Lab-made molecule taps light's energy.

Lab-made molecule taps light's energy

Thanks to photosynthesis, a planetful of life eats and breathes. For decades, scientists have been unraveling the chemistry and physics woven into this biochemical tour de force. Some use the findings as a guide for making synthetic molecules that harvest sunlight in much the same way as the photosynthetic machinery of plants and bacteria.

In the latest step toward artificial photosynthesis, chemists at Arizona State University in Tempe have assembled a five-component molecular machine, or pentad, that harvests light energy and uses it to segregate positive and negative charges on opposite ends of the pentad.

In natural photosynthesis, ensembles of proteins and smaller molecules pull off a similar feat, but then tap the potential energy associated with the separated charges to drive chemical reactions that produce such crucial items as carbohydrates and oxygen.

The 12 chemists, led by Devens Gust and Thomas A. Moore, describe their work in the April 13 SCIENCE.

Gust says his group is already using the pentads to study natural photosynthesis. In the future, they hope to use molecules like these for such applications as harvesting solar energy, driving chemical reactions, making fuels and building molecule-scale electronic devices, he adds.

To make the pentads, the researchers use chemical components similar to ones found in plants and photosynthetic bacteria. Two chlorophyll-like porphyrin molecules, one with a zinc atom in its center, form the light-harvesting antennae of the pentad. A bridged pair of electron-hungry quinone molecules flanks these on the right. A long, zigzagging molecule containing a series of alternating single and double bonds -- known technically as a carotenoid polyene -- flanks on the left and accepts a positive charge. Like the quinones, these polyenes have counterparts in natural photosynthesis molecules.

"This whole thing works because of a multistep electron transfer strategy," Gust says. Initially, the zinc-centered porphyrin absorbs light energy and concentrates it into one of its electrons, which in turn dumps its energy to an electron in the adjacent hydrogen-centered porphyrin. Then, in a series of lightning-fast steps, the electronic energy transforms into a positive charge, which zips to the pentad's polyene end, and a negative charge, which zaps to the rightmost quinone.

"You want to get the charges as far apart as you can as quickly as you can," Gust says. Otherwise, the charges recombine, surrendering their potential for driving chemical reactions. The charge-separated pentads store more than half the original light energy, the chemists report.

"What you have is an energy source, and you could power virtually anything with it," Gust told SCIENCE NEWS.

"It's time to start trying to use these things [pentads and related molecules] to drive reactions," adds chemist Michael R. Wasielewski of Argonne (Ill.) National Laboratory. Wasielewski assembles molecular triads that also mimic initial steps of photosynthesis.