Fluorescence and CD spectroscopic sugar sensing by a cyanine-appended diboronic acid probe

A cyanine dye (4) bearing two boronic acids was designed and synthesized, expecting the selective binding of monosaccharides through the formation of 1:1 intramolecular complexes. While it aggregates in water, it exists discretely in water/methanol 1:1 mixed solvent. In the latter solvent the fluorescence spectra were scarcely affected by the medium pH but efficiently increased when it formed intramolecular 1:1 complexes with monosaccharides. This complexation mode was also corroborated by CD spectroscopy and continuous variation plots. Hence, the saccharide-induced fluorescence increase is rationalized in terms of 'rigidification' of the cyanine skeleton. The association constants (K) were estimated from plots of saccharide concentration vs. fluorescence intensity: the largest K was observed for D-fructose (1.3 x 10⁵ M^-1) and the next for D-arabinose (1.0 x 10⁴ M^-1). This is a novel system for sensitive and selective fluorescence detection of monosaccharides.