A comparative study of the H-1 and N-14 hyperfine interactions between the Cu-A site in an engineered Cu-A center in azurin (WT-Cu(A)Az) and its His120Gly variant (H120G-Cu(A)Az) using the two-dimensional ESEEM technique, HYSCORE, is reported. HYSCORE spectroscopy has clarified conflicting results in previous electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) studies and found clear differences between the two Cu-A azurins. Specifically, a hyperfine coupling A(N perpendicular to) of 15.3 MHz was determined for the first time from the frequencies of double-quantum transitions of N-14 histidine nitrogens coordinated to Cu-A in WT-Cu(A)Az. In contrast, such coupling was not observed in the spectra of H120G-Cu(A)Az, indicating at least a several megahertz increase in A(N perpendicular to) for the coordinated nitrogen in this variant. In addition, N-14 HYSCORE spectra of WT-CuAAz show interaction with only one type of weakly coupled nitrogen assigned to the remote N-epsilon atom of coordinated imidazole residues based on the quadrupole coupling constant (e(2)Qq/4h) of similar to 0.4 MHz. The spectrum of H120G-CuAAz resolves additional features typical for backbone peptide nitrogens with larger e(2)Qq/4h values of similar to 0.7 MHz. Hyperfine couplings with these nitrogens vary between similar to 0.4 and 0.7 MHz. In addition, the two resolved cross-peaks from C-beta protons in H120G-Cu(A)Az display only similar to 1 MHz shifts relative to the corresponding peaks in WT-Cu-AAz. These new findings have provided the first experimental evidence of the previous density functional theory analysis that predicted changes in the delocalized electron spin population of similar to 0.02-0.03 (i.e., similar to 10%) on copper and sulfur atoms of the Cu-A center in H120 variants relative to WT-CuA(A)z and resolved contradicting results between EPR and ENDOR studies of the valence distribution in Cu(A)Az and its variants.