Spectral Properties of Fluorophores Combining the Boronic Acid Group with Electron Donor or Withdrawing Groups. Implication in the Development of Fluorescence Probes for Saccharides

Abstract

We evaluated the spectral properties of four stilbene derivatives containing the boronic acid group [-B(OH) 2 ]: stilbene-4-boronic acid (STBA), 4′-cyanostilbene-4-boronic acid (CSTBA), 4′methoxystilbene-4-boronic acid (MSTBA), and 4′-(dimethylamino)stilbene-4-boronic acid (DSTBA). The emission spectrum of DSTBA displays a large solvent-polarity dependence showing the formation of a photoinduced charge transfer state (CT). This state is weakly present in MSTBA and not present for CSTBA and STBA for the neutral form of the boronic acid group. These results show the donor withdrawing property of the neutral form of the boronic acid group. At higher pH, the boronic acid group is present in the anionic form [-B(OH) 3 − ], resulting in a change of the configuration around the boron atom from the triangular planar (sp 2 hybridization) to the tetrahedral conformation (sp 3 hybridization). This change induced a blue shift of about 50 nm and an increase of intensity in the emission spectrum of DSTBA because of the loss of the electron-withdrawing properties for the anionic form of the boronic acid group, leading to the loss of the CT effect. The same effect is also observed for MSTBA. In contrast, a red shift of about 35 nm and a decrease of intensity are observed for CSTBA from the neutral to the anionic forms of the boronic acid group. These observations lead to the conclusion that the anionic form of the boronic acid group acts as an electron donor group and a photoinduced CT state can be formed when an electron withdrawing group is present on the fluorophore. The usefulness of this effect for the development of saccharide probes is also demonstrated. After addition of sugar, the emission spectra of DSTBA and MSTBA showed a blue shift and an increase of the intensity. On the other hand, a red shift and a decrease of the intensity are observed in the emission spectra of CSTBA after addition of sugar. A change from the neutral to the anionic form of the boronic acid group is used to explain these changes. These results show that the use of the combination of electron donor or withdrawing groups with the boronic acid group is a new and promising way to develop ratiometric fluorescent probes for glucose and other saccharides.

Fluorescence sensors for monosaccharides based on the 6-methylquinolinium nucleus and boronic acid moiety: potential application to ophthalmic diagnostics

“…The pK a of phenyl boronic acid is known to be tunable with the appropriate substituents [ 39 ], for example, an electron withdrawing group reduces the pK a while an electron donating group increases the pK a of the sugar bound form. We therefore considered the interaction between the quaternary nitrogen of the 6-methylquinolinium moiety, and the boronic acid group, which reduces the pK a of the probe.…”

Ophthalmic Glucose Monitoring Using Disposable Contact Lenses—A Review

“…In addition to the environmental parameters and constraints of pH and polarity, the probes also have to be sensitive to the very low concentrations of tear glucose, ≈ 500 µM for a healthy person, increasing up to several mM in diabetics, recalling that the blood glucose levels for a healthy person are ≈10-fold higher [14][15][16][17]. The pK a of phenyl boronic acid is known to be tunable with the appropriate substituents [44], for example, an electron withdrawing group reduces the pK a while an electron donating group increases the pK a of the sugar bound form. We therefore considered the interaction between the quaternary nitrogen of the 6-methyl and methoxy quinolinium moieties, and the boronic acid group, which reduces the pK a of the probe.…”

Effects of Cyclodextrins on Intramolecular Photoinduced Electron Transfer in a Boronic Acid Fluorophore

“…Although chemosensors based on boronic acid are not as selective as enzyme-based sensors, they have attracted much attention because of their greater stabilities. 4,5 Phenylboronic acid can readily form stable cyclic esters with the diol moiety of sugars in water, 6 and various types of boronic acid chemosensors with response mechanisms based on internal charge transfer (ICT), [7][8][9][10] photoinduced electron transfer (PET), [11][12][13] and fluorescent resonance energy transfer (FRET) 14 have been developed. The versatile designs of chemosensors based on supramolecular chemistry are another approach to construct novel sugar sensors.…”