A W-band Electron Nuclear Double Resonance study of single crystals of N-14 and N-15 azurin

We report pulsed Electron-Nuclear-DOuble-Resonance (ENDOR) experiments at 95 GHz and 1.2 K of single crystals of the blue-copper protein azurin and its fully N-15-enriched analogue. The nitrogen ENDOR spectra and their variation with the orientation of the magnetic field with respect to the crystals are described, and it is shown that contributions of five distant nitrogens may be distinguished. For the remote nitrogens of the copper ligands histidines-46 and -117 complete hyperfine and quadrupole tensors are presented and discussed in relation to the structure of the copper site and the extension of the wave function of the unpaired electron. Besides these nuclei three backbone nitrogens show up in the ENDOR spectra, and their hyperfine tensors are reported. One of the backbone nitrogens concerns that of cysteine-112; the others are tentatively assigned to histidine-46 and glycine-45. The wave function of the unpaired electron of oxidized azurin is found to be smeared out over both the copper ligands and parts of the protein backbone.