +/- 0.001, pK(Tricine) = 7.929 +/- 0.004). The ternary system H+-H2VO4--DEA contained one complex with a (0,1,1) composition and a charge of -1 with log beta-0,1,1 = 3.02 +/- 0.02. The ternary system H+-H2VO4--Tricine contained two complexes of the compositions (0,1,1) and (1,1,1), respectively. The (0,1,1) complex had an overall charge of -1, and the (1,1,1) complex was neutral. The log beta-0,1,1 = 3.65 +/- 0.01, and the log beta-1,1,1 = 6.69 +/- 0.07. The structural characteristics of the V-Tricine (0,1,1) complex were further explored by H-1 and C-13 NMR spectroscopy. The Tricine in the V-Tricine complex was found to bind to vanadium through a hydroxyl, the carboxylate, and the amine group. By use of 2D C-13 EXSY NMR, the intramolecular exchange rate constant of the three hydroxyl groups in the V-Tricine (0,1,1) complex was measured to be 3.1 s-1 at 273 K. Correspondingly, the intermolecular exchange rate constant between Tricine and the (0,1,1) complex was determined to be 5.3 s-1 at 298 K. This microscopic rate constant corresponds to a second-order rate constant of 0.76 x 10(4) M-1 s-1. These observations combined with previous studies are in accord with the interpretation that the rate of complex formation is limited by the loss of a hydroxo or aqua ligand in the vanadium complexes. The structural preferences and kinetic properties observed for these complexes may aid in understanding the chemical interactions of vanadate with amino acids, peptides, and proteins and the biological activities of vanadium.