Electron Transport between Photosystem II and Photosystem I Encapsulated in Sol–Gel Glasses

Abstract

Photosystem I (PSI) and photosystem II (PSII) are the primary solar-energy-converting enzymes of oxygenic photosynthetic organisms.[1] PSI is a robust, potent, and highly efficient photosensitizer capable of providing electrons at a high reduction potential (À0.55 V vs. normal hydrogen electrode, NHE) from its reducing end.[2] These properties have prompted many attempts to integrate PSI into biohybrid systems for solar energy conversion and storage.[3] In a biological setting, water photooxidation by PSII provides a source of electrons for the reductive processes driven by PSI, but in contrast to PSI, integration of PSII into nonbiological systems is very difficult.[4] Both photosystems were successfully "wired" to conductive electrode surfaces, but their integration into a photocatalytic bioelectrochemical device has not been reported to date.[5]Herein we demonstrate a simple scheme for mediated coupling of PSII and PSI in solution; this coupling enables electron flow from water photooxidized by PSII all the way to the reducing end of PSI. Furthermore, we show that the same scheme can be reconstituted either when both photosystems are coencapsulated in sol-gel glasses, or when one photosystem is encapsulated and the other is in solution. The solgel trapping technique is a proven method for encapsulating a wide variety of biological materials, from intact whole cells to functional individual enzymes. [6,7] In addition, sol-gel systems are porous and optically transparent, [8] which makes them ideal scaffolds for photoinduced electron transfer systems such as the photosynthetic machinery.[9] The main difference between the sol-gel and solution samples is that the photosystem complexes are immobilized within the sol-gel cavities instead of diffusing freely in solution. This property suggests interesting possibilities of segregating PSI and PSII in distinct microenvironments while maintaining electron flow between the photosystems. PSII is not naturally directly coupled to PSI (Figure 1, top). Instead, electrons flow from PSII to a pool of membrane-soluble plastoquinones that diffuse between the acceptor and donor sides of PSII, and cytochrome b 6 f (b6f), respectively.[10] Only a fraction of the electrons extracted from water at the oxygen-evolving complex of PSII end up at the acceptor side of PSI. The rest are cycled between PSII and b6f whereby directed diffusion of quinones, the release of protons on the water oxidizing face, and their consumption on the reducing face of the membrane actively pumps protons across the membrane, and generates proton-motive force.[11]By using the amphipathic quinone analogue 2,6-dichlorophenolindophenol (DCPIP) as an electron carrier, we were able to bypass b6f and set up an alternative pathway of electron flow from PSII to PSI (Figure 1, bottom). Although it is well established that DCPIP can be reduced by PSII, and the reduced form DCPIPH 2 is an electron donor to PSI, [12] DCPIP-mediated electron flow from PSII to PSI was not reported to date, to the best of our knowledge. Th...

Enhanced Biological Photosynthetic Efficiency Using Light‐Harvesting Engineering with Dual‐Emissive Carbon Dots

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Conjugated Polymer Nanoparticles to Augment Photosynthesis of Chloroplasts

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Sol–gel entrapped light harvesting antennas: immobilization and stabilization of chlorosomes for energy harvesting

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