Surface-confined pyrroloquinoline quinone: Characterizations and interactions with copper ions

Inspired by the unique reactivity of the surface-bound quinone and copper cofactors in copper-containing amine oxidases, we investigated the interactions of pyrroloquinoline quinone (PQQ) with copper ions in solution and adsorbed on indium-doped tin oxide (ITO) electrodes. Characterizations based on atomic force microscopy, electrochemical mode scanning tunneling microscopy, and emission spectroscopy showed that when PQQ was reduced, the resulting two-electron transfer product, PQQH₂, could couple to 3-aminophenylboronic acid and therefore be tethered to ITO. PQQH₂ was also noticed to form multilayer adsorption on the electrodes, featuring reversible changes in adsorption and desorption with potential switching. X-ray photoelectron spectroscopic analysis and X-ray absorption near-edge-structure spectral measurements showed that both PQQ and PQQH₂ could interact with copper ions through the N-1 and N-6 sites. Because of this reactivity, the copper ion exhibited quenching effects on the photoexcited PQQ and PQQH ₂ in solution and on ITO. In addition, current enhancement for PQQ^0/2- was also noticed during the reduction of PQQ as copper ions were added, indicating that PQQH₂ could transfer electrons to Cu ²⁺ ions. The electron transfer rate constant was estimated to be ∼10¹² cm⁶ mol^-2 s^-1 at pH 3. This electron transfer reaction, however, was less influential than the complexation counterpart in quenching the excited PQQH₂. We thus deduce that the electron transfer process may be less energetic or slower than the complexation counterpart or that it takes place only after the latter is complete.