Photosystem I (PS I) is a transmembrane, multi-subunit protein-chlorophyll complex that mediates vectorial, light-induced electron transfer. The nanometer-sized dimensions, an energy yield of approximately 58 %, and the quantum efficiency of almost 1 [1] make the reaction center a promising unit for applications in molecular nanoelectronics. PS I is located in the thylakoid membranes of chloroplasts and cyanobacteria. It mediates light-induced electron transfer from plastocyanin or cytochrome C 553 to ferredoxin. [2,3] The crystalline structure of PS I from Synechococus elongatus and from plants' chloroplast was resolved to 2.5 and 4.4 Å, respectively. [4,5] In cyanobacteria, the complex consists of at least 12 polypeptides, some of which bind 96 light-harvesting chlorophyll molecules. The electron-transport chain contains P700, A 0 , A 1 , F X , F A , and F B , representing a chlorophyll a dimer, a monomeric chlorophyll a, two phylloquinones, and three [4Fe-4S] iron-sulfur centers, respectively. The reaction-center core complex is made up of the heterodimeric PsaA and PsaB subunits, containing the primary electron donor, P700, which undergoes light-induced charge separation and transfers an electron through the sequential carriers A 0 , A 1 , and F X . The final acceptors, F A and F B , are located on another subunit, PsaC. The redox potential of the primary donor, P700, is +0.43 V and that of the final acceptor, F B , is -0.53 V, producing a redox difference of -1.0 V. The charge separation spans about 5 nm of the height of the protein, representing the center-tocenter distance between the primary donor and the final acceptor. The protein complex is 9 nm in height and has a diameter of 21 nm and 15 nm for the trimer and the monomer, respectively.[4] The photoactivity and the nanometer-sized dimensions make this complex a promising unit for applications in molecular nanoelectronics. In earlier works, care was taken to indirectly attach plant PS I [6,7] and bacterial reaction centers [8,9] to solid surfaces in attempts avoid inactivation of selfassembled monolayers.In this work, we devised a system that overcame the problems arising from direct covalent binding of proteins to metal surfaces. We selected the robust PS I reaction centers from the cyanobacteria Synechocystis sp. PCC 6803. The main reason for the structural stability of this PS I is due to the fact that all chlorophyll molecules and carotenoids are integrated into the complex of core subunits, while, in plant and bacterial reaction centers, the antenna chlorophylls are bound to chlorophyll-protein complexes that are attached to the core subunits. Indeed, there was no need to use peptide surfactants, which were essential for stabilization of plant PS I and the bacterial reaction centers. [7] A careful selection of the amino acids, which were modified to cysteines for covalent attachment of the PS I to the gold surface, was the second factor that insured structural and functional stability of the self-assembled, oriented PS I. The rational design was b...
The photoelectronic nature of a dried photosystem I protein attached to a metal surface is studied using various spectroscopic techniques. The proteins are found to be optically active after the chemical adsorption. In addition, energy-resolved photoelectronic measurements indicate that the interaction of photosystem I with the metal surface leads to new molecule/substrate states, yielding energy states different from those of the individual components. Such interactions increase the spectral-response range beyond the absorption spectrum of photosystem I and are expected to improve the energy-conversion efficiency of devices based on this system.
The photosystem I (PS I) reaction center is a chlorophyll protein complex located in thylakoid membranes of chloroplasts and cyanobacteria. PS I mediates a light-induced electron transfer through a serial of redox reactions.[1] It is intriguing to incorporate the PS I into optoelectronic circuits, since the PS I exhibits outstanding optoelectronic properties found only in the photosynthetic systems. The quantum yield for absorbing a photon within the whole complex is determined to be close to 100 %, while the energy yield for the process is approximately 58 %.[1] The nanoscale dimension and the generation of 1 V photovoltage further makes the PS I reaction center a promising unit for applications in molecular optoelectronics. [2][3][4][5][6] Utilizing a unique cysteine (Cys) mutation at the end of PS I, we demonstrate a four-step chemical procedure based on carbodiimide chemistry for covalent binding of PS I proteins to carbon nanotubes (CNTs). [7] The method allows studying hybrid nanosystems for the construction of optoelectronic devices based on PSI-CNTs heterostructures. Three variations in the design of PSI-CNT hybrid structures are presented which allow exploiting the potential of PS I as an integrated part of CNT nanodevice for optoelectronic applications. Recently, we have demonstrated the possibility to covalently bind PS I directly to gold surfaces [5] and indirectly via a small linker molecule to GaAs surfaces. [6] To this end, amino acids in the extra membrane loops of the PS I facing the cytoplasmic side of the bacterial membrane (oxidizing side) were mutated to cysteines (Cys) enabling the formation of covalent bonds with a metal surface or a chemically functionalized GaAs surface. The Cys located at extra membranal loops of the protein do not have steric hindrance, when placed on a solid surface e.g. of a gold electrode or CNTs as shown here.The mutations D235C/Y634C were selected near the special chlorophyll pair P700 to allow close proximity between the reaction center and the CNTs. [5] As depicted by white arrows in Figure 1, here we utilize a PS I with two mutants on the oxidizing side of the PS I. This single sided mutant ensures a high outcome of our chemical self-assembly procedure. For a variation of our chemical scheme we also use bipolar (BM) mutants, where the mutations are located at both the oxidizing (white arrows) and the reducing side of the PS I (gray arrow). Our self-assembly approach facilitates efficient electronic junctions and avoids disturbance in the function of the reaction center. The covalent attachment of the PS I through the Cys further ensures the structural stability of the self-assembled, oriented PS I. As demonstrated recently, [5,6] a dry oriented monolayer of PS I assembled on gold electrodes and GaAs surfaces exhibits charge transfer between PS I and the solid state surface. In this work, we extend the above chemical scheme in order to covalently attach PS I proteins to CNTs. The hybrid systems are characterized by atomic force microscopy and COMMUNICATION
Fabrication of serially-oriented multilayers of photosynthetic reaction center photosystem I (PS I) was mediated by the photo-catalytic specificity that reduced Pt 4+ ions to metal patches on the reducing side of PSI forming junctions with the oxidizing end of the proteins through Pt-sulfide bond of genetically-engineered cysteine mutants. The dry multilayers can be utilized in hybrid bio-solid-state electronic devices in which an increase in photo-voltage, resulting from the larger absorption cross-section and the serial-arrangement of PS I, is required. PS I is a transmembrane multisubunit protein-chlorophyll complex that mediates vectorial light-induced electron transfer. The nano-size dimension, an absorbed light energy yield of approximately 47% (or ca. 23% of solar radiation) and a photovoltage of 1 V with quantum efficiency of almost 1 [1] , make the reaction center a promising unit for applications in molecular nano-electronics. The robust PS I used in these experiments, that was isolated from the thylakoid membranes of cyanobacteria, is sufficiently stable to be used in hybrid solid-state electronic device. The dry PS I monolayer was shown earlier [3] to remain stable for more than three months and it stayed active for over one year in the present experiments. The structural stability is due to hydrophobic interactions that integrates 96 chlorophyll and 22 carotenoid pigment molecules and the trans membrane helixes of the core subunits.[2]The light-induced electron transfer at cryogenic temperatures [3] is an indication of little structural motions during function. We have fabricated self-assembled oriented monolayers by the formation of direct sulfide bonds between unique cysteine mutants of PS I from the cyanobacteria and the metal surface which generated, a photovoltage of 0.45 V under a dry environment. [4] In earlier works, only indirect adsorption of single plant PS I molecules [5] and binding of bacterial reaction center monolayers [6] were functioning in such an environment. Although a Schottky junction with PS I monolayer provides electronic coupling with unique photovoltaic properties, oriented multilayers can be advantageous when a larger light absorption cross section and enhanced photovoltage values are desired. As an efficient oriented multilayer, the PS I complexes need to be physically and electronically coupled and organized in a serial fashion. The use of the unique specificity of a photo-catalytic protein with redox potential of -0.53 V enabled the reduction of Pt 4+ ions and deposition of metallic platinum at the reducing end of PS I (Fig. 1a and b)
The electronic coupling between the photoactive proteins and semiconductors can be used for fabrication of a hybrid biosolid-state electrooptical devices. The robust cyanbacterial nanosized protein-chlorophyll complex photosystem I (PS I) can generate a photovoltage of 1 V with a quantum efficiency of ∼1 and can be used as a phototransistor gate. A functional dry-oriented junction was fabricated by covalently binding genetically engineered cysteine mutants of PS I to a chemisorbed small connecting molecules on the GaAs surface. Kelvin probe force microscopy measurements showed an induced photovoltage of 0.3 and −0.47 V in PS I-coated p- and n-type GaAs, respectively. The photovoltage resulted from an opposite direction of charge transfer between PS I and the semiconductors due to a difference of almost −0.8 eV in the Fermi level energy of the p- and n-GaAs, thus providing direct evidence of an electronically coupled junction useable as a photosensor.
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