Fig. 3.

The 77 K EPR spectra (offset for clarity) of 12.5 μM (black line; g_∥ = 2.22, A_∥ = 196 G, g_⊥ = 2.01) and 1.25 μM (purple line) Cu²⁺ standards, and 125 μM Cu⁺ prepared by diluting the 50 mM Cu⁺ comproportionation stock solution in 5 mM (blue line) or 25 mM (red line, gain ×10) MeCN.