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Earth's oldest photosynthetic organisms provide insight into clean energy solutions

Heliobacterial photosynthetic reaction center

Two-dimensional electronic spectroscopy absorption spectra (bottom) and heliobacterial reaction center model (top).

June 25, 2021

The development of clean energy has historically been an important research area for Arizona State University's School of Molecular Sciences. In this context the study of natural and artificial photosynthetic systems has been of central importance.

In nature, the conversion of light energy into usable chemical energy — photosynthesis — occurs over a series of steps mediated in a complex array of specialized protein, pigment and other molecular structures referred to as the “photochemical reaction center.” Photochemical reaction centers are complex nanomachines that convert light energy into chemical energy usable by life.

Although photosynthesis is usually considered to be a critical process utilized by plants, the earliest photosynthetic organisms on Earth were actually bacteria. Not all bacteria are photosynthetic, but bacteria were the original photosynthetic powerhouses that were primarily responsible for generating the oxygen that life on Earth depends upon, and understanding how they convert light energy into chemical energy is an important area of research.

In a recent article in Nature Communications, ASU scientists, in collaboration with colleagues at the University of Michigan, studied the reaction center from heliobacteria — the simplest, and most similar to the earliest photosynthetic reaction centers that first appeared on Earth over 3 billion years ago.

School of Molecular Sciences' Kevin Redding and William Johnson, together with Jennifer Ogilvie’s research group at University of Michigan, are using advanced spectroscopic techniques to study reaction centers. The team was able to study the primary light-driven molecular structure and identify the very first step in energy conversion, formation of a charge-separated structure.                              

Even this seemingly simple process is very difficult to study in the reaction center due to the complex arrangement of many similar molecular structures, all of which could participate in one way or another. The team used a sophisticated two-dimensional electronic spectroscopy technique to follow the electron path and discovered that the “hole” left behind after electron transfer is delocalized (or spread out) over three to four molecules.

“This is a very clever experimental approach to a problem that researchers have been working on for over 40 years with only limited success," said Ian Gould, School of Molecular Sciences interim director. "It has historically been very difficult to follow electrons around in reaction centers due to the molecular complexity of the system; it is exciting to see how state-of-the-art techniques are now unraveling this in rich kinetic detail.”

“This work demonstrates one of the ways in which nature has specialized the design of the photochemical reaction center, the basic architecture of which emerged over 3 billion years ago, to suit the specific needs of the niche in which it operates, giving us insight into new strategies to make artificial versions that can accomplish similar feats,” Professor Redding said. 

This research builds upon research carried out at ASU through a collaboration between Redding, Raimund Fromme and John Golbeck, and funded by the U.S. Department of Energy to understand the structure of the heliobacterial reaction center. Although energy conversion in heliobacteria has been studied since the late 1980s, the improved understanding of their reaction-center mechanisms can be applied not only to the study of reaction centers in other organisms, but also to the development of clean energy.

The team’s research can in principle be applied to efforts to create photoelectrochemical cells using molecule structures rather than inorganic materials, like silicon. Unlike photovoltaics, which only drive electrical current, photoelectrochemical cells based on photosynthesis may be used in the future to create chemical products, using solar energy. Photochemists at ASU and elsewhere have already created such systems, which can make very simple molecules, like hydrogen (the simplest molecule of all). It can be envisioned that more complex molecules can be created from multiple electron transfer events, if the system can be tuned for maximal efficiency in electron delivery.

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