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 emplo ying a series of H-2-labeled alcohols provides direct evidence that small a lcohols (methanol and ethanol) ligate to the Mn cluster of the oxygen evolv ing complex (OEC) of Photosystem II in the S-2-state of the Kok cycle. A nu merical method for calculating the through-space hyperfine interactions for exchange-coupled tetranuclear Mn clusters is described. This method is use d to calculate hyperfine interaction tensors for protons [deuterons] in the vicinity of two different arrangements of Mn ions in a tetranuclear cluste r: a symmetric cubane model and the EXAFS-based Berkeley "dimer-of-dimers" model. The Mn-H distances derived from the spectroscopically observed coupl ing 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 sim ulate the three-pulse ESEEM time domain pattern with three dipolar hyperfin e interactions of 2.92, 1.33, and 1.15 MHz, corresponding to a range of max imal Mn-H distances in the models of 3.7-5.6 Angstrom (dimer-of-dimers) and 3.6-4.9 Angstrom (symmetric cubane). We also find evidence for limited acc ess of n-propanol, but no evidence for 2-propanol or DMSO access. Implicati ons for substrate accessibility to the OEC are discussed.