Probing the electronic structure of transition metal ion centres in proteins by coherent Raman-detected electron paramagnetic resonance spectroscopy

The simultaneous excitation of a paramagnetic sample with optical (laser) a nd microwave radiation can cause an amplitude or phase modulation of the tr ansmitted light at the microwave frequency. The detection of this modulatio n indicates the presence of coupled optical and electron paramagnetic reson ance (EPR) transitions in the sample. Here we report the first application of this technique to a biomolecule: the blue copper centre of Pseudomonas a eruginosa azurin, Using optical excitation at 686 nm, in the thiol to coppe r(II) charge transfer band, we measure a coherent Raman-detected EPR spectr um of a frozen aqueous solution. Its lineshape is characteristic of the mag netic circular dichroism along each principal g-value axis. This informatio n allows electronic and structural models of transition metal ion centres i n proteins to be tested.