Reversing electrons’ course through nature’s solar cells

Switching tracks.

Photosynthetic organisms use pigment-protein complexes called reaction centers (RCs)—effectively nature’s solar cells—to convert the energy of sunlight into charge-separated species that power life processes. From this point, the electrons could move either to an A-branch (or “right-track”) set of molecules or to a B-branch (“left-track”) set of identical molecules.

Scientists recently coax the electrons in a way that typically doesn’t travel. Doing this could help get detail insights on the early events that occurred in photosynthesis.

For the study, scientists designed many iterations of photosynthetic mutants to achieve charge separation using the B branch instead. They come up with a pathway in a purple photosynthetic bacteria, one of nature’s solar cells.

Christine Kirmaier, a research professor of chemistry in Arts & Sciences, said, “Using molecular biology, we’ve been changing the amino acids around the pigments to try and find the magic combination to make the B branch work.”

The goal was to create structural changes that de-tune, or make less ideal, electron moves along the A side or normal path — and afterward, simultaneously, accelerate the responses along the B side.

Scientists were to step up this experimentation procedure by testing all possible amino acids at a particular objective site on the A or B side, discovering at least one that improves the B-side yield. They, at that point, conveyed that “hit” forward in the mutant background to test the following target site, etc.

Kirmaier said, “It was unexpected. We picked a site, and in one of our best mutant backgrounds, placed all 20 amino acids there — and one of them gave us a 90% yield.”

Deborah K. Hanson of the biosciences division, Argonne National Laboratory said, “This is a breakthrough achievement and something that [everyone in] the field has been actively trying to figure out for decades — ever since we first set eyes on the two tracks in a high-profile structural study in Nature nearly 35 years ago.”

The study highlights basic structure-function principles that moderate effective light-induced electron transfer. The applications of the research involved designing of biohybrid and bioinspired systems for energy conversion and storage.

Dewey Holten, professor of chemistry at Washington University, said, “The results raise lots of questions about what is required to get unidirectional charge separation. In the original history of photosynthesis, maybe such a combination of a fast two-step and slower one-step processes gave an 80 or 90% yield — and then, over time, it optimized.”

The findings were published Dec. 31 in the Proceedings of the National Academy of Sciences (PNAS).

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