Azurin is a protein containing a copper ion that shuttles between the +2 and +1 oxidation states, and cytochrome c is a protein in which a haem-bound iron ion shuttles between the +3 and +2 oxidation states. The rate constant for electron transfer from reduced azurin to oxidized cytochrome c is 1.6 x 103 dm3 mol-1 s-1. Estimate the electron self-exchange rate constant for azurin from the following data: