The Role of Electrode Curvature in Controlling Electron Transfer between the Photosynthetic Reaction Center Protein and Gold Nanoelectrodes

Abstract

The bacterial photosynthetic reaction center (RC) of Rhodobacter sphaeroides is a protein-pigment complex capable of photosynthesis, a process that converts light energy into chemical energy by a chain of photoinduced electron transfer (ET) reactions. [1,2] Recent studies suggest that the reaction center can be used for the construction of new efficient bioinorganic photovoltaic devices converting solar energy into electricity or chemical energy. [3][4][5][6][7] The efficiency of enzyme-based bioinorganic devices critically depends on the rate of electron transfer between the electrode and the protein. Recently, we demonstrated that ET from the flat gold electrode to the immobilized RC increases by several orders of magnitude when one incorporates between them cytochrome c, a natural photosynthetic electron transfer mediator. [7] One of the possible explanations of this effect is that cytochrome penetrates efficiently into RC and thus improves electron coupling between the protein and the electrode. If this is true, then it is possible that optimizing the shape of the electrode itself could improve the conductivity of the RC-electrode interface even without cytochrome. To evaluate this possibility herein, we calculate the efficiency of the ET (electron matrix coupling element) between the electrode and the RC when cytochrome c is replaced with metal nanoparticles of various diameters.For these calculations, we develop a new approach based on recent advances in the theory of molecular electron transfer and optimization theory. It is based on the shortest path algorithm that incorporates the pathway model of Beratan and Onuchic, [8] developed to estimate the ET rates in proteins. In addition, our approach considers the electronic tunneling through water and incorporates the possibility of a simultaneous ET to any part of a larger partner such as a metal nanocluster.In our calculations, the gold nanocluster was assumed to have an Fm3m group symmetry (space group number 225), the atomic coordinates for the RC were obtained from the Protein Database (http://www.rcsb.org/pdb/, file "1PCR''), and the structure of RC-nanocluster complexes was optimized by minimization of the electrostatic potential energy calculated as the result of induced charge distribution on the nanoparticle under the constraints that do not allow the overlapping of the van der Waals surfaces of the protein and the nanoparticle. We approximate the metal nanoparticles by fitting geometrical spheres of various radii and consider the flat surface as a plane. The smallest possible distance R 0 between protein and the nanoparticle was taken as R 0 ¼ 0:286 nm, which is equal to the sum of the van der Waals radii of a hydrogen atom belonging to surface of the RC and an atom of gold. We have chosen water to be the surrounding medium. The calculation of the positions of the metal relative to the RC shows that the shortest distance between the metal cluster and the protein special pair is about 1 nm ( Figure 1). This distance is similar to the distance betwee...

On the electron transfer through Geobacter sulfurreducens PilA protein

“…The third parameter determining the directions (and thus the efficiency) of ET within a molecule is the ET path. 2,41,42 Consistent with the results gained from analysis of PDOS, the strongest transmission path for each peptide fragment goes through the peptide backbone (Fig. 8).…”

“…2 dE around the peaks using Newton's method implemented in the modified package LOQO. 35 Although the presence of water might substantially affect protein conductivity, 2,10,25 in the present work we did not analyze the contribution of water to PilA molecular conductance since all studied parts of the protein are hydrophobic, and since most experimental data about individual pili conductivity were obtained with dry samples. 14,28,30 PilA 3D structure estimated by NMR (PDB accession number 2M7G) consists of 18 conformers demonstrating flexibility of the peptide N-terminal and especially C-terminal ends (Fig.…”

“…Although the presence of water might substantially affect protein conductivity, in the present work we did not analyze the contribution of water to PilA molecular conductance since all studied parts of the protein are hydrophobic, and since most experimental data about individual pili conductivity were obtained with dry samples …”

“…INTRODUCTION Utilization of proteins and their components (e.g., amino acids, peptides, and redox cofactors) as molecular conductors could make possible new types of super small flexible electronic devices for energy harvesting, sensing, and signal transduction at bioinorganic interfaces. [1][2][3] Extracellular proteinaceous filaments (pili) of Geobacter sulfurreducens, composed of the type IV pili structural peptide, PilA, have received considerable attention owing to their reported ability to conduct electrons over long distances (>1 micron) from cells to insoluble oxidants including Fe(III) oxides and anodes. [4][5][6] Two possible mechanisms have been proposed to explain their conductivity.…”

Near‐IR Absorbing Solar Cell Sensitized With Bacterial Photosynthetic Membranes

“…These biological systems have evolved over billions of years to assemble naturally optimized light harvesting (LH) and photochemical reaction center (RC) complexes that absorb available light and convert this radiant energy into chemical potential with strikingly high efficiencies. This has led to the development of biohybrid photoelectric cells based on natural algal photosynthetic systems (3,4) for the ultimate conversion of sunlight into hydrogen and the generation of electrical currents (5,6), as well as light‐driven current formation from photosynthetic membrane vesicles (chromatophores; 7) of purple photosynthetic bacteria and their isolated RC (8–11) and RC‐light harvesting 1 (LH1) protein core components (12).…”