Scientists from the Indian Institute of Science (IISc) and the California Institute of Technology (Caltech) have finally solved a long-standing puzzle concerning the earliest moments of photosynthesis — the vital process through which plants, algae, and certain bacteria capture sunlight to generate oxygen and energy-rich compounds.
Their research reveals why the primary movements of electrons, that are crucial for transferring energy, occur through just one side of a key protein-pigment structure. The findings were published within the Proceedings of the National Academy of Sciences.
Photosynthesis is a sequence of reactions by which electrons pass between multiple pigment molecules. Even though it has been examined for many years, the method stays difficult to totally explain since it involves quite a few intricate components, operates at extremely fast timescales, and varies barely across different species. Gaining a deeper understanding of those steps could help scientists develop efficient artificial systems, akin to synthetic leaves and solar-based fuel technologies, that replicate nature’s design.
In most life forms that use photosynthesis, the method begins with a protein-pigment complex often called Photosystem II (PSII). This complex captures sunlight and splits water molecules, releasing oxygen and sending electrons onward to other molecules within the chain of energy transfer.
PSII accommodates two nearly an identical branches, often called D1 and D2, surrounded by 4 chlorophyll molecules and two related pigments called pheophytins. These are symmetrically arranged and connected to electron carriers often called plastoquinones. In theory, electrons should move from chlorophyll to pheophytin after which to plastoquinone along each branches.
Nevertheless, experiments have consistently shown that electrons move only through the D1 branch — a finding that has baffled scientists for years. “Despite the structural symmetry between the D1 and D2 protein branches in PSII, only the D1 branch is functionally energetic,” explains Aditya Kumar Mandal, the study’s first creator and a PhD student within the Department of Physics at IISc.
To analyze this imbalance, the team combined molecular dynamics simulations, quantum mechanical analyses, and Marcus theory (a Nobel Prize-winning model that describes how electrons are transferred) to chart the energy patterns in each pathways. “We assessed the electron transfer efficiency step-by-step through each D1 and D2 branches,” says Shubham Basera, PhD student within the Department of Physics and one in all the authors.
The team found that the D2 branch has a much higher energy barrier, which makes electron transport energetically unfavourable. Specifically, the transfer of electrons from pheophytin to plastoquinone in D2 requires twice as much activation energy as D1 — a barrier that electrons seem unable to beat, stopping energy from flowing forward.
The researchers also simulated the current-voltage characteristics of each branches and located that the resistance against electron movement in D2 was two orders of magnitude higher than that in D1.
The asymmetry in electron flow might also be influenced by subtle differences within the protein environment across the PSII and the way the pigments are embedded in it, the researchers suggest. For instance, the chlorophyll pigment in D1 has an excitation state at a lower energy than its D2 counterpart, suggesting that the D1 pigment has a greater likelihood of attracting and transferring electrons.
The researchers also suggest that tweaking a few of these components can boost or rewire electron flow across PSII. For instance, swapping chlorophyll and pheophytin in D2 could overcome the electron block, because chlorophyll needs lower activation energy than pheophytin.
“Our research presents a big step forward in understanding natural photosynthesis,” says Prabal K Maiti, Professor on the Department of Physics and one in all the corresponding authors of the study. “These findings may help design efficient artificial photosynthetic systems able to converting solar energy into chemical fuels, contributing to revolutionary and sustainable renewable energy solutions.”
That is an exquisite combination of theory at various levels to deal with a long-standing problem culminating in a brand new level of understanding, but still leaving mysteries to be challenged, says Bill Goddard, Professor at Caltech and one in all the corresponding authors.