Azurin proteins with redox potentials that span the entire range of physiological redox potentials (from -950 mV to + 972 mV vs. standard hydrogen electrode) that can be used as tunable and water soluble redox agents for biochemical and biophysical studies. Highlights: Highly pure, Pseudomonas aeruginosa azurin expressed in and purified from Escherichia coliDisplay a wide range of reduction potentials from -954 to +972 mV (+/-10 mV) vs. SHE (the variants with negative reduction potentials are not stable in their reductive form)Water soluble and relatively stable in organic solventsPossess same overall structure and surface properties of native azurin and contain either a Cu center (as in azurin) or Ni in the place of the Cu center