Insights into PSII's S 3 Y Z • State: An Electronic and Magnetic Analysis.

Using BS-DFT (broken-symmetry density functional theory), the electronic and magnetic properties of the S 3 Y Z • state of photosystem II were investigated and compared to those of the S 3 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 Mn 1 and O6. Similarly, a peroxo species is found to exhibit significant exchange couplings between the Y Z • center and the Mn cluster through Mn 1 . Mechanistic changes in O-O bond formation in S 3 Y Z • are highlighted by analysis of IBOs (intrinsic bonding orbitals) showing deviation for Mn 1 and O6 centered IBOs. This change in coupling interactions throughout the complex as a result of S 3 Y Z • formation presents implications for the determination of the mechanism spanning the end of the S 3 and the start of the S 4 states, affecting both electron movement and oxygen bond formation.