A fluorescent glucose sensor binding covalently to all five hydroxy groups of α-D-glucofuranose. A reinvestigation

Abstract

The structures of the complexes between a fluorescent bisboronic acid 7 and glucose have been determined. Shinkai et al.¹ previously studied the complex between 7 and glucose and they deduced a 1,2∶4,6-α-D-glucopyranose bisboronate structure. We have shown that this structure is only valid as an initial complex formed under completely nonaqueous conditions. In the presence of water the pyranose complex rearranges rapidly into an α-D-glucofuranose-1,2∶3,5,6-bisboronate in which all five free hydroxy groups of glucose are covalently bound by the sensor molecule. A favourable B–N interaction around the 1,2-binding site and the effect of an o-ammoniomethyl group on the pK_a value of the second boronic acid group allow for the observed binding at neutral pH. The structure evaluations are based on ¹H and ¹³C NMR data as well as information obtained from ¹J_CC coupling constants. The fluorescence spectra of both complexes were measured and discussed. MALDI TOF-MS experiments showed competitive formation of 1∶2 (boronic acid∶glucose) complexes under conditions of physiological glucose levels.