Non-damaging x-ray technique unveils protein complex that uses sunlight to split water


Sun to Split Water nondamagingxx250A more accurate view of the structure of the oxygen-evolving complex that splits water during photosynthesis is now in hand thanks to a study involving researchers from the RIKEN SPring-8 Center, Okayama Univ. and the Japan Science and Technology Agency. The new model of natural photosynthesis provides a blueprint for synthesizing water-splitting catalysts that mimic this natural process.

Natural photosynthesis starts in an elaborate pigment–protein complex known as photosystem II, which is found in cellular membranes of higher plants, algae and cyanobacteria. This complex catalyzes the splitting of water into oxygen, hydrogen ions and electrons. In principle, this reaction is a solar-powered fuel cycle, with the only end product being the water formed when the hydrogen and oxygen are recombined to release energy.

“The bottleneck in producing sustainable and clean fuel energy from sunlight and water is the availability of efficient artificial catalysts for water splitting,” says Hideo Ago from the RIKEN research team. This explains why there is such intense interest in understanding exactly how nature splits water so efficiently.

The structure of photosystem II has been examined before at very high resolution by x-ray diffraction crystallography, and detailed models of the system have been obtained. However, the high-energy x-rays used in such analyses tend to damage the structure of the complex, resulting in some discrepancy among findings and a lack of clarity for certain atomic structures.

The research team avoided the problem of radiation damage by using a series of ultrashort femtosecond x-ray pulses generated by RIKEN’s ‘SACLA’ x-ray free-electron laser facility. The short pulses prevented structural damage and the more accurate results allowed the researchers to identify several new features of the catalytic center of photosystem II from the bacterium Thermosynechococcus vulcanus. The results indicate that catalysis occurs in a cluster of manganese and oxygen atoms with one calcium atom. With more accurate measurement of the distances between the atoms, the researchers revealed that one of the oxygen atoms may actually form part of a hydroxide ion and could be derived from a split water molecule.

Sun to Split Water nondamagingxx250

The molecular architecture of the photosynthetic oxygen-evolving complex (gray: manganese, blue: calcium, red and yellow: oxygen, orange: water). Image: M. Suga et al

“We solved the structure in the ‘dark-stable’ state before the water-splitting reaction,” explains Jian-Ren Shen, one of the researchers from Okayama Univ. “We now hope to use the same method with light illumination to follow the process through the reaction cycle.” Such a study would provide a complete picture of how the biological photosynthesis reaction proceeds and greatly assist efforts to use the process in a fuel cycle.

Source: RIKEN

Organic semiconductors open path for flexible electronics


News & Trends • Posted:16 October 2013

mix-id328072.jpgScientists from the Emergent Molecular Function Research Group at the RIKEN Centre for Emergent Matter Science have developed a synthetic procedure that according to them makes it easier to tailor the chemical structure of an important organic semiconductor.

 

Organic semiconductors made from small aromatic molecules can be dissolved and screen-printed onto many substrates, including plastics, opening the path for ‘flexible’ electronic devices such as low-cost polymer solar cells.

Kazuo Takimiya and colleagues, in collaboration with researchers from Hiroshima University, have now developed a synthetic procedure that makes it easier to tailor the chemical structure of an important organic semiconductor.

Kazuo Takimiya and his team, in collaboration with researchers from Hiroshima University, were studying molecules known as diacene-fused thienothiophenes when they discovered their new synthetic procedure. Diacene-fused thienothiophenes are composed of interlocking benzene and sulphur-containing aromatic rings and are more durable, and have higher charge carrier mobilities, than most other organic semiconductors. Although current schemes to make these compounds are relatively straightforward, they are also difficult to modify. Thus, chemists have a hard time producing derivatives based on this ring system with more desirable properties.

The researchers devised a creative synthesis that, instead of relying on bulky aromatic precursors, generates diacene-fused thienothiophenes from small molecules through two consecutive ring-forming reactions. First, they generated an active reagent called phenylsulfenyl chloride that joins to a benzene–acetylene molecule and transforms it into a three ring system. Then, they used selective carbon–hydrogen bond activation to set off a rare intramolecular coupling that produces a molecule with four fused rings known as benzothieno-benzothiophene (BTBT). Takimiya explained that this approach produces excellent yields and makes it possible to scrutinise numerous BTBT derivatives by making simple changes to the starting reagents.

Trials revealed that this technique was particularly useful for extending the ring structure of BTBT-type molecules. For example, by substituting double- and triple-fused benzene molecules into the synthetic procedure, the team linearly constructed the BTBT substructure to form five, six and seven aromatic rings. Intriguingly, these new derivatives have an asymmetric structure that may dramatically improve their solubility—an important processing feature for printed electronics and one that is difficult to achieve using existing synthetic techniques.

Lengthening the BTBT framework to an eight-ringed symmetric structure also yielded a potent new organic semiconductor with excellent thermal stability and a charge carrier mobility five times higher than that of BTBT. “This mobility is among the highest recorded for thin film organic field-effect transistors, meaning that this molecule could be a candidate for real flexible electronics applications in the future,” stated Takimiya.