Redox switching of carbohydrate binding to ferrocene boronic acid

The redox properties of ferrocene boronic acid in aqueous buffers from pH 4 to pH 12 have been investigated using cyclic voltammetry. It is shown that the proton transfer equilibrium is under thermodynamic control over the ra nge of scan rates employed (20-5000 mV/s), leading to pK(a) values of 10.8 and 5.8 for the ferrocene and ferrocenium forms, respectively. In the prese nce of sorbitol, fructose, or glucose, the voltammetric behaviour of ferroc ene boronic acid at pH 7 is consistent with reversible formation of a carbo hydrate - boronic acid complex. However, in all cases, the complexation-dis sociation is under kinetic control. Analysis of the voltammetric response a s a function of scan rate and carbohydrate concentration, together with an independent spectroscopic determination of the binding constant to the ferr ocene form, allows all of the kinetic and thermodynamic constants for each system to be determined. In general, it is found that the binding constants ferrocenium form are about two orders of magnitude greater than those for the ferrocene form. It is possible that this redox dependent switching of t he carbohydrate binding can be exploited in amperometric or potentiometric sensing devices.