APPLICATION OF TIME-RESOLVED V-51 2D NMR FOR QUANTITATION OF KINETIC EXCHANGE PATHWAYS BETWEEN VANADATE MONOMER, DIMER, TETRAMER, AND PENTAMER

A two-dimensional 51V homonuclear NMR exchange experiment (2D-EXSY) has been used to study the oligomerization reactions vanadate undergoes in aqueous solutions. This manuscript describes the first quantitative measurement of complex intermolecular chemical exchange rates by using 51V (I = 7/2) in a 2D-EXSY experiment. Microscopic (pseudo-first-order) rate constants for intermolecular exchange were obtained by using a numerical procedure to solve the 2D exchange matrix. The 2D exchange matrix was converted to a rate matrix that could be used in a kinetic analysis of the four exchanging vanadium species. The major pathway for monomer formation is unimolecular decomposition of the dimer. The major pathway for dimer formation is dimerization of the monomer. The tetramer forms mainly from two monomers and one dimer. At low vanadate concentrations, the pentamer forms from tetramer and either monomer or dimer with similar rate. At higher vanadate concentration, the pentamer exchanges more rapidly with the tetramer. The vanadate monomer is involved in more significant reaction pathways than any other species. The vanadate dimer is inherently more labile than the tetramer and pentamer as illustrated by its rapid hydrolysis rate. Our analysis demonstrates an approach that is applicable to solving other multiexchange systems. The 2D-EXSY method is versatile and may become central to determining the major reaction pathways by which vanadium acts in both chemical and biological systems. © 1990, American Chemical Society. All rights reserved.