Electron Paramagnetic Resonance in the Study of Binuclear Centres of Copper Proteins

Abstract

Metalloproteins often use binuclear metal centres to perform important biological functions. This is the case of electron transfer proteins, such as ferredoxins, oxygen carriers, such as hemerythrin and hemocyanin, and oxygenases, such as tyrosinase. Hemocyanins contain copper pairs as the functional unit (Lontie and Witters, 1973) and are known since long time to show no electron paramagnetic resonance (EPR) signal in the native state, either oxy or deoxy (Nakamura and Mason, 1960). Though EPR can only detect paramagnetic copper and has therefore been extensively used in the study of copper proteins (Vanngard, 1972), the absence of EPR signals in molecules containing more than one copper, does not mean that the copper is in the diamagnetic cuprous state. In fact two paramagnetic ions, which would give EPR signals in mononuclear centres, may become EPR silent if they are very close to each other or are bridged by a common ligand. In the case, as for Cu(II), that they have each total electron spin S = 1/2, they can couple to each other according to the scheme shown in Figure 1. The coupling will result in Open image in new window Fig. 1 Magnetic states produced as a result of coupling of two paramagnetic centres with total spin S = 1/2 a ground state with antiparallel spins, which will be diamagnetic (S = 0), and in an excited state, with parallel electron spins, which will behave as a triplet state (S = 1). The coupling between unpaired electrons will always have a dipolar component, that is an interaction between magnetic dipoles depending on their distance (α1/r3) and on their orientation with respect to the magnetic field. This kind of interaction will produce zero field splitting of the triplet state (D), which will give rise to the magnetic field dependent separation of energy levels shown in Figure 2.