Magnetic circular dichroism anisotropy of the Cu-A centre of nitrous oxide reductase from coherent Raman detected electron spin resonance spectroscopy

Coherent Raman detected electron spin resonance spectroscopy is a technique that bridges the established fields of magnetic resonance and magneto-optics. By exploiting the orientational selectivity of the microwave resonance condition it becomes possible to measure the relative orientations of the magnetic and optical anisotropies of paramagnetic chromophores, and thereby to test models of their electronic structure. This paper reports the application of this method to the Cu-A centre from Paracoccus pantotrophus nitrous oxide reductase, an unusual mixed valence copper, Cu(I)/Cu(II), dimer centre also found in some heme-copper terminal oxidases. Data from the principal visible bands (at 476, 514 and 750 nm) shows that their magnetic circular dichroism is almost entirely aligned with the g-value z-axis. This is consistent with previous models of the electronic structure in which the optical transitions are polarized within the copper-thiolate plane of the centre, and the g-value z-axis is orientated normal to this plane.