Thursday, December 29, 2011

Souped up photosynthesis produces H2


Scientists have taken photosynthesis to the next level, by creating a tiny solar-powered device that works twice as fast as nature to produce hydrogen biofuel.
But the researchers add further research is needed before we can start using it to fuel our cars.
Hydrogen is seen as an alternative fuel source because it burns to produce water as waste. However, sources of hydrogen are difficult to tap.
Some scientists are turning to biomimicry, designing devices based on photosynthesis to use light to split water into oxygen and hydrogen.
"I think there's good prospects for using some of these biological photosynthesis systems to produce biofuels for the future," says Professor Donald Bryant, of Pennsylvania State University and co-author of a paper appearing inProceedings of the National Academy of Sciences.
In nature, a light-capturing enzyme known as Photosystem I uses light energy to energise electrons. Those electrons are moved, relatively slowly, to another enzyme known as FNR. The FNR enzyme combines these electrons with a biological molecule (NADP+) and a hydrogen ion to produce an energy-storage molecule called NADPH, which is used to make sugars.
To make their biofuel device the reserachers at Pennsylvania State University replaced the FNR enzyme with hydrogenase, an enzyme that combines two electrons with two hydrogen ions to make molecular hydrogen.
To increase its efficiency the hydrogenase was 'tethered' directly to the Photosystem I enzyme with a carbon chain. This chain, which acts like a molecular wire, speeds up the movement of electrons between enzymes, boosting hydrogen production.

Useful for other enzymes

Lead author of the study, Dr Carolyn Lubner, also of Pennsylvania State University, says the design can be adapted for other enzymes.
"For example, we could potentially use formic acid dehydrogenase to make formic acid, which is also a biofuel," she says.
Beyond biofuel, using light to inject single electrons one by one into enzymes gives scientists a unique ability to study the process of photosynthesis, says Bryant.
"For example, the hydrogenase enzyme has to accumulate two electrons, and they come one at a time - even in natural settings," he says. "What happens when one electron comes, and where it goes, is something that is currently unknown."
Lubner adds that "the more insight we can gain into enzymes found in nature, that make compounds ... such as hydrogen or ethanol, the better we can design mimics of them."
Associate Professor John Stride, of the University of New South Wales, says it makes sense to look to nature when designing new processes. "Nature has had millennia to solve problems, and photosynthesis is very efficient."
"One can imagine a biogenerator of hydrogen fuel that works just by using sunlight," he says. "Though they do have to feed it sodium ascorbate [vitamin C], so it's not quite energy for nothing."
Bryant says that while they used ascorbate in this experiement, "We can feed it electrons from anywhere, including the ground. We just have to have electrons from 'somewhere'."

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