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Abstract
The initial light-driven electron transfer event in plant photosynthesis creates an electron-deficient pigment molecule with sufficient oxidative potential to strip electrons from water. The process of photosynthetic water oxidation releases molecular oxygen as a byproduct. The water oxidation process is cyclic, with intermediate states designated $S\sb0$ through $S\sb4$. Each photo-oxidation of the primary pigment induces a transition in the cycle. Molecular oxygen is released after four photo-oxidation events, and the complex resets to the least oxidized state, $S\sb0$. The water oxidation chemistry is performed by a membrane-bound protein complex containing the transition metal Manganese. The structure of the Mn complex has been partially characterized by X-ray spectroscopy and electron paramagnetic resonance (EPR).
We have constructed a high-power pulsed EPR spectrometer to perform Electron Spin Echo (ESE) experiments on the Mn complex. In particular, we are interested in measuring the magnetic dipolar and electric quadrupolar parameters of paramagnetic nuclei magnetically coupled to the Mn complex. High power microwave pulses induce quantum mechanical coherences in the nuclear spin sublevels. Interference effects due to these sublevel coherences can be measured by monitoring the amplitude of the ESE signal as a function of the interval between the applied microwave pulses. Fourier analysis of the resulting Electron Spin Echo Envelope Modulation (ESEEM) reveals the nuclear sublevel splittings.
The ESEEM technique has provided details of the chemical environment of $\sp1\rm H$, $\sp2\rm H$, and $\sp{14}\rm N$ nuclei in the vicinity of the photosynthetic Mn cluster. The specific $\sp{14}\rm N$ sublevel frequencies observed indicate Mn coordination to an imidizole group from a histidine residue. We discuss possible roles for coordinated imidizole in the functioning of the water oxidation complex. Additional $\sp{14}\rm N$ modulation is observed when oxygen evolution is inhibited by ammonia. The increase in $\sp{14}\rm N$ modulation may result from directly coordinated ammonia. ESEEM studies following incubation in $\sp2\rm H\sb2\rm O$ buffers indicate water coordination to Mn in the $S\sb2$ state. However, water does not bind to Mn during the $S\sb1$ $\to$ $S\sb2$ transition. The coordinated water may be present in the least-oxidized state, $S\sb0$, or water may bind during the $S\sb0$ $\to$ $S\sb1$ transition.
We also present ESEEM data on di-$\mu$-oxo bridged Mn(III)Mn(IV) dimers with 2,2,-bipyridine or 1,10-phenanthroline terminal ligands. (Abstract shortened with permission of author.)
