Role of Vibronic Coupling and of Conformational Substate Distribution in Determining the Features of Copper-Protein EPR Spectra

It is stressed that the presence of a metal ion directly bound to a protein can introduce elements of complexity, such as vibronic coupling, which cannot be ruled out a priori. As a typical example, two copper proteins, azurin and plastocyanin, have been considered. Electron paramagnetic resonance spectra of the two proteins, recorded in the range 4-200 K and with different cooling rates, support the hypothesis of a prominent role played by vibronic interactions as well as by the conformational substate distribution.