C. Carra, N. Iordanova, S. Hammes-Schiffer

Department of Chemistry, 152 Davey Laboratory, The Pennsylvania State University,
University Park, PA 16802 (USA)

Abstract: Theoretical calculations of a model for tyrosine oxidation in Photosystem II are
presented.  In this model system, an electron is transferred to ruthenium from tyrosine, which
is concurrently deprotonated.  This investigation is motivated by experimental measurements
of the dependence of the rates on pH and temperature (Sjödin et al., J. Am. Chem. Soc. 2000,
122, 3932).  The mechanism is proton-coupled electron transfer (PCET) at pH<10 when the
tyrosine is initially protonated and is single electron transfer (ET) for pH>10 when the tyrosine
is initially deprotonated.  The PCET rate increases monotonically with pH, whereas the single
ET rate is independent of pH and is two orders of magnitude faster than the PCET rate.  The
calculations reproduce these experimentally observed trends.  The pH dependence for the
PCET reaction arises from the decrease in the reaction free energies with pH.  The calculations
indicate that the larger rate for single ET arises from a combination of factors, including the
smaller solvent reorganization energy for ET and the averaging of the coupling for PCET over
the reactant and product hydrogen vibrational wavefunctions (i.e., a vibrational overlap factor
in the PCET rate expression).  The temperature dependence of the rates, the calculated solvent
reorganization energies, and the deuterium kinetic isotope effects are consistent with the
experimental results.