Detailed Reaction Mechanism of Phenylboronic Acid with Alizarin Red S in Aqueous Solution: Re-Investigation with Spectrophotometry and Fluorometry

Re-investigation of the reaction of phenylboronic acid (PhB(OH)₂) with Alizarin Red S (ARS, H₂L) was carried out using absorption and emission spectrophotometry to correct the currently proposed mechanism. Kinetic results obtained by both methods, which essentially show no significant differences, showed that the kinetic reactivity of ARS toward PhB(OH)₂ was in the order H₂L < HL^– and that of phenylboronic acid toward HL^– was PhB(OH)₂ > PhB(OH)₃ ^–. Acetic acid/acetate buffer accelerated the present reaction, though HEPES buffer did not. A detailed reaction mechanism involving the acceleration effect of the acetate buffer is proposed for the present reaction on the basis of a recent universal reaction mechanism.