Voltammetry and Single‐Molecule In Situ Scanning Tunnelling Microscopy of the Redox Metalloenzyme Human Sulfite Oxidase

Human sulfite oxidase ( h SO) is a homodimeric two‐domain enzyme central in the biological sulfur cycle. A pyranopterin molybdenum cofactor (Moco) is the catalytic site and a heme b 5 group located in the N‐terminal domain. The two domains are connected by a flexible linker region. Electrons produced at the Moco in sulfite oxidation, are relayed via heme b 5 to electron acceptors or an electrode surface. Inter‐domain conformational changes between an open and a closed enzyme conformation, allowing “gated” electron transfer has been suggested. We first recorded cyclic voltammetry (CV) of h SO on single‐crystal Au(111)‐electrode surfaces modified by self‐assembled monolayers (SAMs) both of a short rigid thiol, cysteamine and of a longer structurally flexible thiol, ω‐amino‐octanethiol (AOT). h SO on cysteamine SAMs displays a well‐defined pair of voltammetric peaks around −0.207 V vs. SCE in the absence of sulfite substrate, but no electrocatalysis. h SO on AOT SAMs displays well‐defined electrocatalysis, but only “fair” quality voltammetry in the absence of sulfite. We recorded next in situ scanning tunnelling spectroscopy (STS) of h SO on AOT modified Au(111)‐electrodes, disclosing, a 2–5 % surface coverage of strong molecular scale contrasts, assigned to single h SO molecules, notably with no contrast difference in the absence and presence of sulfite. In situ STS corroborated this observation with a sigmoidal tunnelling current/overpotential correlation.