Redox and spectroscopic (electronic absorption, multifrequency electron paramagnetic resonance (EPR), and X-ray absorption) properties together with X-ray crystal structures are reported for the type 2 Cu-II C112D/M121E variant of Pseudomonas aeruginosa azurin. The results suggest that Cu-II is constrained from interaction with the proximal glutamate; this structural frustration implies a "rack" mechanism for the 290 mV (vs NHE) reduction potential measured at neutral pH. At high pH (similar to 9), hydrogen bonding in the outer coordination sphere is perturbed to allow axial glutamate ligation to Cu-II, with a decrease in potential to 119 mV. These results highlight the role played by outer-sphere interactions, and the structural constraints they impose, in determining the redox behavior of transition metal protein cofactors.