Boronic acid fluorophore/beta-cyclodextrin complex sensors for selective sugar recognition in water

A novel boronic acid fluorophore 1/beta -cyclodextrin (beta -CyD) complex s ensor for sugar recognition in water has been designed. The probe 1 bearing pyrene moiety as a fluorescent signal transducer exhibits no fluorescence emission, due to its aggregation in water containing 2% DMSO; however, the addition of beta -CyD to this solution largely changes UV-vis and fluoresce nce spectra of 1 by forming an inclusion complex with beta -CyD, and an eff icient fluorescence emission response of 1/beta -CyD complex upon sugar bin ding is found to be obtained at pH 7.5. The pH-fluorescence profile of the 1/beta -CyD complex reveals that the boronate ester formation with fructose induces the apparent pK(a) shift from 7.95 +/- 0.03 in the absence of fruc tose to 6.06 +/- 0.03 in the presence of 30 mM fructose, resulting in the f luorescence emission response under the neutral condition. The spectral pro perties of 1 in 95% methanol:5% water (v/v), as well as the fluorescence qu enching study of l-methylpyrene with 4-methoxycarbonylphenyl-boronic acid 2 , demonstrate that the response mechanism is based on the photoinduced elec tron transfer (PET) from the pyrene donor to the acid form of phenylboronic acid acceptor in 1, and thus, the proton dissociation of phenylboronic aci d induced by sugar binding inhibits the PET system while increasing the flu orescence intensity of the pyrene moiety. To evaluate the binding ability a nd selectivity of the 1/beta -CyD complex for monosaccharides in water, the response equilibria have been derived. The 1:1 binding constants of the 1/ beta -CyD complex obtained from the equilibrium analysis are in the order: D-fructose (2515 +/- 134 M-1) much greater than L-arabinose (269 +/- 28 M-1 ) > D-galactose (197 +/- 28 M-1) > D-glucose (79 +/- 33 M-1), which is cons istent with the binding selectivity of phenylboronic acid.