Early Fluorescent Sensors for Saccharides

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. 

The corresponding isomer 9-anthrylboronic 37, was also examined but displayed smaller changes in fluorescence emission, a feature attributed to the unfavourable peri-interactions that would be expected at the 9-position. The observed stability constant (Afobs) for boronic acid 36 was 270 with d-fructose in 1 99 (v/v) DMSO/water at pH 7.4 (phosphate buffer).  

The research group of Aoyama investigated the fluorescence properties of 

5-indolylboronic acid 38 with mono-, di- and higher saccharides. It was found that the stability constants of the monosaccharides mirrored the 

It was postulated that the increased stabilisation imparted to complexes with the longer-chained saccharides could be derived from secondary effects such as advantageous hydrophobic interactions with the extended aromatic 7i-system or from CH-ti interactions. 

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