Effects of Distance and Driving Force on Photoinduced Electron Transfer between Photosynthetic Reaction Centers and Gold Electrodes

Abstract

The electron-transfer (ET) parameters for oriented and aligned monolayers of the bacterial photosynthetic reaction center (RC) from Rhodobacter sphaeroides formed on the top of self-assembled monolayers (SAMs) of alkanethiols of various lengths immobilized on gold electrode are estimated using cyclic voltammetry and photoelectrochemistry. Utilization of the unique polyHis tag in the protein and the Ni-NTA chelator complex in SAMs allows for specific protein orientation with the RC primary donor facing the electrode. To improve the efficiency of ET between the RC special pair and the electrode, an RC-Cyt complex was formed (J. Am. Chem. Soc. 2006, 128, 12044-12045). The results are analyzed in terms of integrated Marcus formalism, taking into account the density of electronic states in the metal. The dependence of the ET rate on the distance between RC and electrode demonstrates an adiabatic region up to 10 Å, typical for other proteins, followed by a nonadiabatic area of electron tunneling with a β-factor of ∼0.8 per methylene group of the alkanethiol. Reorganization energy of the system is rather low (0.23 eV) and indicates negligible (if any) system conformational change or protein tilting in the course of ET. Scanning probe microscopic (SPM) examination of the constructed surfaces confirmed a high density of surface coverage by the protein and the absence of RC structural deformation in the monolayers. Estimated reduction potential of the RC primary donor for the immobilized protein is +0.5 V (vs NHE), which is close to that observed for the protein in solution and in vivo. These results open the possibility for the measurements and detailed analysis of the mechanisms of ET in photosynthetic proteins in precisely organized monolayers on the surfaces of inorganic electrodes.

On the mechanism of ubiquinone mediated photocurrent generation by a reaction center based photocathode

“…Native ubiquinones are confined to the lipid bilayer and have an isoprenoid tail comprising 10 isoprene units (50 carbons), rendering this electron acceptor completely insoluble in an aqueous cell. As a result shorter chain analogues have been used, including UQ2 [7,22,24,26,28,29] and UQ0 [5,10,13,20]. Examination of ubiquinones with isoprenoid tails of varying length showed that the photocurrent density decreased with increasing numbers of isoprene units.…”

“…Major preoccupations of this work have been the development of strategies for effective interfacing of the positive and negative terminals of the photovoltaic protein to the working and counter electrode, either through direct binding or the use of diffusional mediators [4][5][6][7][8][16][17][18][19][20][21][22][23][24][25][26][27]. In the case of RC's from purple bacteria a particularly effective strategy has been to use analogues of the natural electron donor and electron acceptor, cytochrome (cyt) c2 and ubiquinone-10 (UQ10), to connect the photovoltaic protein to the two electrodes [18,20,22,[27][28][29]. At the positive terminal, cyt c2, or more often the commercially-available mitochondrial equivalent cyt c, has been used to "wire" the transfer of electrons from the working electrode to an RC protein adhered via the natural electrostatic protein-protein interactions [18,[27][28][29].…”

“…In the case of RC's from purple bacteria a particularly effective strategy has been to use analogues of the natural electron donor and electron acceptor, cytochrome (cyt) c2 and ubiquinone-10 (UQ10), to connect the photovoltaic protein to the two electrodes [18,20,22,[27][28][29]. At the positive terminal, cyt c2, or more often the commercially-available mitochondrial equivalent cyt c, has been used to "wire" the transfer of electrons from the working electrode to an RC protein adhered via the natural electrostatic protein-protein interactions [18,[27][28][29]. The same strategy has also been used for the larger complex in which the RC is surrounded by a light-harvesting 1 antenna protein (termed the RC-LH1 complex) [5,7].…”

“…The same strategy has also been used for the larger complex in which the RC is surrounded by a light-harvesting 1 antenna protein (termed the RC-LH1 complex) [5,7]. At the negative terminal, short-chain water-soluble analogues of the natural UQ10, such as ubiquinone-2 (UQ2) [7,22,26,28,29] or ubiquinone-0 (UQ0) [5,10,19], have been used to connect electron flow through the RC or RC-LH1 complex to the counter electrode.…”

Two‐Dimensional Protein Crystals for Solar Energy Conversion

“…Self assembled monolayers (SAMs) are typically used in many PSI photocurrent studies [ 8,9 ] to preserve the protein structure on the electrode surface and maintain activity but they suffer from poor electrical conductivity, thereby limiting photocurrent. [ 10 ] PSI has also been shown to produce photocurrents on semiconductor electrodes such as TiO 2 /ZnO [ 11 ] and p-doped silicon, [ 12 ] as well as in osmium redox polymers. [ 13 ] Photocurrents in these devices range from ∼ 10 nA cm −2 [ 9 ] to 875 µA cm −2 .…”