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 nearest 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-' is due to a n -n* transition, where the n and n* orbitals are extended in the trigonal plane with Cu 3&, S 3pn, and, to a lesser extent, N 2p0 character. Strong absorption at other energies is due to ligand field transitions which borrow intensity from the n -n* transition because of symmetry lowering from C2". Comments, based on calculations, are offered on copper-zinc superoxide dismutase mutants, whose spectral properties to some extent resemble those of the blue proteins.

The structure of ribonuclease A has been refined jointly with the neutron and X-ray data extending to 2.0 A. The results of an earlier X-ray refinement provided the starting model [Wlodawer, A., Bott, R., & Sjölin, L. (1982) J. Biol. Chem. 257, 1325-1332]. The final R factors were 0.159 (X-ray) and 0.183 (neutron) for a model containing all of the atoms expected in the protein, 128 waters, and a phosphate molecule in the active site. The joint refinement necessitated modifications in the orientation of a number of side chains, including the catalytically active lysine-41, which is now thought to form a salt link to the phosphate. Major modifications of the previous model of the bound solvent were necessary. The refinement of all atom occupancies with the neutron data only provided the information about the amide hydrogen exchange. A fourth of all amide hydrogens were found to be at least partially protected from exchange after a year of exchange with D2O.

No abstract