Electrochemistry of surface wired redox protein: Axial ligation and control of redox potential

Electrochemical characteristics of surface wired heme protein, microperoxidase-11 (MP-11) and the influence of the axial binding of ligand on the redox potential are described. MP-11 was covalently wired on a carboxylic acid terminated self-assembled monolayer (SAM) preassembled on a polycrystalline Au electrode. Well-defined reversible voltammetric response was obtained at −0.43 V for the surface wired MP-11 in phosphate buffer solution (PBS). The effect of axial ligation on the redox properties of surface wired MP-11 has been investigated using amino acids and various N-donor ligands. These ligands replace water molecule axially coordinated to the heme iron and control the formal potential (E0′E0′) of surface confined MP-11. The axial ligands control the E0′E0′ of MP-11 and the E0′E0′ depends on the nature of axial ligand. Histidine, methionine, and pyridines induce a positive shift in the E0′E0′ whereas tryptophan and imidazole cause negative shift in the E0′E0′. Positive shift in the redox potential implies that the Fe(II) bind these ligands more firmly than Fe(III). The negative shift indicates that the Fe(III) state is stabilized over Fe(II) state. The heterogeneous electron transfer rate constant for the surface wired MP-11 is significantly enhanced upon axial coordination of histidine and 4-N,N′-dimethylaminopyridine (DMAP). This ligand responsive redox behavior demonstrates that the surface wired MP-11 can be successfully used as redox-receptor for recognizing target ligands. The surface wired redox receptor shows many advantages over the solution based receptor. The axial coordination of exogeneous ligands were further confirmed by spectral measurements in homogeneous solution.