Binding Constants and the Influence of Lewis Bases

The final point to be considered in boronic acid - saccharide binding is the predisposition of boronic acids to interact with different kinds of diols. The stability constants (K) between various polyols and boronic acids were first quantified by Lorand and Edwards and it is the case that the trends established now appear inherent in all monoboronic acids.  

It is well known that boronic acids readily form five-membered rings with vicinal cw-diols in basic aqueous media. It is also the case that six-membered rings can be formed with trans-l,3-diol groups, although the stability of these cyclic diesters is somewhat lower than their five-membered counterparts.  

These trends have been heavily utilised in the design of new sensors and it has been found that the apparent values determined for monoboronic acid sensors concur with the trends of the equilibrium constants reported in Table 1. Apparent values depend heavily on the environments in which the sensors have been studied and are influenced by factors such as pH, buffer, solvent composition, the concentration of the reactants and the presence of any Lewis basic components. 

In response to the growing demand for an accurate interpretation of the complex multi-component equilibria involved in these systems, an extensive investigation was conducted using the potentiometric titration technique. The 

It was shown that in buffered solutions binary (boronate Lewis base) complexes as well as ternary (boronate Lewis base saccharide) complexes would be formed. Under acidic conditions these ternary complexes are significant and under certain stoichiometric conditions can become the dominant components in solution. 

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