Decades of Research at Caltech Illuminate a Key Mystery of Photosynthesis

Photosynthesis begins in a sophisticated protein complex named Photosystem II. Present in plants, algae and certain bacteria, it plays a fundamental role by capturing light energy and by triggering a cascade of chemical reactions. This system is based on two identical branches, called D1 and D2, which host pigments such as chlorophyll and pheophytin. Theoretically, these two routes should allow the transfer of electrons in an equivalent manner. However, only those in the D1 branch actually get in motion.

This functional asymmetry has long confused the researchers, especially since the architecture of the two branches seemed rigorously symmetrical. But by studying the energy dynamics of each of them, scientists discovered an unsuspected imbalance. At the atomic scale, the electrons encounter variable resistances, and this is precisely what makes the branch D2 inactive. This energy barrier prevents all traffic, despite the presence of the same components.

What the intimate structure of photosystem II reveals

To explore this phenomenon, the researchers combined simulations of molecular dynamics and quantum calculations, based on Marcus' theory, a reference in the study of electron transfers. They have thus reconstructed, step by step, the energy landscape that the electrons must cross inside the D1 and D2 branches. The results reveal that the last link in the chain, in the D2 branch, has a barrier almost twice higher than that of D1. This difference is enough to block the flow.

By analyzing the electrochemical properties of the pigments, the researchers also noted that the states of excitement of the chlorophyll were different between the two branches. The pigment housed in D1 absorbs and more easily transmits the energy than its equivalent in D2. In addition, the surrounding proteins, although apparently similar, create slightly distinct environments. These micro-deferences modify the organization of electrical charges and influence the behavior of electrons.

These works were published in the Revue Proceedings of the National Academy of Sciences by the IISC and Caltech team, providing an expected response to a biological enigma of several decades.

Towards more efficient bio-inspired technologies

This advance is not only of fundamental interest. Understanding the specific reasons that promote the transfer of electrons in a single way opens up concrete perspectives for materials science. By adjusting the components or imitating the effective structures of nature, it becomes possible to envisage artificial systems capable of exploiting light more efficiently.

Devices such as artificial leaves, which aim to convert solar light into clean fuel, could benefit from this knowledge. By manipulating the layout of the pigments or by reproducing the optimal protein environment, engineers could reduce energy losses and increase overall yield.

The simulations have also shown that a simple exchange of position between certain pigments could unlock the passage of electrons in the D2 branch. This type of adjustment, if applied to synthetic systems, could considerably improve their energy efficiency.

The conclusions resulting from this research show that the fine observation of natural functioning makes it possible to imagine innovative solutions to meet the challenges of the energy transition. What nature has been optimizing for millions of years today becomes an engineering model.