Abstract
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.