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Using Au(111) and Au(100) single-crystal electrodes modified with self-assembled monolayers (SAMs), the direct electron transfer reaction of bilirubin oxidase (BOD) adsorbed onto their surfaces was investigated. The BOD adsorbed onto the Au(111), Au(100) and gold /mica electrodes, and the BOD adsorbed onto Au(111) electrodes modified with C(3)-SO(3)H and C(n)-COOH (n = 2, 5 and 7), showed the electrocatalytic currents of dioxygen reduction based on the direct electron transfer reaction. The BOD adsorbed onto Au(111) electrodes modified with C(6)-NH(2), C(6)-OH and C(5)-CH(3) did not show any electrocatalytic current. Negatively charged electrode surfaces can give a suitable molecular orientation for the direct electron transfer of BOD. The k degrees values evaluated by an analysis of the steady-state voltammogram with a simulated fitting method did not depend on the crystal structure of the gold electrode surface. Using a C(n)-COOH (n = 2, 5, 7) modified Au(111) electrode, the k degrees values decreased with an increasing alkyl chain length of C(n)-COOH. Based on the k degrees values obtained from the C(n)-COOH (n = 2, 5, 7) modified Au(111) electrodes, the electron tunneling distance was evaluated. The average distance between the type 1 Cu site of BOD and the outside of the BOD protein structure was evaluated to be 17 (+/-2) A.

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