Photophysical model of primary charge separation in green algal photosystem I based on combined analysis of time-resolved absorption and fluorescence spectra.

The mechanism of primary charge separation in photosynthetic reaction centers is still not fully understood. In the case of photosystem I (PSI), the study of this process is particularly difficult due to the integration of the reaction center (RC) with the core antenna and presence of so-called red chlorophylls in the antenna system. Their excitation dynamics overlap spectrally and temporarily with the primary charge separation in RC. Therefore, we performed time-resolved absorption measurements (femtosecond pump-probe technique) for the red chlorophyll-free PSI core from green algae Chlamydomonas reinhardtii, both under reducing (open RC) and oxidizing (closed RC) conditions. Kinetic modeling of the recorded absorption changes indicated the same trapping rate of excitations by open and closed RC. Detailed spectral analysis showed, in turn, that processes related to the primary charge separation can be described by temporal evolution of several main well-defined spectral bands (with different contributions in the case of open and closed RC). We explained and thoroughly discussed their origin in the context of the excitonic coupling between the RC chlorophylls. We also implemented the identical kinetic model to the time-resolved fluorescence data published previously for exactly the same PSI preparation (Giera et al. 2014; Biochim Biophys Acta – Bioenergetics 1837:1756–1768). This combined analysis allowed us to identify different emission properties of the primary state generated in the open and closed RC after excitation trapping. The nature of this state is discussed.