| Abstract
| - The biological function of metalloproteins stems from the electronic and geometric structures oftheir active sites. Thus, in blue copper proteins such as plastocyanins, an unusual electronic structure ofthe metal site is believed to contribute to the rapid, long-range electron-transfer reactivity that characterizesthese proteins. To clarify this structure−function relationship, numerous quantum chemical calculations ofthe electronic structure of the blue copper proteins have been made. However, the obtained structuresdepend strongly on the applied model. Experimental approaches based on ENDOR spectroscopy andX-ray absorption have also been used to elucidate the electronic structure of the blue copper site. Still, thedetermination of the electronic structure relies on a calibration with quantum chemical calculations, performedon small model complexes. Here we present an approach that allows a direct experimental mapping of theelectron spin delocalization in paramagnetic metalloproteins using oxidized plastocyanin from Anabaenavariabilis as an example. The approach utilizes the longitudinal paramagnetic relaxation of protons closeto the metal site and relies on the dependence of these relaxations on the spatial distribution of the unpairedelectron of the metal ion. Surprisingly it is found that the unpaired electron of the copper ion in plastocyaninis less delocalized than predicted by most of the quantum chemical calculations.
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