An extensive assignment of the H-1 NMR spectra of a Cu-A domain from the cytochrome c oxidase of Paracoccus denitrificans has been obtained on the basis of dipolar connectivities coupled to the available structural information. The assignment has been extended with the aid of H-2 NMR spectra on a protein sample with cysteines selectively labeled at the beta position. The spectra have been compared to those published earlier for a similar Cu-A fragment from the Thermus thermophilus oxidase and for a Cu-A construct in a blue copper protein, amicyanin, The shifts and their temperature dependence are discussed in terms of molecular orbital descriptions available for these systems. The mechanisms for the fast electron relaxation, which make the obtainment of relatively sharp NMR signals possible, are discussed. It is shown that the proton shifts of one histidine are variable from one system to the other, possibly reflecting changes in its interaction with the Cu2S2 diamond structure. Large electron delocalization onto the S atoms is confirmed. This is reflected in the large shifts experienced by the cysteine beta-CH2 protons, and these shifts are in turn tuned by the Cu-S-C-H torsion angles. It is suggested that the electronic structure of Cu-A is ideal for its function in electron transfer.