Fluorescent boronic acids chemosensors

Chiral Recognition Using Fluorescent Boronic Acids Chemosensors... 

Figure 6.1 Typical Wolf-type fluorescent boronic acid chemosensors (1-3) for recognition of saccharides.

In 1995, James and Shinkai et al. ° used Binol based fluorescent boronic acid chemosensors for enantioseleetive recognition of monosaccharides, such as glucose. The Binol-based boronie acid sensors are [R)- and (Sj-isomers, whieh demonstrates mirror responses to the enantiomers of o-glueose and L-glueose. The enantioselectivity ean be unambiguously demonstrated by this mirror response. The origin of the enantioselectivity was attributed to the different strain for the cyclic adducts of the bis-boronic aeid with glueose. 

Figure 6.2 Binol-based fluorescent chiral bis boronic acid chemosensors (4).

Determination of the binding constants confirmed the enantioselec-tivity. For instance, at pH 5.6, in the presence of o-tartaric acid, the fluorescence enhancement for Rfi]-2 and (S,S)-2 are 9.05- and 3.61-fold, respectively. The binding constants are log AT = 5.78 and 4.20, respectively. At pH 2.5, the fluorescence intensity of (Rfi]-2 and (5,S)-2 increased by 3.31-and 1.49-fold, respectively, whereas the a-methylbenzylamine-based chiral bis-boronic acid chemosensors hardly give any fluorescence enhancement (Figure 6.11). 

A new PET-based chemosensor for uronic and sialic acids utilizing the cooperative action of boronic acid and metal chelate was reported by Shinkai and co-workers. This group synthesized a novel fluorescent chemosensor molecule bearing both an o-aminomethylphenylboronic acid group for diol binding to a saccharide and a l,10-phenanthroline-Zn(II)chelate moiety for the carboxylate binding, which enables this sensor to discriminate between neutral monosaccharides and acidic compounds [110],... 

Fluorescent Chemosensors Based on Boronic Acid Derivatives... 

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. 

Normal a-PET fluorescent chemosensors are not effective in the acidic pH region because of the fluorescent switch unit, usually a nitrogen atom in an alkylamino moiety, can be fully protonated. As a result, the fluorescence is already fully switched on by protonation. Adding analytes and the binding of the analytes will not result in further fluorescence enhancement. Based on the anthracene-a-methylbenzylamine chiral molecular profile, a bulkier chirogenic centre was introduced into the boronic acid (Figure 6.10). ... 

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