High-precision quantum chemistry is being used to investigate super-efficient energy transfer in photosynthesis.

All life on Earth is driven by photosynthesis. The sunlight-powered transformation of carbon dioxide and water into energy-rich sugar and oxygen necessitates intricate processes. These cycles are driven by two protein edifices, photosystems I and II. Nearly all of the sunlight in photosystem I is utilized. The key here is a 288-chlorophyll-strong complex network.

A group led by LMU scientific expert Regina de Vivie-Riedle has now portrayed these chlorophylls with the assistance of high-accuracy quantum compound estimations — a significant achievement toward a far-reaching comprehension of energy move in this framework. Its future application in artificial systems may benefit from this discovery.

Photosystem By capturing sunlight in an antenna complex, chlorophyll transfers energy to a reaction center. The redox process, or chemical process by which electrons are transferred, is sparked there by the use of solar energy. Since photosystem I has a quantum yield of almost 100 percent, almost every photon that is absorbed results in a redox reaction in the reaction center.

“While the intricate energy transmission inside the photosystem has been studied for decades, there is still no agreement on the actual mechanism,”

LMU chemist Regina de Vivie-Riedle

Simulation under natural conditions

According to de Vivie-Riedle, “Although the complicated energy transfer inside the photosystem has been studied for decades, there has been no consensus up until today about the exact mechanism.” The researchers used a model of photosystem I embedded in a lipid membrane to simulate the light excitation of all chlorophylls in order to gain a deeper understanding. The electronic excitations were calculated with a highly accurate multireference method. This method makes it possible to describe the photosystem I using cutting-edge methods, which sets it apart from previous research. The Leibniz Supercomputing Center’s supercomputer made the difficult calculations possible.

The study’s findings, which are featured on the cover of Chemical Science, reveal so-called “red chlorophylls” that, due to ambient electrostatic effects, absorb light at slightly lower energies than their neighbors. Their absorption spectrum is consequently red-shifted. Similar to this, the researchers also found energy barriers between the reaction center and the antenna complex. ” Sebastian Reiter, the lead author, explains, “This seems surprising at first because there is no obvious gradient along which energy is transferred from the antenna complex to the reaction center.”

Vacillations conquer energy boundaries

Under physiological circumstances, nonetheless, the whole photosystem I is dependent upon warm variances that defeat these energy boundaries, as the general energies of the chlorophylls change regarding one another. This allows for the continuous opening and closing of new entrances to the reaction center. “Our atomistic simulation of these processes enables a microscopic understanding of the system and its dynamics in its natural environment, complementary to experimental approaches,” concludes Regina de Vivie-Riedle, who is also a member of the e-conversion cluster of excellence. This, according to the authors’ core thesis, may be the key to Photosystem I’s high efficiency.

One of the cluster’s objectives is to eventually transfer the effectiveness of natural photocatalysts to artificial nano-bio hybrid systems for use in processes like producing hydrogen as an energy carrier or converting carbon monoxide into fuel. A deeper understanding of the energy transfer mechanism is required for this. With their findings on photosystem I, the researchers have made a significant progress toward achieving this objective.

More information: Sebastian Reiter et al, Thermal site energy fluctuations in photosystem I: new insights from MD/QM/MM calculations, Chemical Science (2023). DOI: 10.1039/D2SC06160K

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