F-19 NMR-STUDIES OF FLUOROBENZENEBORONIC ACIDS .1. INTERACTION KINETICS WITH BIOLOGICALLY SIGNIFICANT LIGANDS

Kinetic studies of the interaction of 4-fluorobenzeneboronic acid and the 3-chloro-4-fluoro analog with hydroxyl ions and with several biologically significant ligands have been performed using F-19 line width and magnetization transfer measurements. Analysis of pH-dependent line-width data for the boronic acid-boronate equilibrium indicates that at millimolar concentrations the boronate dissociation rates are dominated by intermolecular hydroxyl ion transfer. Dissociation rate constants for the fluorobenzeneboronates due to intermolecular hydroxyl ion transfer to the corresponding boronic acid were determined to be 2.9 X 10(6) M-1 s-s and 1 X 10(7) M-1 s-1 for 4-fluorobenzeneboronate (FBA) and 3 chloro-4-fluorobenzeneboronate (CFBA), respectively, indicating a threefold greater rate for the 3-chloro derivative. This result presumably reflects a greater tendency of CFBA toward intermolecular association. Analogously, the presence of various buffers enhanced the rate of dissociation of hydroxyl ion from 4-fluorobenzeneboronate, with values of 2 X 10(7) M-1 s-1 and 2.5 x 10(8) M-1 s-1 for the effects of phosphate and imidazole, respectively. The proton-dependent dissociation pathway in the absence of buffers is apparently not significant in the region near neutral pH. A number of bidentate boron ligands studied, including a series of carbohydrates, catechols, and alpha-hydroxy acids, form complexes with sufficient stability to result in slow exchange kinetics on the F-19 time scale. At higher pH values, multiple resonances corresponding to the various boronate complexes could often be resolved. The F-19 shifts for these adducts are typically close to that of the boronate anion, consistent with an sp3-hybridized boronate structure. However, data obtained for the FBA-sorbitol adduct at lower Ph values support the presence of a trigonal boronic acid-sorbitol adduct. Dissociation of several ligand-FBA complexes was studied using magnetization transfer techniques. As in the case of hydroxyl ion dissociation, the presence of buffers significantly accelerated the dissociation rate of boronate-ligand complexes. Results were generally in close agreement with previous stopped-flow kinetic measurements, although the dissociation rate constant from the catechols is about an order of magnitude faster than the rate extrapolated from lower pH stopped-flow measurements.