Application of a nitrospiropyran-FAD-reconstituted glucose oxidase and charged electron mediators as optobioelectronic assemblies for the amperometric transduction of recorded optical signals: Control of the 'on'-'off' direction of the photoswitch

Apo-glucose oxidase is reconstituted with the semisynthetic nitrospiropyran-FAD cofactor to yield a photoisomerizable glucose oxidase, 3a-GOx. The nitrospiropyran-FAD-reconstituted GOx, 3a-GOx, undergoes reversible photoisomerization to the protonated nitromerocyanine-FAD GOx, 3b-GOx. The photoisomerizable enzyme was assembled as a monolayer on a Au electrode. The bioelectrocatalyzed oxidation of glucose in the presence of ferrocenecarboxylic acid (4), fenocene-1,1'-dicarboxylic acid (5), or 1-[1-(dimethylamino)ethyl]ferrocene (6), acting as electron mediators, was examined in the different photoisomer states of the enzyme. With 4 and 5 acting as electron mediators, the enzyme in state 3b-GOx exhibited bioelectrocatalytic activities for the oxidation of glucose. The bioelectrocatalytic functions of the enzyme in state 3a-GOx were blocked in the presence of 4 or 5. With 6 acting as electron mediator, the enzyme in state 3a-GOx exhibits bioelectrocatalytic activities for the oxidation of glucose, whereas the bioelectrocatalytic functions of the 3b-GOx state are blocked. The directionality of the enzyme photoswitch is controlled by the affinities of the oxidized electron mediators to diffusionally penetrate the protein and reach appropriate electron transfer distances to oxidize the FADH2 redox center of the enzyme. In all of the systems, cyclic amperometric transduction of optical signals recorded by the photoisomerizable enzyme is accomplished. The enzyme photoisomer 'on' or 'off' switching direction is controlled by tuning the electrical interactions of the electron mediator and the photoisomerizable enzyme.