This mini-review focuses on recent experimental results and questions, which came up since the last more comprehensive reviews on the subject. We include a brief discussion of the different techniques used for time-resolved studies of electron transfer in photosystem I (PS I) and relate the kinetic results to new structural data of the PS I reaction centre.

Dedicated to Professor P. Gouzerh on the occasion of his 65th birthday Homogeneous light-driven catalytic systems for hydrogen production and, more generally, efficient photoactivated synthetic multielectron catalysts remain relatively scarce. [1] Such systems [2][3][4] generally consist of 1) a photosensitizer, often based on the ruthenium tris(diimine) moiety, [5] 2) a metal-based catalytic center, and in some cases 3) an additional redox mediator. However, their efficiency remains to be improved in terms of both turnover numbers (stability) and turnover frequencies, and these systems should preferably rely on inexpensive first-row transition-metal catalysts rather than unsustainable noble metals. We and others recently reported that cobaloximes are very efficient and cheap electrocatalysts for hydrogen evolution. [6][7][8][9] We thus decided to couple cobaloximes with ruthenium tris(diimine) moieties in order to make a supramolecular variant of the system previously studied by Lehn et al. for photochemical production of hydrogen. [3] In such a molecular device, the intramolecular electron transfer from the photoactivated center to the catalytic center can potentially be controlled, and the charge-recombination processes limited, to an extent larger than in intermolecular systems, by fine-tuning both the distance between metal centers and the nature of the bridge. [2,10] Such an organized assembly is found in hydrogen-evolving green algae, where the photosystem I is tightly coupled to hydrogenase enzymes. [11] In this paper we describe the synthesis and activity of a series of novel heterodinuclear ruthenium-cobaloxime photocatalysts able to achieve the photochemical production of hydrogen with the highest turnover numbers so far reported for such devices. Compounds 1-3 (Scheme 1) were synthesized in good yields [12] by replacing one axial ligand of cobaloxime moieties with the pyridine residue of the previously reported [(bpy) 2 Ru(l-pyr)] 2+ complex (l-pyr = (4-pyridine)oxazolo-[4,5-f]phenanthroline).[13] NMR measurements and ESI-MS analysis are consistent with the l-pyr ligand connecting the ruthenium and cobalt centers. This was further supported by cyclic voltammetry: [12] in addition to ruthenium-centered processes, which are not significantly modified upon complexation to the cobalt center, cyclic voltammograms of 1-3 show Co II /Co I reversible processes shifted by % 80 mV to more positive potentials relative to the starting cobaloximes, probably because of the overall 2 + charge of the compounds.We checked by cyclic voltammetry that the cobaloxime moieties retain their electrocatalytic properties for hydrogen production in all three heterobinuclear complexes: an electrocatalytic wave corresponding to proton reduction develops at À0.45 V vs. Ag/AgCl upon addition of increasing amounts of p-cyanoanilinium tetrafluoroborate to a solution of 1 in CH 3 CN [12] (electrocatalytic waves are observed at À0.9 V vs.

The effect of global (15)N or (2)H labeling on the light-induced P700(+)/P700 FTIR difference spectra has been investigated in photosystem I samples from Synechocystis at 90 K. The small isotope-induced frequency shifts of the carbonyl modes observed in the P700(+)/P700 spectra are compared to those of isolated chlorophyll a. This comparison shows that bands at 1749 and 1733 cm(-)(1) and at 1697 and 1637 cm(-)(1), which upshift upon formation of P700(+), are candidates for the 10a-ester and 9-keto C=O groups of P700, respectively. A broad and relatively weak band peaking at 3300 cm(-)(1), which does not shift upon global labeling or (1)H-(2)H exchange, is ascribed to an electronic transition of P700(+), indicating that at least two chlorophyll a molecules (denoted P(1) and P(2)) participate in P700(+). Comparisons of the (3)P700/P700 FTIR difference spectrum at 90 K with spectra of triplet formation in isolated chlorophyll a or in RCs from photosystem II or purple bacteria identify the bands at 1733 and 1637 cm(-)(1), which downshift upon formation of (3)P700, as the 10a-ester and 9-keto C=O modes, respectively, of the half of P700 that bears the triplet (P(1)). Thus, while the P(2) carbonyls are free from interaction, both the 10a-ester and the 9-keto C=O of P(1) are hydrogen bonded and the latter group is drastically perturbed compared to chlorophyll a in solution. The Mg atoms of P(1) and P(2) appear to be five-coordinated. No localization of the triplet on the P(2) half of P700 is observed in the temperature range of 90-200 K. Upon P700 photooxidation, the 9-keto C=O bands of P(1) and P(2) upshift by almost the same amount, giving rise to the 1656(+)/1637(-) and 1717(+)/1697(-) cm(-)(1) differential signals, respectively. The relative amplitudes of these differential signals, as well as of those of the 10a-ester C=O modes, appear to be slightly dependent on sample orientation and temperature and on the organism used to generate the P700(+)/P700 spectrum. If it is assumed that the charge density on ring V of chlorophyll a, as measured by the perturbation of the 10a-ester or 9-keto C=O IR vibrations, mainly reflects the spin density on the two halves of the oxidized P700 special pair, a charge distribution ranging from 1:1 to 2:1 (in favor of P(2)) is deduced from the measurements presented here. The extreme downshift of the 9-keto C=O group of P(1), indicative of an unusually strong hydrogen bond, is discussed in relation with the models previously proposed for the PSI special pair.

CO2‐Reduktion Harnstoffgruppen in einem Eisenporphyrinkatalysator führen zu einer mehrfachen Stabilisierung von CO2 durch Wasserstoffbrücken. Wie Z. Halime, A. Aukauloo et al. in ihrer Zuschrift auf S. 4552 zeigen, hat der Katalysator eine hohe Aktivität in der CO2‐Reduktion.

The Photosystem I complex catalyses the transfer of an electron from lumenal plastocyanin to stromal ferredoxin, using the energy of an absorbed photon. The initial photochemical event is the transfer of an electron from the excited state of P 700 , a pair of chlorophylls, to a monomer chlorophyll serving as the primary electron acceptor. We have performed a systematic survey of conserved histidines in the last six transmembrane segments of the related polytopic membrane proteins PsaA and PsaB in the green alga Chlamydomonas reinhardtii. These histidines, which are present in analogous positions in both proteins, were changed to glutamine or leucine by site-directed mutagenesis. Double mutants in which both histidines had been changed to glutamine were screened for changes in the characteristics of P 700 using electron paramagnetic resonance, Fourier transform infrared and visible spectroscopy. Only mutations in the histidines of helix 10 (PsaA-His676 and PsaB-His656) resulted in changes in spectroscopic properties of P 700 , leading us to conclude that these histidines are most likely the axial ligands to the P 700 chlorophylls.

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In this study we report a strategy to attach methylimidazolium fragments as ionic liquid units on an established iron porphyrin catalyst for the selective reduction of CO2 to CO in water, precluding the need for external proton source.

International audienceArtificial photosynthesis: The first photoelectron trade between P680 and the TyrZ-His190 pair of Photosystem II was modeled by a ruthenium(II) trisbipyridine type complex that contains a phenol hydrogen atom bonded to an imidazole group. The photogenerated phenoxyl radical has been characterized. This opens up the way for a more complete biomimetic model of Photosystem II