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 PQQ0/2- 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.
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