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A new bisboronic acid based glucose sensor 7 was synthesized from 9,10-bis(chloromethyl)anthracene and 2,2-dimethyl-1,3-propanediol protected 3-pyridineboronic acid. Due to its ionic structure 7 was found sufficiently water soluble for carbohydrate binding studies at neutral pH by means of NMR spectroscopy, fluorometry, and potentiometry. The pK a's of 7 have been determined to 3.7 and 4.7 by potentiometric titration. From a solution of 7 and glucose (1:1) in water (pH 7.4) we observed the formation of a bisdentate boronic acid complex 7·Glu which has been assigned to a 1,2:3,5 bound α-d-glucofuranose complex. The evidence for this furanose structure comprises 1H and 13C NMR data with emphasis on the information from 1 J C - C coupling constants. Complex 7·Glu shows an increased fluorescence compared to 7. The stability constant for the 1:1 complex (log K = 3.4) was determined from fluorometric titration, potentiometry, and NMR spectroscopy. Boronic acid 7 shows good selectivity for glucose compared to fructose and galactose.

Complex formation between p-tolylboronic acid and D-fructose was investigated by I3C NMR spectroscopy both in neutral nonaqueous and alkaline aqueous solutions to evaluate the possibilities of boronic acid based fructose sensors. Under both conditions a mixture of complexes was observed. The structures of the complexes in solution were assigned on the basis of 'Jcc coupling constants which provided the information of the binding sites for the formed cyclic boronic esters. In alkaline aqueous solution seven different complexes were observed. At a 1 : 1 boronic acid : fructose ratio the major complex (82%) was J3-D-fructofuranose 2,3,6-tri-O-@-tolylorthoboronate) 1. The minor complex was p-Dfructofuranose 2,3-@-tolylhydroxyboronate) present in two diastereomeric forms 2a and 2b. At higher boronic acid : fructose ratios four additional complexes were observed namely the four possible diastereomeric forms of fi-D-fructopyranose 2,3:4,5-bis(p-tolylhydroxyboronate) 3a-d. Under nonaqueous conditions signals arising from 13 different compounds were observed. Five of these complexes were structurally assigned. At a 1 : 1 boronic acid : fructose ratio the major complex was fl-D-fructofuranose 2,3-@-tolylboronate) 5 present in 46%. At a ratio of 4: 1 the major species was p-D-fructopyranose 2,3:4,5bis@-tolylboronate) 7 present in 67%. Two additional monoesterified pyranoses and one furanose were observed. They were assigned to the ~-~-fructopyranosel,2-(p-tolylboronate) 8, the a-D-fructopyranose 1,3-@-tolylboronate) 9 and the Jh-fructofuranose 1,3-@-tolylboronate) 6. It is concluded that results of earlier studies on binding constants bear no relation to the actual formed complexes.

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