A new study published in Nature Communications offers a new perspective on one of the long-standing questions surrounding the first steps of photosynthesis. Researchers from Cnr Nano, Princeton University, the University of L’Aquila and the Max Planck Institute suggest that the different timescales observed for primary charge separation in Photosystem II may not reflect the existence of alternative reaction pathways, as commonly proposed, but rather the constantly changing molecular environment in which the reaction takes place.
Primary charge separation is the first reaction event following the absorption of a photon in the reaction centre of Photosystem II. The energy of light is converted into a separation of electric charge when an electron is transferred between two molecules, creating a pair of opposite charges. This ultrafast process marks the first step in converting light energy into a form that can drive the subsequent reactions of photosynthesis.
Understanding how this first step occurs is challenging due to spectral congestion and the concurrent involvement of excitation energy transfer processes. Experiments have reported characteristic times ranging from a few hundred femtoseconds to several picoseconds. One of the most widely discussed explanations is that the reaction can proceed through different pathways involving distinct intermediate states, each characterised by a different timescale.
The new study proposes an alternative picture: there is essentially one dominant reaction pathway, while its rate varies depending on the molecular environment in which the reaction takes place. Cnr Nano researchers Matteo Capone and Laura Zanetti-Polzi contributed to the computational study. “Our calculations show that the variability in reaction times can emerge even when charge separation proceeds through a single pathway, whose rate is modulated by conformational fluctuations of the protein and by the dynamics of the water molecules within it,” explains Laura Zanetti-Polzi, researcher at Cnr Nano in Modena.
“We can imagine the reaction as a journey through a forest from one shelter to another,” says Zanetti-Polzi. “For years, the idea was that there were different paths, with different lengths. Our results suggest instead that there is essentially one path: the time needed to travel along it depends on the conditions of the terrain.”
“The continuous fluctuations of the Photosystem II complex and the reorganisation of the water molecules surrounding the reaction centre modify the local energetic conditions under which the electron transfer reaction takes place,” says Matteo Capone. “As a result, the same charge-separation pathway can become faster or slower, explaining the different timescales observed experimentally.”
The researchers reached this conclusion through an entirely computational approach, combining classical molecular-dynamics simulations, quantum-chemical calculations and the perturbed matrix method. Molecular-dynamics simulations were used to reproduce the fluctuations of Photosystem II in its native environment, including the membrane, water and ions, and to follow how the molecular configuration changes over time.
These configurations were then used to calculate how the energetics of charge separation varies with the movements of the protein and water molecules. The study also incorporated numerically exact quantum-dynamics simulations carried out at Princeton University to describe energy transfer between the possible electron donors in the reaction centre, a process that competes with charge separation and therefore needs to be considered when interpreting the observed timescales. The different computational results were finally integrated into a kinetic model and compared with experimental observations.
The findings provide a new framework for understanding how the molecular environment contributes to the remarkable efficiency of photosynthetic energy conversion. Their implications are currently primarily fundamental, but a better understanding of these mechanisms could eventually inspire biomimetic approaches to solar-energy conversion, in which the environment surrounding an artificial catalyst actively contributes to controlling the reaction. “A natural next step is to extend this approach to other stages of the electron-transfer chain in Photosystem II,” says Capone, “to investigate whether the dynamics of water and of the protein matrix also influence the stabilisation of subsequent states.”
The researchers also plan to explore whether the modulatory role of water observed in Photosystem II may be relevant to other systems characterised by ultrafast reactions.
Reference article: M. Capone, A. Benny, G. Dell’Orletta, G. D. Scholes, D. A. Pantazis, L. Zanetti-Polzi, I. Daidone, Water-modulated conformational heterogeneity underlies multiple timescales of primary charge separation in photosystem II. Nat Commun 17, 7795 (2026). https://doi.org/10.1038/s41467-026-73961-w
