title: Photocurrents from photosystem II in a metal oxide hybrid system: Electron transfer pathways.
abstract: We have investigated the nature of the photocurrent generated by Photosystem II (PSII), the water oxidizing enzyme, isolated from Thermosynechococcus elongatus, when immobilized on nanostructured titanium dioxide on an indium tin oxide electrode (TiO2/ITO). We investigated the properties of the photocurrent from PSII when immobilized as a monolayer versus multilayers, in the presence and absence of an inhibitor that binds to the site of the exchangeable quinone (QB) and in the presence and absence of exogenous mobile electron carriers (mediators). The findings indicate that electron transfer occurs from the first quinone (QA) directly to the electrode surface but that the electron transfer through the nanostructured metal oxide is the rate-limiting step. Redox mediators enhance the photocurrent by taking electrons from the nanostructured semiconductor surface to the ITO electrode surface not from PSII. This is demonstrated by photocurrent enhancement using a mediator incapable of accepting electrons from PSII. This model for electron transfer also explains anomalies reported in the literature using similar and related systems. The slow rate of the electron transfer step in the TiO2 is due to the energy level of electron injection into the semiconducting material being below the conduction band. This limits the usefulness of the present hybrid electrode. Strategies to overcome this kinetic limitation are discussed.
authors.affiliation: Department of Life Sciences, Imperial College London, London SW7 2AZ, UK.
authors.forename: Katharina
authors.initials: K
authors.lastname: Brinkert
authors.affiliation: Department of Chemistry, Imperial College London, London SW7 2AZ, UK.
authors.forename: Florian
authors.initials: F
authors.lastname: Le Formal
authors.affiliation: Department of Chemistry, Imperial College London, London SW7 2AZ, UK.
authors.forename: Xiaoe
authors.initials: X
authors.lastname: Li
authors.affiliation: Department of Chemistry, Imperial College London, London SW7 2AZ, UK.
authors.forename: James
authors.initials: J
authors.lastname: Durrant
authors.affiliation: Department of Life Sciences, Imperial College London, London SW7 2AZ, UK. Electronic address: a.rutherford@imperial.ac.uk.
authors.forename: A William
authors.initials: AW
authors.lastname: Rutherford
authors.affiliation: Department of Life Sciences, Imperial College London, London SW7 2AZ, UK. Electronic address: a.fantuzzi@imperial.ac.uk.
authors.forename: Andrea
authors.initials: A
authors.lastname: Fantuzzi
mesh_terms: D004566:Electrodes; D004579:Electron Transport; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011809:Quinones / Q000737:chemistry; D017971:Tin Compounds / Q000737:chemistry*; D014025:Titanium / Q000737:chemistry*
chemical_list: D045332:Photosystem II Protein Complex; D011809:Quinones; D017971:Tin Compounds; C009495:titanium dioxide; C109984:indium tin oxide; D014025:Titanium
keywords: Photosynthesis; Photosynthetic reaction centre; Protein electrode interface; Protein film photoelectrochemistry; Quinone; Water oxidizing enzyme