Boronic acid-appended polymers

Another type of boronic acid-appended polymer exhibiting saccharide-induced conformational changes was developed by Kanekiyo et aV The synthesized copolymer 46 consists of boronic acid and pyrene units. In aqueous solution, binding with saccharide turns the polymer conformation from a contracted to an expanded form due to electrostatic repulsion between ionized boronate groups. This conformational change is conveniently detected 

A label-free detection of saccharides was proposed by Chung et A boronic acid-containing copolymer was prepared from APBA and acrylamide. The polymer was mixed with a cationic platinum complex 49 in aqueous solution buffered at pH 9.0, and the fluorescence spectra were recorded by excitation at 448 nm. In the absence of saccharides, an emission band at 577 nm is observed. With increasing glucose concentration, this band decreases while a new emission band at 800 nm grows. These spectral changes are caused by the saccharide-induced aggregation of cationic 49 onto the anionic boronate polymer. 

3 Self-Assembly of Boronie Aeids onto Polymers 

Fluorescence spectra of 52 ([52] = 1 pM) in aqueous polycation solution ([8] = 31 pM) buffered with Na2COj and NaHCOj (0.5 mM each, pH 10.2) at various ATP concentrations. The spectra are normalized at 377 nm. Excitation 342 nm. Reproduced from ref. 63 with permission from The Royal Society of Chemistry. 

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Figure 9.1 Polymer-polymer interactions assisted by boronic acid-diol reaction (a) boronic acid-appended polymer-hydroxylated polymer interaction, (b) boronic acid-appended polymer-saccharide interaction, and (c) boronic acid-functionalized molecule-hydroxylated polymer...

Figure 9.2 Formation of a three-dimensional polymeric network through esterification between boronic acid-appended polymers and hydrated polymers.

Shinkai and Kanekiyo discuss the development of boronic acid-based supra-molecular systems. Supramolecular systems discussed include sugar-responsive gels, porphyrin-boronic acid, systems that exhibit guest-induced spectroscopic changes, two-dimensional self-assembly at the air-water interface, boronic acid-functionalized metal nanoparticles and boronic acid-appended polymers. 

Tandem esterification of boronic acid-appended molecules with the appropriate diol molecules leads to formation of polymers with boronic ester linkages in the main chain. The three-dimensional tandem repeat from esterification forms covalent moleeular frameworks. Indeed, this type of polymeric materials is fascinating. Related discussions are exeluded from this chapter. 

Nagasaki and Kimura have used polymers of poly(lysine) with boronic acids appended to the amine residue as saccharide receptors [158-160]. On saccharide com-plexation these polymers are converted from neutral sp boron into anionic sp hybridized boron. The anionic polymer thus formed interacts with added cyanine dye. Saccharide binding can then be read-out by changes in the absorption and induced circular dichroism (ICD) spectra of the cyanine dye molecule. 

FIGURE 30. (a) Poly(lysine) appended boronic acids interact with fructose to form helix-rich super-structure architectures, (b) Alternatively ditopic glucose intramole-cularly cross-links to the polymer to introduce P-turns. 

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