11B-NMR Study of the Complex Formation of Borate with Catechol and L-Dopa

In the solution of catechol and borax at pH 11, the equilibria between catechol and the monomeric borate anion to form the 1 : 1 and 2 : 1 (catechol: boron) anionic complexes have been demonstrated by the existence of 11B-NMR signals for the two complexes and the monomeric borate anion. In the solution of L-dopa and borax at pH 11, however, the signal for the 2 : 1 anionic complex was not observed, though it was observed below pH 7. By the 11B-NMR spectra at pH 6.5, the complex formation constants, logK1 and logK2, have been estimated as 3.9 and 4.4 for catechol, and as 4.3 and 5.0 for L-dopa, respectively.