Screening of boronic acids for strong inhibition of the hydrolytic activity of α-chymotrypsin and for sugar sensing associated with a large fluorescence change

The inhibitory effect of arylboronic acids which act as a transition state analogue for certain peptidases is efficiently intensified by added saccharides: this finding enables us to control the enzyme activity by a combination of arylboronic acids and polyols including sugars. In particular, the combined system of 3-biphenylboronic acid and D-glucose strongly inhibited the hydrolysis reaction mediated by α-chymotrypsin, the inhibitory effect (K_i = 1.1×10^-7 mol dm^-3) being stronger than that of a specific inhibitor, chymostatin (K_i = 4.8×10^-7 mol dm^-3). Hence, saccharides act as a 'co-inhibitor' in the boronic acid inhibition system. To search for such fluorescent boronic acids that are useful for sensitive detection of sugars in water, 8 aromatic boronic acids have been screened. Among them biphenyl-3-boronic acid and naphthalene-2-boronic acid could fully satisfy three prerequisites for sugar sensing: strong fluorescence intensity, large pH-dependent change in I_max, and shift of the pH-I_max profile to lower pH region in the presence of sugars.