Photosynthesis is responsible for the sunlight-powered conversion of carbon dioxide and water into chemical energy in the form of carbohydrates and the release of O2 as a by-product. Although many proteins are involved in photosynthesis, the fascinating machinery of Photosystem II (PSII) is at the heart of this process. This tutorial review describes an emerging technique named protein film photoelectrochemistry (PF-PEC), which allows for the light-dependent activity of PSII adsorbed onto an electrode surface to be studied. The technique is straightforward to use, does not require highly specialised and/or expensive equipment, is highly selective for the active fractions of the adsorbed enzyme, and requires a small amount of enzyme sample. The use of PF-PEC to study PSII can yield insights into its activity, stability, quantum yields, redox behaviour, and interfacial electron transfer pathways. It can also be used in PSII inhibition studies and chemical screening, which may prove useful in the development of biosensors. PSII PF-PEC cells also serve as proof-of-principle solar water oxidation systems; here, a comparison is made against PSII-inspired synthetic photocatalysts and materials for artificial photosynthesis.

The integration of the water-oxidation enzyme, photosystem II (PSII), into electrodes allows the electrons extracted from water-oxidation to be harnessed for enzyme characterization and driving novel endergonic reactions. However, PSII continues to underperform in integrated photoelectrochemical systems despite extensive optimization efforts. Here, we performed proteinfilm photoelectrochemistry on spinach and Thermosynechococcus elongatus PSII, and identified a competing charge transfer pathway at the enzyme-electrode interface that short-circuits the known water-oxidation pathway: photo-induced O 2 reduction occurring at the chlorophyll pigments. This undesirable pathway is promoted by the embedment of PSII in an electron-conducting matrix, a common strategy of enzyme immobilization. Anaerobicity helps to recover the PSII photoresponses, and unmasked the onset potentials relating to the Q A /Q B charge transfer process. These findings have imparted a fuller understanding of the charge transfer pathways within PSII and at photosystem-electrode interfaces, which will lead to more rational design of pigmentcontaining photoelectrodes in general.Photosystem II (PSII) is a 700 kDa dimeric pigment-protein complex that resides in the membranes of oxygen evolving photosynthetic organisms.1 It is unique in nature in its ability to photocatalyze water-oxidation to liberate H + , O 2 and electrons, which is the first step of oxygenic photosynthesis. The study of PSII functionality is important in both fundamental and applied sciences,2 in particular for providing lessons for solar fuel technologies where water-oxidation is a bottleneck.3Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use

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Coumarins are a class of UV absorbing compounds which exhibit fast, photoinduced cyclobutane ring formation and cleavage reactions. The photophysics behind such processes hold significant relevance for biomedical and photoresponsive materials research. In order to further understand the underlying dynamics of the cleavage reaction, and develop strategies for increasing the reaction efficiency, UV transient absorption spectroscopy was applied to three unsubstituted, isomeric coumarin dimers: anti-head-to-head (anti-hh), syn-head-to-head (syn-hh) and syn-head-to-tail (syn-ht). The experiments performed under 280 nm excitation and broadband (300-620 nm) probing revealed that the cleavage reaction of coumarin dimers occurs through non-radiative, short-lived (<200 fs) singlet states. From the data, two branched kinetic models were developed to describe the monomer formation and dimer relaxation dynamics, identify possible intermediate states, and determine the quantum yields of the dimer cleavage. The anti-hh dimer shows the highest cleavage efficiency with a value of about 20%. The differences in the cleavage efficiency for the three isomers are interpreted in terms of differing steric hindrances of the benzene groups attached to the cyclobutane ring and charge delocalisation of the intermediate state.

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