Deconvolution and assignment of different optical transitions of the blue copper protein azurin from optically detected electron paramagnetic resonance spectroscopy

Magnetic circular dichroism is a powerful spectroscopic tool for the assign ment of optical resonance lines. An extension of this technique, microwave- modulated circular dichroism, provides additional details, in particular in formation about the orientation of optical transition moments. It arises fr om magnetization precessing around the static magnetic field, excited by a microwave field, in close analogy to electron paramagnetic resonance (EPR). In this paper we investigate the visible and near-infrared spectrum of the blue copper protein Pseudomonas aeruginosa azurin. Using a nonoriented sam ple (frozen solution), we apply this technique to measure the variation of the optical anisotropy with the wavelength. A comparison with the optical a nisotropies of the possible ligand-field and charge-transfer transitions al lows us to identify individual resonance lines in the strongly overlapping spectrum and assign them to specific electronic transitions. The technique is readily applicable to other proteins with transition metal centers.