Teaming Spectroscopy and Quantum Theory of Photosystem II

In photosynthesis of cyanobacteria and higher plants carbon dioxide, water and sun light are used to synthesize sugar, which is an important building block of nature. For this purpose two photosystems (I and II) work in series. Although the overall reaction scheme is known and can be found in standard biochemistry textbooks, many molecular details of the underlying photophysical and chemical reactions still need to be deciphered. In the present project we and our experimental collaboration partner Petar Lambrev from Szeged (Hungary) will study Photosystem II, which consists of the photosynthetic reaction center, where the photochemistry starts, and core and peripheral light- harvesting complexes, which absorb the sun light and transfer the excitation energy to the reaction center to drive electron transfer reactions. In the course of the latter, water is split into oxygen that we all breath, protons that are used by the ATPase to create the biological currency ATP, and electrons that are shuttled to photosystem I to create NADPH, a reductant needed for the next stages of sugar synthesis. The energy transfer to the reaction center occurs with high (90%) quantum efficiency under low light conditions. Under high light, the light-harvesting complexes can switch from a light-harvesting to a photoprotective mode, where the excitation energy is quenched in order to protect the reaction center from receiving too much energy, which could lead to photochemical damage. In the present project we aim at a multiscale theoretical description of the excitation energy and primary charge transfer reactions in photosystem II and the optical experiments probing these reactions under low and high-light conditions. For this purpose, existing model Hamiltonians of subunits of photosystem II from the literature (including our own work) will be tested against new spectroscopic experiments and later combined to study excitation energy- and charge transfer in the whole photosystem II. Our collaboration partner has recently introduced a powerful new technique, termed Anisotropic Circular Dichroism (ACD) spectroscopy, where one measures the circular dichroism spectrum on a sample with oriented complexes. We have developed an exciton theory of ACD that will be applied to photosystem II and its subunits in the present project. We expect valuable insight into the electronic structure of excited states that will help us to refine existing Frenkel exciton/charge transfer Hamiltonians that will pave the way for a detailed modelling of excitation energy and primary charge transfer reactions in photosystem II in the present project.