The electronic structures of the perturbed blue copper proteins stellacyanin (STC) and cucumber basic protein (CBP, also called plantacyanin, PNC) are defined relative to that of the well-understood “classic” site found in plastocyanin (PLC) by combining the results of low-temperature optical absorption, circular dichroism, and magnetic circular dichroism spectra with density functional calculations. Additionally, absorption and magnetic circular dichroism spectra of Alcaligenes denitrificans wild-type and M121Q azurin are presented and compared to PLC and STC, respectively. These studies show that the principal electronic structure changes in CBP/PNC, with respect to PLC, are a small shift of the ligand field transitions to higher energy and a rotation of the Cu dx2-y2 half-filled HOMO which increases the pseudo-σ and decreases the π interactions of the cysteine (Cys) sulfur with Cu dx2-y2 and, in addition, mixes some methionine (Met) sulfur character into the HOMO. The geometrical distortion responsible...