Fabrication of Oriented Multilayers of Photosystem I Proteins on Solid Surfaces by Auto‐Metallization

Fabrication of serially-oriented multilayers of photosynthetic reaction center photosystem I (PS I) was mediated by the photo-catalytic specificity that reduced Pt 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, 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 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. The light-induced electron transfer at cryogenic temperatures 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. In earlier works, only indirect adsorption of single plant PS I molecules and binding of bacterial reaction center monolayers 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 ions and deposition of metallic platinum at the reducing end of PS I (Fig. 1a and b). The metC O M M U N IC A IO N