Relative reactivity of phenylboronic acid to phenylboronate ion toward L-lactate and mandelate at physiological pH

Kinetic studies were performed on the reactions of phenylboronic acid with L-lactic acid and mandelic acid in acidic aqueous and alkaline solutions in order to specify reactive species in these reactions. It was confirmed that the diprotonated ligand (H2L: L-lactic acid or mandelic acid) is less reactive than the monoprotonated ligand (HL−: L-lactate ion or mandelate ion), which made possible direct determination of the rate constants of phenylboronic acid (PhB(OH)2) and its conjugate base, phenylboronate ion (PhB(OH)3−). It was found that PhB(OH)2 is more reactive than PhB(OH)3−. On the basis of kinetic results, it was concluded that the most reactive species are PhB(OH)2 and HL− at physiological pH 7.4, so the reaction in the boronic acid-based sensor for L-lactate mainly would occur between these species.