Dalton Transactions

doi: 10.1039/b922496npmid: N/A

doi: 10.1039/b922498jpmid: N/A

doi: 10.1039/b921891mpmid: 19904419

Lomoth, Reiner; Ott, Sascha

doi: 10.1039/b911129hpmid: 19904420

The light-driven splitting of water into its constituting elements gives access to a valuable fuel from an abundant substrate, using sunlight as the only energy source. Synthetic diiron complexes as functional models of the [FeFe] hydrogenase H2ase enzyme active site have moved into the centre of focus as potentially viable catalysts for the reductive side of this process, i.e. the reduction of protons to molecular hydrogen. The active site of the enzyme, as well as its mimics in an artificial system, are required to accumulate two electrons from single electron transfer events and to combine them with two protons to form hydrogen. Whereas in biology this reaction is not coupled to photosynthesis and thus proceeds in the dark, additional aspects need to be considered when designing a functional artificial system for the light-driven reduction of protons. Suitable photosensitizers have to be chosen that not only provide sufficient driving force for the reduction of the synthetic diiron catalyst, but also allow for selective excitation to minimize photodegradation. Electron transfer efficiencies have to be optimized for all steps and the sequential nature of the catalyst reduction requires a sufficient stability of potentially labile intermediates of the catalytic cycle.In this perspective, systems for the light-driven conversion of protons to molecular hydrogen are discussed where the catalyst is based on model complexes of the [FeFe] H2ase active site. Covalently linked dyads, supramolecular assemblies and multi-component systems will be examined with an emphasis on mechanistic electron transfer schemes, the properties of the individual components, their scope and their potential limitations.

Stripp, Sven T.; Happe, Thomas

doi: 10.1039/b916246apmid: 19904421

Green algae are the only known eukaryotes capable of oxygenic photosynthesis which are equipped with a hydrogen metabolism. Hydrogen production is light-dependent, since the [FeFe] hydrogenases are coupled to the photosynthetic electron transport chain via ferredoxin. Algal [FeFe] hydrogenases are one of the most active biocatalysts for the evolution of hydrogen. Therefore, special interest exists in the biophysical characterization and biotechnological usage of these [Fe-S] enzymes. This review traces the discovery of this interesting class of proteins. Recent findings allow insight into the electronic structure and configuration of the [FeFe] hydrogenase active site (H-cluster). Emphasis is placed on novel discoveries of the hydrogenase interaction with its natural electron donor ferredoxin and the mechanism of enzyme inactivation through oxygen.

English, Christine M.; Eckert, Carrie; Brown, Katherine; Seibert, Michael; King, Paul W.

doi: 10.1039/b913426npmid: 19904422

This review focuses on recent progress in developing heterologous and recombinant expression as well as in vitro maturation systems for the biosynthesis of active [FeFe] and [NiFe]-hydrogenases, which catalyze the reversible reaction, H2↔ 2e− + 2H+. Activities of [FeFe] and [NiFe]-hydrogenases produced from different recombinant and in vitro maturation approaches are compared. Examples of how hydrogenase expression supports basic and applied studies of these enzymes are presented, and barriers to achieving more viable biological and synthetic H2-production systems and catalysts are addressed.

Hambourger, Michael; Kodis, Gerdenis; Vaughn, Michael D.; Moore, Gary F.; Gust, Devens; Moore, Ana L.; Moore, Thomas A.

doi: 10.1039/b912170fpmid: 19904423

A photoelectrochemical biofuel cell has been developed which incorporates aspects of both an enzymatic biofuel cell and a dye-sensitized solar cell. Photon absorption at a porphyrin-sensitized n-type semiconductor electrode gives rise to a charge-separated state. Electrons and holes are shuttled to appropriate cathodic and anodic catalysts, respectively, allowing the production of electricity, or a reduced fuel, via the photochemical oxidation of a biomass-derived substrate. The operation of this device is reviewed. The use of alternate anodic redox mediators provides insight regarding loss mechanisms in the device. Design strategies for enhanced performance are discussed.

Oliveira, Paulo; Lindblad, Peter

doi: 10.1039/b908593apmid: 19904424

The overall processes of transcription and its regulation have advanced significantly in the last years, making our understanding of prokaryotic biology more complex and detailed. In fact, a systematic study of different aspects of transcriptional regulation opens up outstanding opportunities to improve and develop the perception of complex reaction mechanisms, genetic processes and cell functions. In close connection to the cyanobacterial bidirectional hydrogenase, the main hydrogen-evolving enzyme in non-nitrogen fixing strains, two novel transcription factors have received increasing attention over the past five years: a LexA-related protein and the AbrB-like family members. Recent work on these regulators has produced new insights and advances towards the understanding (and possible interconnection) of several regulatory networks in cyanobacteria, namely nitrogen metabolism, redox response, toxin production, CO2 concentrating mechanisms and hydrogen metabolism. The fact that a LexA-related protein and AbrB-like family members have been co-purified in independent laboratories studying different sets of cyanobacterial genes suggests a possible common and/or complementary function of these regulators. In this review, we summarize the knowledge gained thus far regarding the transcriptional regulation of the cyanobacterial bidirectional hydrogenase, with special focus on the above mentioned transcription factors. Moreover, we discuss several additional points that warrants further investigation to increase our knowledge in this fast evolving research field.

La Ganga, Giuseppina; Nastasi, Francesco; Campagna, Sebastiano; Puntoriero, Fausto

doi: 10.1039/b907257hpmid: 19904425

A multimetallic ruthenium(ii) dendrimer is used for the first time to photosensitize dioxygen production from water by IrO2 nanoparticles; the system is more efficient than an analogous system based on the more commonly used [Ru(bpy)3]2+-type photosensitizers, in particular for the ability of the dendrimer to take advantage of the red portion of the solar spectrum.

Benson-Smith, Jessica J.; Ohkita, Hideo; Cook, Steffan; Durrant, James R.; Bradley, Donal D. C.; Nelson, Jenny

doi: 10.1039/b910675hpmid: 19904426

Transient and steady state optical spectroscopies were used to study thin films made from a series of polyfluorene polymers blended with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) in order to determine the influence of polymer ionisation potential on photoinduced charge separation. We find that the energy of the charge separated state ΔECS, given by the energy difference between the ionisation potential of the polymer and the electron affinity of the fullerene, must be smaller than a threshold value of about 1.6 eV for charge separation to occur. When ΔECS is greater than this threshold, PCBM triplet formation is observed in preference to charge pair generation. If ΔECS is similar to the threshold value, both PCBM triplet formation and charge separation occur in the blend film, with a tendency for charge separation to dominate over PCBM triplet formation as PCBM concentration increases. The mechanism of triplet formation is believed to be energy transfer to the PCBM singlet state followed by intersystem crossing. The threshold value of ΔECS is found to be similar to the PCBM singlet energy.