Mahdi Hejazi

Biohybrid architectures for efficient light-to-current conversion based on photosystem I within scalable 3D mesoporous electrodes

The combination of advanced materials and defined surface design with complex proteins from natural photosynthesis is currently one of the major topics in the development of biohybrid systems and biophotovoltaic devices. In this study transparent mesoporous indium tin oxide (μITO) electrodes have been used in combination with the trimeric supercomplex photosystem I (PSI) from Thermosynechococcus elongatus and the small redox protein cytochrome c (cyt c) from horse heart to fabricate advanced and efficient photobiocathodes. The preparation of the μITO via spin coating allows easy scalability and ensures a defined increase in the electrochemically active surface area with accessibility for both proteins. Using these 3D electrodes up to 40 μm thickness, the immobilization of cyt c and PSI with full monolayer coverage and their electrical communication to the electrode can be achieved. Significant improvement can be made when the heterogenous electron transfer rate constant of cyt c with the electrode is increased by an appropriate surface treatment. The photocurrent follows linearly the thickness of the μITO and current densities of up to 150 μA cm−2 can be obtained without indications of a limitation. The internal quantum efficiency is determined to be 39% which demonstrates that the wiring of PSI via cyt c can be advantageously used in a system with high protein loading and efficient electron pathways inside 3D transparent conducting oxides.

Structure of the isoaspartyl peptidase with l-­asparaginase activity from Escherichia coli

The crystal structure of the Escherichia coli enzyme (EcAIII) with isoaspartyl dipeptidase and L-asparaginase activity has been solved and refined to a resolution of 1.65 angstroms, with crystallographic R-factor and Rfree values of 0.178 and 0.209, respectively. EcAIII belongs to the family of N-terminal hydrolases. The amino-acid sequence of EcAIII is homologous to those of putative asparaginases from plants. The structure of EcAIII is similar to the structures of glycosylasparaginases. The mature and catalytically active form of EcAIII is a heterotetramer consisting of two alpha-subunits and two beta-subunits. Both of the equivalent active sites present in the EcAIII tetramer is assisted by a metal-binding site. The metal cations, modelled here as Na+, have not previously been observed in glycosylasparaginases. This reported structure helps to explain the inability of EcAIII and other plant-type asparaginases to hydrolyze N4-(beta-N-acetylglucosaminyl)-L-asparagine, the substrate of glycosylasparaginases.

Unidirectional Photocurrent of Photosystem I on π-System-Modified Graphene Electrodes: Nanobionic Approaches for the Construction of Photobiohybrid Systems

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.

Bioelectronic Circuit on a 3D Electrode Architecture: Enzymatic Catalysis Interconnected with Photosystem I

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.