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 S3YZ center dot 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 dot) center through Mn-1 and O6. Similarly, a peroxo species is found to exhibit significant exchange couplings between the Y-Z(center dot) center and the Mn cluster through Mn-1. Mechanistic changes in O-O bond formation in S3YZ center dot 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 S3YZ center dot 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.