Boronic formation from saccharides

S-2, in which the spacer between the two boronic acids is flexible, has the additional capability of forming excimers. The 1 1 binding of a saccharide leads to an increase in the monomer fluorescence intensity. This increase has two origins the decrease in excimer formation, and the increase in fluorescence quantum yield resulting from suppression of the PET process. The 1 1 complex is formed at low saccharide concentrations, but increasing the concentration leads to the formation of the 1 2 complex, as revealed by the increase in the ratio of the intensities of the excimer band to the monomer band. The selectivity of S-2 was found to be similar to that of S-l. 

However, these four coupled equilibria from Scheme 12.1 are not the full story. Boronic acids readily form stable complexes with buffer conjugate bases (phosphate, citrate and imidazole) [20], In fact, both binary boronate-X complexes are formed with Lewis bases (X), as well as ternary boronate-X-saccharide complexes. In some cases, these previously unrecognized species persist into acidic solution and tmder some stoichiometric conditions they can be the dominant components of the solution. These complexes suppress the boronate and boronic acid concentrations, leading to a decrease in the measured apparent formation constants (f pp). As a consequence, the scope of the simple diol-boronate recognition system is greatly expanded over the simple picture of Scheme 12.1. 

As discussed in Section 3.2 the formation of a boronic acid-saccharide complex acidifies the boron atom, making the resultant boronic ester more acidic than the initial uncomplexed boronic acid. In this instance, a of 8.8 was reported for the neutral 2-anthrylboronic acid, and a p a of 5.9 was reported for the 2-anthrylboronic acid complex formed in saturated fructose solution. Exploiting this phenomenon, the system was bulfered to a pH of 7.4, a value between the corresponding pA a and pKj values reported. With this constraint in place, a high-fluorescence emission intensity was observed from the uncomplexed boronic acid (pH < pKf). However, imder these buffered conditions, addition of a saccharide to the solution formed the boronic ester, lowering the acidity of the boronic species below the pH of the solution pKf < pH). As a direct result, the boronate anion was generated inducing the decrease in fluorescence observed on addition of saccharide. 

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