A W-Band Electron Spin Echo Envelope Modulation Study of a Single Crystal of Azurin

At 95 GHz, deep electron spin echo envelope modulations of a single crystal of the blue copper protein azurin have been observed. The modulations arise from the coordinated nitrogens of the histidines that ligate to copper. From the ESEEM frequencies, hyperfine and quadrupole tensors of these nitrogens have been deduced. The isotropic hyperfine coupling of the copper-bound nitrogen of histidine-117 is 1.4 times larger than that of histidine-46. The anisotropic hyperfine tensors show that the wave function of the unpaired electron on both coordinated nitrogens mainly concerns the σ bonds with copper.