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