Dissertations, Theses, and Capstone Projects

Date of Degree


Document Type


Degree Name





Marilyn Gunner

Committee Members

Garry Brudvig

Victor Batista

Stephen O’brien

Ronald Koder

Marilyn Gunner

Subject Categories

Biological and Chemical Physics


Photosynthesis, Photosytem II, OEC, Water splitting, Oxygen Evolving Complex, PSII


The Kok cycle is catalytic process by which the oxygen-evolving complex (OEC) of photosystem II (PSII) oxidizes two water molecules forming oxygen. Four OEC oxidation states (S0 to S3) in the Kok cycle precede the final product formation in the S4 state. Here a semi-empirical classical electrostatics analysis is applied to S0 to S3 states of the OEC is used to estimate the electrochemical midpoints for each S-state transition and the proton loss coupled to oxidation. To account for geometrical rearrangement within the cluster during Kok cycle optimized QM/MM geometries are used for each S state. To obtain the electrochemical midpoint potentials for each transition, the obtained results for consecutive states are averaged using LRA methodology. Protonation state changes between S-states are determined as a function of pH. The role of tyrosine Z (Yz) oxidation is investigated. Light absorption by P680, followed by its oxidation, initiates the overall PSII reaction. Prior to each S state transition Yz is oxidized by P680+, leaving His190+Yz. The manner in which His190+Yz shifts the OEC redox potential and proton release is determined in the S0 to S3 states.