Figure 6.

Tunnelling simulation of the mixed valence state (i.e. Cu_A and haem a initially oxidized). (a) With average redox midpoint potentials, O₂ reduction is comparable to the O₂ reduction rate in the reduced system and not 10 times slower as experimentally observed. Electron back flow to haem a and a small amount of Cu_A are observed. (b) Raising the redox midpoint of haem a₃ and Cu_B to 320 mV, as expected for a partly reduced system, slows the O₂ reduction rate to 300 μs, comparable to the observed mixed valence rate. Only a small amount of haem a is reduced on the nanosecond time-scale.