"This seems surprising at first glance because there is no obvious gradient along which energy is transferred from the antenna complex to the reaction center," explains lead author Sebastian Reiter. Analogously, the researchers also identified energy barriers between the antenna complex and the reaction center, among other places. As a result, their absorption spectrum is red-shifted. The results of the study, which is featured on the cover of the journal Chemical Science, reveal so-called "red chlorophylls" that absorb light at slightly lower energies than their neighbors due to ambient electrostatic effects. The complicated calculations were made possible by the supercomputer at the Leibniz Supercomputing Center.
Compared to earlier studies, this approach allows the photosystem I to be described on the basis of state-of-the-art methodology.
A highly accurate multireference method was used to calculate the electronic excitations. To gain deeper insights, the researchers simulated the light excitation of all chlorophylls in a model of photosystem I embedded in a lipid membrane. "Although the complicated energy transfer inside the photosystem has been studied for decades, there is no consensus up to today about the exact mechanism," says de Vivie-Riedle. The quantum yield of photosystem I is almost 100%, meaning that almost every absorbed photon leads to a redox event in the reaction center. There, the solar energy is used to trigger a redox process-that is to say, a chemical process whereby electrons are transferred. The chlorophylls in photosystem I capture sunlight in an antenna complex and transfer the energy to a reaction center. This discovery may help exploit its efficiency in artificial systems in the future. A team led by LMU chemist Regina de Vivie-Riedle has now characterized these chlorophylls with the help of high-precision quantum chemical calculations-an important milestone toward a comprehensive understanding of energy transfer in this system.
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