Interfacial electron transfer kinetics of myoglobins chemically modified with succinic anhydride at an indium oxide electrode

We modified native horse heart myoglobin (Mb or Mb F1) with succinic anhydride to introduce negative charge(s) onto the surface of the protein, and obtained four kinds of Mbs in which the modification site (i.e. local charge) was different (Mb(F2)–Mb(F5)). The chemical modification significantly affected the electron transfer kinetics at an In2O3 electrode in a bis–tris buffer solution (pH 6.8). The heterogeneous electron transfer rate constant (k0′) of Mb decreased depending on the modification site for succinic anhydride. The calculated k0′ values were 1.8 (±0.3) × 10−4, 1.6 (±0.3) × 10−4, 5.5 (±0.5) × 10−5, 4.5 (±0.5) × 10−5 and 1.5 (±0.7) × 10−5 cm s−1 for native Mb (or Mb(F1)), Mb(F2), Mb(F3), Mb(F4) and Mb(F5), respectively, which showed a clear reciprocal relationship to the rate of photoinduced electron transfer reactions of Zn–Mbs(F2–F5) with methyl viologen. This effect was also confirmed using cyano-forms of modified Mbs. The present results indicate that the local charge of the Mb molecule is important for its electron transfer kinetics at an electrode surface.