Conformational triggering in voltammetry and single-molecule conductivity of two-centre redox metalloproteins: Cytochrome c4 and copper nitrite reductase

Blue copper enzymes often show no voltammetry themselves, whereas substrate binding triggers strong electrocatalytic signals. Similarly, electrochemical STM only gives strong contrasts when substrate (O2, NO2-) is present. AFM shows that CuNIR on Au(111)-electrodes modified by self-assembled cysteamine monolayers (SAMs) maintains constant height throughout the electrocatalytic range, while NO2- triggers substantial enzyme ‘swelling’. ‘Swelling’ does not accord with the crystalline state, which, however, is not the relevant catalytic environment.

With a view on understanding these patterns, we present ab initio quantum chemical studies of CuNIR/OH2 and CuNIR/NO2- 740-atom fragments including the type I and type II Cu-centres. Replacing water at the type II centre by nitrite triggers 2-Å Cu–Cu distance increase, according with enzyme ‘swelling’. 2 Å Cu–Cu increase would close intramolecular ET entirely, but is compensated by efficient superexchange alignment of closely interacting LUMOs and HOMOs. In the water-bound enzyme these orbitals are separated by unfavourable through-space tunneling regions.