A synthetic cysteine oxidase based on a ferrocene-cyclodextrin conjugate

We report a novel synthetic cysteine oxidase consisting of a ferrocene-beta-cyclodextrin conjugate in which the ferrocene moiety is bound to the secondary hydroxyl side of the cyclodextrin cavity through an ethylenediamine linker. Cysteine oxidation occurs after the ferrocene group is electrochemically oxidized to the ferricinium form, and this generates a voltammetric electrocatalytic wave, the magnitude of which is related to the rate constant for cysteine oxidation. Comparison of cysteine oxidation rates for the primary and secondary beta-cyclodextrin derivatives (105 and 1470 M-1 s-1, respectively) shows that the secondary derivatives are more effective synthetic enzymes. Substrate selectivity of the secondary derivative is demonstrated by comparison of oxidation rates for cysteine (1470 M-1 s-1) and glutathione (260 M-1 s-1) at pH 7.0. The rate constant for cysteine oxidation was 3-fold higher at pH 8.0. With a constant synthetic enzyme concentration, electrocatalytic limiting currents increased linearly with increasing cysteine concentration to a maximum at 6 mM cysteine; above this concentration, the current decreased significantly. These and other results suggest that product inhibition of the catalytic cycle occurs as a result of cystine binding more strongly to the cyclodextrin than cysteine.