Electron‐Transfer Reactions through the Associated Interaction between Cytochrome c and Self‐Assembled Monolayers of Optically Active Cobalt(III) Complexes: Molecular Recognition Ability Induced by the Chirality of the Cobalt(III) Units

Self‐assembled monolayers (SAMs) of optically active Co III complexes (( S )‐ 2 /( R )‐ 2 ) that contain ( S )‐ or ( R )‐phenylalanine derivatives as a molecular recognition site were constructed on Au electrodes (( S )‐ 2 –Au/( R )‐ 2 –Au). Molecular recognition characteristics induced by the S and R configurations were investigated by measurements of electron‐transfer reactions with horse heart cytochrome  c (cyt  c ). The electrochemical studies indicate that the maximum current of cyt  c reduction is obtained when the Au electrode is modified by 2 with a moderate coverage of approximately 4.0×10 −11  mol cm −2 . Since the Au electrode is not densely packed with the Co III units at this concentration, we conclude that the penetrative association process between cyt  c and the Co III unit plays an important role in this electron‐transfer system. The differences in the electron‐transfer rates of ( S )‐ 2 –Au and ( R )‐ 2 –Au increase with increasing scan rates, a result indicating that the chiral ligand has an influence on the rate of association of the complexes with cyt  c . 3 –Au has a mixed monolayer composed of 2 and hexanethiol and exhibits electron‐transfer behavior comparable to 2 –Au. The difference in the association rates of ( S )‐ 3 –Au and ( R )‐ 3 –Au is larger than that between ( S )‐ 2 –Au and ( R )‐ 2 –Au, which indicates that the molecular recognition ability of 3 –Au has been enhanced by filling the gap between molecules of 2 with hexanethiols. The differences in the oxidation rates of cyt  c II between ( S )‐ 2 –Au and ( R )‐ 2 –Au and between ( S )‐ 3 –Au and ( R )‐ 3 –Au were larger than the differences in the rates of the reduction of cyt c III ; this suggests that the size of the heme crevice varies according to the oxidation state of cyt  c .