Effect of pH on the enantiospecificity of homocysteine monolayer on Au(111) for the redox reaction of 3,4-dihydroxyphenylalanine

The effect of pH on the enantiospecificity of Au (111) electrode modified with L-homocysteine was evaluated for the electrochemical redox reaction of 3,4-dihydroxyphenylalanine (DOPA). Cyclic voltammetric peaks clearly exhibited enantiospecificity at pH 0.6 and 2, whereas no enantiospecificity was observed at pH 3, 4, and 5.5. Scanning tunneling microscopy confirmed the highly ordered (2√3x3√3)R30° structure of L-homocysteine at pH 0.6, at which lhomocysteine molecules form a dimer through the hydrogen-bond between carboxy groups (COOH), while a disordered structure was observed at pH 5.5. These results suggest that the dimer formed in the acidic solutions at pH below 3 plays an important role in providing the enantiospecificity to the Au(111) surface.