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.
- DFT and Ab initio calculations
- Single-molecule in situ AFM
- Single-molecule in situ STM
- Substrate-triggered single-molecule enzyme swelling
- Two-centre redox metalloproteins