Chiral boronic acid chemosensors

BINOL-Derived Chiral Boronic Acid Chemosensors... 

Chiral Recognition Using Fluorescent Boronic Acids Chemosensors... 

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

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. 

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). 

Boronic acid-derived fluorescent chemosensors are unique in that the inter-molecular interaction is a covalent bond, and not hydrogen bonding as is the case for most conventional fluorescent molecular sensors used for the selective reeognition of hydroxyl carboxylic acids. This chapter summarizes the development of the boronic acid-based chiral fluorescent chemosensors over recent years and the enantioselective fluorescent reeognition of chiral a-hydroxyl carboxylic acids analytes in aqueous solutions. The fundamental scaffolds of these chiral sensors include a fluorophore, an arylboronie aeid binding site, and linker between the two units. The systems usually consist of a bis-boronic acid unit, which is required for enantioselective recognition of the chiral a-hydroxyl carboxylic acid analytes. However, mono-boronic acid fluorescent chemosensors have also been developed. All three components of the chiral boronic acid sensors play an important role in determining the... 

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). ... 

In 2009 a d-PET boronic acid was devised by Zhao and James et al (Figure 6.15). An electron-rich carbazole was used as the fluorophore of the chiral bis-boronic acid sensor 9 and a-methylbenzylamine was used as the chirogenic centre of the sensor. To achieve chiral recognition a bis-boronic acid system was used in designing the fluorescent sensor. The fluorescence response of the sensor toward protonation is in contrast to the normal a-PET chemosensors. The fluoreseenee of the sensor is weaker in the acid pH region (Figure 6.16). 

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