The connections between site structure, electronic structure and spectra, and electron-transfer properties of type 1 or blue copper proteins are investigated. The theoretical model includes the neatest neighbors of the Cu ion and the residues to which these neighbors are attached. The electronic structure is calculated using an extended CNDO/S method adapted to spin doublet states. The calculated spectra agree reasonably well with the experimental ones as well as with the calculations of Solomon et al. The strong absorption at about 16 000 cm(-1) is due to a pi --> pi(*) transition, where the pi and pi(*) orbitals are extended in the trigonal plane with Cu 3d pi, S 3p pi, and, to a lesser extent, N 2p sigma character. Strong absorption at other energies is due to ligand field transitions which borrow intensity from the pi --> pi(*) transition because of symmetry lowering from C-2v. Comments, based on calculations, are offered on copper-zinc superoxide dismutase mutants, whose spectral properties to some extent resemble those of the blue proteins.