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

Abstract

Blue copper enzymes often show no voltammetry themselves, whereas substrate binding triggers strong electrocatalytic signals. Similarly, electrochemical STM only gives strong contrasts when substrate (O 2 , NO 2 - ) 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 NO 2 - 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/OH 2 and CuNIR/NO 2 - 740-atom fragments including the type I and type II Cu-centres. Replacing water at the type II centre by nitrite triggers two-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. • Single-crystal voltammetry of cytochrome c 4 and copper nitrite reductase (CuNIR). • Electrochemical STM and AFM of cyt c 4 and CuNIR. • DFT of intramolecular ET in cyt c 4 . • Ab initio quantum chemistry calculations of 740 atoms CuNIR fragment . • Nitrite triggered CuNIR intramolecular ET channel. • Nitrite triggered CuNIR conformational enzyme swelling in AFM.