A high-resolution structure of trimeric cyanobacterial Photosystem I (PSI) from Thermosynechococcus elongatus was reported as the first atomic model of PSI almost 20 years ago. However, the monomeric PSI structure has not yet been reported despite long-standing interest in its structure and extensive spectroscopic characterization of the loss of red chlorophylls upon monomerization. Here, we describe the structure of monomeric PSI from Thermosynechococcus elongatus BP-1. Comparison with the trimer structure gave detailed insights into monomerization-induced changes in both the central trimerization domain and the peripheral regions of the complex. Monomerization-induced loss of red chlorophylls is assigned to a cluster of chlorophylls adjacent to PsaX. Based on our findings, we propose a role of PsaX in the stabilization of red chlorophylls and that lipids of the surrounding membrane present a major source of thermal energy for uphill excitation energy transfer from red chlorophylls to P700.

Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semiartificial energy conversion devices,c apable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces.W ep resent mixed photosystem I(PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures ah igh density of photoactive protein complexes per unit area, providing the basis to effectively minimize shortcircuiting processes that typically limit the performance of PSIbased bioelectrodes.T he PSI film is further interfaced with redox polymers for optimal electron transfer,e nabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a[ NiFeSe]-hydrogenase confirms the possibility to realizel ight-induced H 2 evolution.

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Integration of a 10 MDa light-harvesting photosystem II super-complex into macro-puorous indium tin oxide biophotoanodes for improved energy conversion.

Photosystem I (PSI), a robust and abundant biomolecule capable of delivering high‐energy photoelectrons, has a great potential for the fabrication of light‐driven semi‐artificial bioelectrodes. Although possibilities have been explored in this regard, the true capabilities of this technology have not been achieved yet, particularly for their use as bioanodes. Here, the use of PSI Langmuir monolayers and their electrical wiring with specifically designed redox polymers is shown, ensuring an efficient mediated electron transfer as the basis for the fabrication of an advanced biophotoanode. The bioelectrode is rationally implemented and optimized for enabling the generation of substantial photocurrents of up to 17.6 µA cm−2 and is even capable of delivering photocurrents at potentials as low as −300 mV vs standard hydrogen electrode, surpassing the performance of comparable devices. To highlight the applicability of the developed light‐driven bioanode, a biophotovoltaic cell is assembled in combination with a gas‐breathing biocathode. The assembly operates in a single compartment cell and delivers considerable power outputs at large cell voltages. The implemented biophotoanode constitutes an important step toward the development of advanced biophotovoltaic devices.