Insights into PSII's S3YZ• State: An Electronic and Magnetic Analysis.

ABSTRACT

Using BS-DFT (broken-symmetry density functional theory), the electronic and magnetic properties of the S3YZ• 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 YZ• center through Mn1 and O6. Similarly, a peroxo species is found to exhibit significant exchange couplings between the YZ• center and the Mn cluster through Mn1. Mechanistic changes in O-O bond formation in S3YZ• 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 S3YZ• 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.