APPLICATION OF BIFUNCTIONAL REAGENTS FOR IMMOBILIZATION OF PROTEINS ON A CARBON ELECTRODE SURFACE - ORIENTED IMMOBILIZATION OF PHOTOSYNTHETIC REACTION CENTERS

A new kind of bifunctional reagent was used to immobilize covalently m onolayers of photosynthetic reaction centers (RCs) on a carbon electro de surface. Condensed aromatic rings were used as an anchor group for chemisorption on the basal-plane surface of a pyrolytic graphite elect rode and chemically active functional groups were used to immobilize t he RCs covalently via the amino acid residues of the protein. The RCs were randomly immobilized via lysine residuals when the bifunctional r eagent activated for the reaction with amino groups was applied. An or iented immobilization of the RCs via the cysteine residual located at their accepting side was achieved when an electrode surface activated for thiol binding was used. A dramatic difference in the photoinduced currents was observed for different orientations of the RCs immobilize d on the electrode surface. The small separation between the quinone s ites inside the RCs and the electrode surface in the case of oriented RCs provides efficient non-diffusional electron transfer, and applicat ion of an additional solubilized electron transfer mediator does not a ffect the photocurrent. Electrochemical oxidation of the immobilized e lectron transfer mediator was shown to be the limiting step of photocu rrent formation and a quantum efficiency of ca. 60% (for the absorbed light) was calculated for the photocurrent generation. In the case of randomly oriented RCs the photocurrent was much smaller, but it could be increased by application of a diffusionally mobile electron transfe r mediator.