Fluorescent monoboronic

It is known that the acidity of monoboronic acids is intensified when they form covalent complexes with diols[3-7,l 1-13]. Hence, at constant pH the saccharide addition can change neutral boronic acids to anionic boronate esters. In fluorescent monoboronic acids this change is reflected as a decrease in the fluorescence intensity (/) of the neighboring fluorophore[l 1-13]. Typical examples are compounds 1-5. 

Cao H, Diaz DI, DiCesare N, Lakowicz JR, Heagy MD. Monoboronic acid sensor that displays anomalous fluorescence sensitivity to glucose. Organic Letters 2002, 4, 1503-1505. 

Sun, X. Y. Liu, B. Jiang, Y. B. An Extremely Sensitive Monoboronic Acid Based Fluorescent Sensor for Glucose. Anal. Chim. Acta 2004, 515, 285-290... 

Zhang X, Chi L, Ji S et al (2009) Rational design of d-PeT phenylethynylated-carbazole monoboronic acid fluorescent sensors for the selective detection of a-hydroxyl carboxylic acids and monosaccharides. J Am Chem Soc 131 17452-17463... 

Figure 6.6 a-Methylbenzylamine based fluorescent chiral boronic acid chemosen-sors (i ,i )-(-)-5 and (S,S)-(+)-5. The monoboronic acid chemosensor 6 is also presented. Anthracene was used as fluorophore. 

Figure 6.8 Relative fluorescence intensity of sensors 5 and 6 versus concentration of D- or L-tartaric acid, (a) Bis-boronic acid 5 with d- and L-tartaric acid, Aex at 365 nm, Agm at 429 nm, pH 8.3 (b) monoboronic acid 6 with D- and L-tartaric acid, Ae at 373 nm, Ag, at 421 nm, at pH 7.0 3.0 x lO" mol dm" of sensors in 5.0 x lo mol dm" NaCl solution (52.1% methanol in water), 22 °C. (Reproduced by permission of the American Chemical Society.)...

Based on the fitting of the fluorescence responses, the binding constant for the combination of (/ )-sensor/D-tartaric acid is logATp 2.79 0.12 whereas for (i )-sensor/L-tartaric acid it has not been determined due to the negligible fluorescence change. Reference chiral chemosensors with a mono-chirogenic centre and monoboronic binding site were also prepared. Indeed,... 

In this case, the sensing mechanism with the bis-boronic acid is open to question. With monoboronic acid sensor 6, fluorescence enhancement was observed in the presence of both d- and L-tartaric acid (Figure 6.8b). For the combination of D-bis-boronic acid sensor and L-tartaric acid, however, no fluorescence enhancement was observed. Herein, we proposed that 1 1 cyclic binding complexes form, otherwise fluorescence enhancement should be observed with the 1 2 binding complexes. Interestingly, the putative 1 1 binding complexes is weakly fluorescent, therefore no significant fluorescence enhancement was observed. The reason for the weak fluorescence of the D-bis-boronic acid sensor/D-(or L)-tartaric acid is unclear. 

Interestingly, we observed a much larger fluorescence enhancement for (iS)-8 compared to the monoboronic acid chemosensor without an intramolecular boronate structure (Figure 6.14). An enhanced fluorescence... 

James and co-workers have prepared 12a, a monoboronic acid fluorescent sensor that shows large shifts in emission wavelength on saccharide binding [50]. The dual fluorescence of 12a, can be ascribed to locally excited (LE) and twisted internal charge transfer (TICT) states of the aniline fluorophore [51]. When saccharides interact with sensor 12a in aqueous solution at pH 8.21 the emission maxima at 404 nm (TICT state) shifts to 362 nm 274 nm, LE state). The band at 404 nm is due to the TICT state of 12a containing a B-N bond i.e. the lone pair is coordinated with the boron and perpendicular to the jt-system. The band at 274 nm (LE state) corresponds to the situation where the B-N bond in 12a has been broken with formation of the boronate (Scheme 12.3). 

Figure12.3 Monoboronic acid photoinduced electron transfer (PET) fluorescent sensors.

Hayashita and Teramae have prepared an interesting fluorescent ensemble that consists of compound 38 and p-cyclodextrin [98], The system displays fluorescence enhancement on saccharide binding and, as expected for a monoboronic acid, the highest binding was observed with D-fructose. Observed fC pp for 38 were 2515 for D-fructose and 79 M for D-glucose in 2% (v/v) DMSO-water at pH 7.5. 

Fluorescent sensors based on monoboronic acid often exhibit selectivity for fructose. The development of glucose-selective chemosensors in the medical field is being pursued vigorously because of the need to measure blood glucose levels for the management of diabetes. 

The fluoreseence titrations of sensor 147(phenanthrene pyrene) (2.5 X 10 mol dm , Xex=299 nm for phenanthrene and Xex=342 nm for pyrene) with different saccharides were carried out in a pH 8.21 aqueous methanolic buffer, as described above (page 86). Absorption V5. concentration plots of sensor 147(phenanthrene pyrene) and the monoboronic add reference compounds 146(pyrene) and 153(phenanthiene) Confirmed that the Jt-Jt stacking of sensor 147(piienanthrene pyrene) was solely iotramolecular. The fluorescence intensity of sensor 147(phenanthrene pyrene) at 417 nm increased with added saccharide when excited at both 299 and 342 nm, while the excimer emission at 460 nm decreased with added saccharide. The change in excimer emission indicates that the Jt-Jt interaction between phenanthrene and pyrene is disrupted on saccharide binding. 

The fluorescence titrations of the monoboronic acid reference compounds... 

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