Die Kombination von fortschrittlichen Materialien und kontrolliertem Oberflächendesign mit komplexen Proteinen aus der natürlichen Photosynthese ist derzeit eines der Hauptthemen bei der Entwicklung von Biohybridsystemen und Biophotovoltaik. In dieser Studie werden transparente makroporöse Indium-Zinn-Oxid-(μITO-) Elektroden mit dem trimeren Superkomplex Photosystem I (PSI) aus dem Cyanobakterium Thermosynechococcus elongatus sowie dem kleinen Redoxprotein Cytochrom c (Cyt c) kombiniert, um neuartige und effiziente biohybride Photokathoden herzustellen. Mit diesen bis zu 40 μm hohen 3D-Strukturen können beide Proteine in einer annähernden Monolage abgeschieden werden und die elektrische Kommunikation mit der Elektrode kann erzielt werden. Der generierte Photostrom folgt dabei linear der kontrollierbaren Schichtdicke der μITO-Elektrode, wobei Stromdichten von bis zu 150 μA cm –2 erhalten werden. Eine effiziente elektrische Kopplung der Proteine kann durch die hohe interne Quanteneffizienz von 30 % gezeigt werden.

The binding of photosystem I (PS I) from Thermosynechococcus elongatus to the native cytochrome (cyt) c(6) and cyt c from horse heart (cyt c(HH)) was analyzed by oxygen consumption measurements, isothermal titration calorimetry (ITC), and rigid body docking combined with electrostatic computations of binding energies. Although PS I has a higher affinity for cyt c(HH) than for cyt c(6), the influence of ionic strength and pH on binding is different in the two cases. ITC and theoretical computations revealed the existence of unspecific binding sites for cyt c(HH) besides one specific binding site close to P 700. Binding to PS I was found to be the same for reduced and oxidized cyt c(HH). Based on this information, suitable conditions for cocrystallization of cyt c(HH) with PS I were found, resulting in crystals with a PS I: cyt c(HH) ratio of 1: 1. A crystal structure at 3.4-angstrom resolution was obtained, but cyt c(HH) cannot be identified in the electron density map because of unspecific binding sites and/or high flexibility at the specific binding site. Modeling the binding of cyt c 6 to PS I revealed a specific binding site where the distance and orientation of cyt c(6) relative to P-700 are comparable with cyt c 2 from purple bacteria relative to P-870. This work provides new insights into the binding modes of different cytochromes to PS I, thus facilitating steps toward solving the PS I-cyt c costructure and a more detailed understanding of natural electron transport processes.

Connection of photosystem I (PSI) with electrodes has been shown to create artificial photosynthetic systems that hold promise for the synthesis of solar fuels. The high quantum yields of PSI require efficient electron transfer from the electrode to the reaction center of PSI in order to restock the light-induced holes, a task which in nature is performed by small redox proteins. Here, we have investigated the potential “wiring” properties of a dihemic c-type cytochrome (cyt c4), in order to efficiently connect PSI with electrodes. Cyt c4 has shown direct electron transfer (DET) with both hemes in electrical communication with two different electrode materials (ITO and Au) and on the basis of cyt c4-multilayer electrodes “self-exchange” properties can also be deduced. Investigation of cyt c4 in combination with PSI within an inverse opal ITO electrode has shown the dihemic protein to be a valuable molecular electron conduit, able to interconnect the photoenzymatic reaction with the 3D electrode. The properties have been compared with those of electrodes based on monohemic cyt c derived from horse heart.

Artificial light-driven signal chains are particularly important for the development of systems converting light into a current, into chemicals or for light-induced sensing. Here, we report on the construction of an all-protein, light-triggered, catalytic circuit based on photosystem I, cytochrome c (cyt c) and human sulfite oxidase (hSOX). The defined assembly of all components using a modular design results in an artificial biohybrid electrode architecture, combining the photophysical features of PSI with the biocatalytic properties of hSOX for advanced light-controlled bioelectronics. The working principle is based on a competitive switch between electron supply from the electrode or by enzymatic substrate conversion.

The structure of monomeric and trimeric photosystem I (PS I) of Thermosynechococcus elongatus BP1 ( T. elongatus ) was investigated by small-angle X-ray scattering (SAXS). The scattering data reveal that the protein–detergent complexes possess radii of gyration of 58 and 78 Å in the cases of monomeric and trimeric PS I, respectively. The results also show that the samples are monodisperse, virtually free of aggregation, and contain empty detergent micelles. The shape of the protein–detergent complexes can be well approximated by elliptical cylinders with a height of 78 Å. Monomeric PS I in buffer solution exhibits minor and major radii of the elliptical cylinder of about 50 and 85 Å, respectively. In the case of trimeric PS I, both radii are equal to about 110 Å. The latter model can be shown to accommodate three elliptical cylinders equal to those describing monomeric PS I. A structure reconstitution also reveals that the protein–detergent complexes are larger than their respective crystal structures. The reconstituted structures are larger by about 20 Å mainly in the region of the hydrophobic surfaces of the monomeric and trimeric PS I complexes. This seeming contradiction can be resolved by the addition of a detergent belt constituted by a monolayer of dodecyl-β-D-maltoside molecules. Assuming a closest possible packing, a number of roughly 1024 and 1472 detergent molecules can be determined for monomeric and trimeric PS I, respectively. Taking the monolayer of detergent molecules into account, the solution structure can be almost perfectly modeled by the crystal structures of monomeric and trimeric PS I.

In this work, we report on the successful assembly of cyanobacterial photosystem I (PSI) on carbon nanotubes for light-to-current conversion applications. For this purpose, glassy carbon electrodes (GCE) have been modified with multi-walled carbon nanotubes (MWCNTs). The surface of the MWCNTs has been adjusted in a non-invasive way by the use of a carboxylated pyrene derivative to achieve a covalent fixation of PSI. Our results show a cathodic photocurrent response and functionality of the biohybrid electrode upon illumination. The experiments verify that the photocurrent generation can clearly be attributed to a functional PSI on the electrode interface. An additional implementation of cytochrome c (cyt c) into this electrode architecture results in a 25-fold enhancement of cathodic photocurrent response (0.8 to 18Acm(-2) at -100mV and 100mWcm(-2)), which can be attributed to an improved connection of PSI with the underlying electrode.[GRAPHICS]. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The engineering of renewable and sustainable protein-based light-to-energy converting systems is an emerging field of research. Here, we report on the development of supramolecular light-harvesting electrodes, consisting of the redox protein cytochrome c working as a molecular scaffold as well as a conductive wiring network and photosystem I as a photo-functional matrix element. Both proteins form complexes in solution, which in turn can be adsorbed on thiol-modified gold electrodes through a self-assembly mechanism. To overcome the limited stability of self-grown assemblies, DNA, a natural polyelectrolyte, is used as a further building block for the construction of a photo-active 3D architecture. DNA acts as a structural matrix element holding larger protein amounts and thus remarkably improving the maximum photocurrent and electrode stability. On investigating the photophysical properties, this system demonstrates that effective electron pathways have been created.

A 3D inverse-opal mesoporous scalable electrode utilizing photosystem I with high efficiency for photocurrent generation and providing insights into protein-surface electrochemistry.

One major vital element of the oxygenic photosynthesis is photosystem I (PSI). We report on the construction of graphene-based nanohybrid light-harvesting architectures consisting of PSI supercomplexes adsorbed onto π-system-modified graphene interfaces. The light-driven nanophotobioelectrochemical architectures have been designed on a modified carbon surface, on the basis of π-π-stacking interactions between polycyclic aromatic compounds and graphene. As a result of the remarkable features of graphene and the feasibility of purposeful surface property adjustment, well-defined photoelectrochemical responses have been displayed by the nanophotohybrid electrodes. In particular, the PSI-graphene electrodes utilizing naphthalene derivatives provided a suitable surface for the adsorption of PSI and display already at the open circuit potential (OCP) a high cathodic photocurrent output of 4.5 ± 0.1 μA/cm(2). By applying an overpotential and addition of a soluble electron acceptor (methyl viologen), the photocurrent density can be further magnified to 20 ± 0.5 μA/cm(2). On the contrary, the investigated anthracene-based PSI-graphene electrodes exhibit considerably smaller and not very directed photoelectrochemical responses. This study grants insights into the influences of different polycyclic aromatic compounds acting as an interface between the very large protein supercomplex PSI and graphene while supporting the electrochemical communication of the biomolecule with the electrode. It needs to be emphasized that solely the naphthalene-based photoelectrodes reveal unidirectional cathodic photocurrents, establishing the feasibility of utilizing this advanced approach for the construction of next-generation photovoltaic devices.