Boronate-diol interactions

The molecular beacon fluorophore—quencher pairs methodology used in quantitative PCR (polymerase chain reaction) assays such as Taqman —may also be a useful signaling regime in the study of boron diol interactions. In appended quencher (Figure 10). 

Similar to borax, boronic acid-containing polymers can also be used to crosslink polyhydroxy polymers, such as PVA. Kitano et al. reported the first example of an interpolymer complex based on boronate-diol interactions. The complex was formed by mixing a PVA solution and an alkali solution of poly(AI-vinyl-pyrrolidone-co-3-acrylamidophenylboronic acid). Complex formation leads to an increase in solution viscosity. Above a critical polymer concentration, the complex solution loses its fluidity to become a transparent gel. The authors later increased the solubility of the polymer under physiological and acidic aqueous conditions by incorporation a third comonomer, Af,Af-dimethylaminopropylacrylamide (DMAPAA). Therefore, an interpolymer complex can form at physiological pH. 

Similar to borate-polyol gels, the interpolymer complex gels based on boronate-diol interactions also show pH- and temperature-dependent behaviors. Kiser et synthesized interpolymer complex gels using a pair... 

A remarkable property of the interpolymer complex gels based on boronate-diol interactions is their self-healing behavior. Because the polymers are crosslinked with reversible boronate ester bonds, the network can form again once it is broken. As Kiser et al. showed, the network structure can... 

Zinc carboxylate interactions have been exploited as part of a fluorescent molecular sensor for uronic acids. The sensors feature two interactions coordination of the carboxylate to the zinc and a boronic acid diol interaction.389 Photoluminescent coordination polymers from hydrothermal syntheses containing Zn40 or Zn4(OH)2 cores with isophthalate or fumarate and 4,4 -bipyridine form two- and three-dimensional structures. Single X-ray diffraction of both dicarboxylates identified the network structure.373... 

Scheme 5 Catecholamine sensors utilizing both amine-aldehyde and boronic acid-diol interactions.

In conclusion, covalent interactions have proven to be very valuable in the constrnction of supramolecular assemblies and sensors. Especially important are the reversible ones including (i) boronic acid interactions with diols... 

Figure 10 A fluorophore-appended boronic acid interacting with a diol-appended quencher.

Boronic acid-containing gels have recently been shown to have potential application in HIV-1 prevention. As mentioned above, the hydrogels developed by Kiser et al, which are interpolymer complexes of a SHA-bearing polymer and a PBA-bearing polymer based on PBA-diol interaction, exhibit... 

Boronic acids can be incorporated into polymers in order to improve polymer-polymer interactions and develop polymer based materials. The incorporation of specific boronic acid-diol interactions, dramatically improves the polymer properties. Such that, structural changes caused by the complex-ation are amplified and readily detected. Details of polymer-polymer interactions prompted by the boronic acid-diol interaction are reviewed in this chapter by Numata. 

The observed stability constants (Afobs) for compounds 140( =3)-145( =g) and 146(pyrene) with saccharidcs, which have the ability to form a furanose ring with a jjn-periplanar anomeric hydroxyl pair (o-glucose, melibiose, o-fructose and lactulose) are large, consistent with our understanding of the boronic acid diol interaction. Equally, the low or zero values of the observed stability constants (Afobs) for saccharides unable to form a furanose ring with a. syw-periplanar anomeric hydroxyl pair (maltose and leucrose) are also consistent with current thinking. 

Growing from simple polymeric assemblies, more complex structures arise from the interactions between polyvalent boronic acids interacting with poly-functional diols. These products may be dynamic, highly cross-linked polymer networks, analogous to slime. Alternatively, these assemblies have taken the form of highly ordered frameworks with persistent pores. Regardless of the degree of order inherent in these systems, the key assembly motif still relies on boronate ester formation. 

Though the earliest references associated with boronate-linked materials date back to the 1950s, it has only been in the last 10-20 years that great strides have been made using these covalent-reversible boronic acid-diol interactions. 

The affinity method may be biospecific, for example as an antibody-antigen interaction, or chemical as in the chelation of boronate by ci5-diols, or of unknown origin as in the binding of certain dyes to albumin and other proteins. 

Studies have established that the partition between transition states 3 and 4 depends on the nature of the diol unit bound to boron and on the steric and electronic effects of the a-sub-stituent X23. The data shown below demonstrate that the reactions of2-(l-methyl-2-propenyl)-4,4,5,5-tetramethyl-l,3,2-dioxaborolane proceed with a moderate preference for transition state 3 with the C2 methyl group in an axial position. Selectivity diminishes with 2-(l-methyl-2-propenyl)-l,3,2-dioxaborolane and reverses with dimethyl (l-methyl-2-propenyl)boronale, suggesting that steric interactions (gauche interactions in the case of the tetramethyl-1,3,2-diox-aborolane) between X and the diol unit on boron are capable of destabilizing transition state 4 relative to 3. 

Shinkai (65) exploited the boronic acid-diol motif mentioned above to self-assemble a polymer based on pyridine-magnesium porphyrin interactions. In this case, the evidence for polymerization comes from lightscattering experiments in dilute solution which yielded an average molecular weight of 109 g mol-1 for this system. 

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