Electron tunnelling through azurin is mediated by the active site Cu ion

Cu– and Zn–azurin chemisorbed on Au(1 1 1) have been comparatively investigated by electrochemical scanning tunnelling microscopy in buffer solution. Cu–azurin shows a marked tunnelling current resonance upon substrate potential at −0.21 V (vs SCE), whereas Zn counterparts do not. These data, discussed in terms of current theories on electron tunnelling through redox adsorbates, demonstrate the role of the electroactive metal ion present in the active site in assisting electron transfer via this metalloprotein.