ESEEM Studies of Alcohol Binding to the Manganese Cluster of the Oxygen Evolving Complex of Photosystem II

An ESEEM (electron spin−echo envelope modulation) spectroscopic study employing a series of 2 H-labeled alcohols provides direct evidence that small alcohols (methanol and ethanol) ligate to the Mn cluster of the oxygen evolving complex (OEC) of Photosystem II in the S 2 -state of the Kok cycle. A numerical method for calculating the through-space hyperfine interactions for exchange-coupled tetranuclear Mn clusters is described. This method is used to calculate hyperfine interaction tensors for protons [deuterons] in the vicinity of two different arrangements of Mn ions in a tetranuclear cluster: a symmetric cubane model and the EXAFS-based Berkeley “dimer-of-dimers” model. The Mn−H distances derived from the spectroscopically observed coupling constants for methanol and ethanol protons [deuterons] and interpreted with these cluster models are consistent with the direct ligation of these small alcohols to the OEC Mn cluster. Specifically, for methanol we can simulate the three-pulse ESEEM time domain pattern with three dipolar hyperfine interactions of 2.92, 1.33, and 1.15 MHz, corresponding to a range of maximal Mn−H distances in the models of 3.7−5.6 Å (dimer-of-dimers) and 3.6−4.9 Å (symmetric cubane). We also find evidence for limited access of n -propanol, but no evidence for 2-propanol or DMSO access. Implications for substrate accessibility to the OEC are discussed.