Insights into PSII's S₃Y_Z^• State: An Electronic and Magnetic Analysis

Using BS-DFT (broken-symmetry density functional theory), the electronic and magnetic properties of the S₃Y_Z^• state of photosystem II were investigated and compared to those of the S₃ state. While the O5 oxo-O6 hydroxo species presents little difference between the two states, a previously identified [O5O6]^3- exhibits reduced stabilization of the O5-O6 shared spin. This species is shown to have some coupling with the Y_Z^• center through Mn₁ and O6. Similarly, a peroxo species is found to exhibit significant exchange couplings between the Y_Z^• center and the Mn cluster through Mn₁. Mechanistic changes in O-O bond formation in S₃Y_Z^• are highlighted by analysis of IBOs (intrinsic bonding orbitals) showing deviation for Mn₁ and O6 centered IBOs. This change in coupling interactions throughout the complex as a result of S₃Y_Z^• formation presents implications for the determination of the mechanism spanning the end of the S₃ and the start of the S₄ states, affecting both electron movement and oxygen bond formation.