Frozen solutions of azurin and of its mutant His117Gly in the presence of [N-15]imidazole have been investigated by 9 GHz three-pulse electron spin-echo envelope modulation (ESEEM) and two-dimensional ESEEM spectroscopy. It is found that the interaction of the unpaired electron with a backbone nitrogen leads to a sizable contribution to the ESEEM spectrum. This observation provides the clue to the interpretation of the ESEEM spectrum of the prototypical blue-copper site of azurin. The modulations stem largely from the interaction of the electron spin with three nitrogen nuclei. The quadrupole bands of the remote nitrogen of histidine-117 dominate the spectrum. Smaller but significant contributions result from the backbone nitrogen and the remote nitrogen of histidine-46. Simulations of the ESEEM spectra of azurin confirm this interpretation and yield complete hyperfine and quadrupole tensors for all three nitrogens. The significant contribution of the backbone nitrogen to the ESEEM spectra of azurin nicely brings out the delocalized character of the unpaired-electron wave function of the oxidized copper center.