High-Resolution Electronic Structure of the Primary Electron Acceptor A0 of Photosystem I

Abstract

The Type I photosynthetic reaction center, Photosystem I, is an exquisitely tuned protein complex comprised of multiple polypeptide subunits and protein-bound electron-transfer (ET) cofactors. Upon illumination, the special chlorophylls, P700, are photoexcited which results in the rapid formation of the charge-separated state, P700+A0-. In order to prevent charge recombination, the electron is transferred to the phylloquinone cofactor, A1, and subsequently to three four iron-four sulfur [4Fe4S] clusters, FA, FB, and FX, respectively. Through the ET reactions of PSI, the reducing equivalents that are required for the carbon fixation reactions are generated and stored as NADPH or ‘biohydrogen’. The PSI reaction center displays pseudo-C2 symmetry, such that the ET cofactors, A0 and A1, are duplicated in what is termed the A- and B-branch. Although ET is bidirectional along the A- and B-branch, it has been demonstrated that ET in the A-branch is highly preferred. This is thought to be due to the tuning of the redox-potential of the ET cofactors by smart matrix effects from the surrounding protein environment. This research explores the electronic structure of the primary electron acceptor A0 in the A-branch (A0A) through the use of two-dimensional (2D) hyperfine sublevel correlation (HYSCORE) spectroscopy in tandem with density functional theory calculations. The application of 2D HYSCORE spectroscopy allows for the identification of the 14N atoms and protons that are magnetically interacting with the paramagnetic center of A0A-. The 14N and 1H hyperfine parameters obtained here provide a direct measure of the electron spin density distribution of A0- in the A-branch of PSI. These are then be compared with simulations of the electronic structure of the A-branch of Photosystem I in order to better understand the mechanism of electron transfer from P700 to the primary electron acceptor, A0A.