Low-temperature optical absorption, circular dichroism, magnetic circular dichroism, and sulfur K-edge X-ray absorption spectra have been measured for the green "blue" copper center (type 1) in Achromobacter cycloclastes nitrite reductase. Combined with density functional calculations, the results of these spectroscopies have been used to define the extremely "perturbed" electronic structure of this site relative to that of the prototypical "classic" site found in plastocyanin. Experimentally calibrated density functional calculations have been further used to determine the specific geometric distortions which generate the perturbed electronic structure. These studies indicate that the principal electronic structure changes in nitrite reductase, relative to plastocyanin, are a rotation of the Cu d(x2-y2) half-filled, highest occupied molecular orbital (HOMO) and an increase in the ligand field strength at the Cu center.