Molecular recognition, by boronic acid

Interfacial Molecular Recognition by Boronic Acid-Appended Amphiphiles... 

On the basis of the simple boronic acid-diol reaction at the moleeular level, this chapter focuses on polymer-polymer interaetions triggered by boronic acid-diol esterification. When boronie aeid-diol reaetions oeeur cooperatively along a polymer chain, the structural changes caused by the complexation can easily amplify over hierarchies of nanometer to mierome-ter scale. Even without any signal such as fluorescence emission, the boronic acid-diol reaction based on precise molecular recognition is detectable as phase transition at the micrometer scale. 

Very recently, a Japanese team reported an elegant example of molecular recognition of vesicles. It was shown by agglutination assays that vesicles equipped with boronic acid specifically bind to vesicles containing diol lipid such as phosphatidylinositol. It was demonstrated that the interaction of boronic acid vesicles and inositol vesicles is the result of selective coordination of the boronic acid and the carbohydrate diol. 

The most popnlar class of the boronic acid-based sensors utilize an amine gronp proximal to boron coupled to a fluorescence outpnL The Lewis acid-Lewis base interaction between the boronic acid and the tertiary amine has a dual role. Firstly, it enables molecular recognition at nentral pH. Secondly, it can be nsed to signal binding by modnlating the intensity of flnorescence emissions. 

Porphyrin is a useful scaffold for developing molecular recognition elements, since it shows highly sensitive UV-vis absorption and fluorescence emission. By combining porphyrin and boronic acid, one can construct supramolecular systems that exhibit unique guest-induced spectroscopic changes. 

This section focuses on molecular recognition of small diol molecules by a series of boronic acid-functionalized polymers in most cases, the target molecule is glucose because of the need for a glucose-sensing system or an insulin delivery system. 

Specific control of LCST behavior by guest molecules is accomplished when the balance of the hydrophilicity and hydrophobicity of a polymer is changed by molecular recognition corresponding to interactions of the polymer and small molecules. In particular, molecular recognition combinations such as (1) cyclodextrins and hydro-phobic guests, (2) crown ethers and cations, and (3) boronic acids and sugars have been used. 

The design of a molecular recognition site that allows the capture of one glucose molecule by two boronic acid residues is expected to increase the selectivity for glucose. As it is difficult for other monosaccharides such as fructose to form esters in a 1 2 ratio, glucose selectivity can be enhanced. James et al. developed a boronic acid fluorescent probe 1 (Figure 13.2) that exhibits glucose selectivity via a two-point... 

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