Molecular dimensions of dried glucose oxidase on a Au(1 1 1) surface studied by dynamic mode scanning force microscopy

We have investigated the molecular dimensions of a dried single glucose oxidase (GO) molecule adsorbed on a Au(1 1 1) surface with the UHV non-contact atomic force microscopy (NC-AFM) and tapping mode atomic force microcopy (TMAFM). The smallest air-dried GO particles in a TMAFM-measured size distribution are found to be 10–11 nm wide and 0.3–0.4 nm high. We find each collapsed ellipsoidal feature with a groove in a NC-AFM image, which measured 12 nm × 10 nm × 0.5 nm. The lateral dimensions (12 nm × 10 nm) of the observed feature is close to those of a GO monomer measured by scanning tunneling microscopy (STM) [Quijin et al., 12.2 nm × 8.9 nm as the size of one wing of an opening butterfly (dimer) appeared in a STM image] and by contact mode AFM [Quinto et al., 14 nm × 8 nm]. Our value of the vertical dimension (0.5 nm) is consistent with AFM results and molecular dynamics simulations that suggest a surface-induced complete unfolding, showing the average diameter of amino acid residues.