Electroanalytical chemistry of myoglobin with modification of distal histidine by cyanated imidazole

In order to understand the effect of the heme environment on the electron t ransfer reaction of Mb, modification of distal histidine (His-64, E7) of na tive sperm whale (SW) and horse heart (HH) myoglobin (Mb) molecules by cyan ated imidazole (CN-E7-Mb) was made by the reaction of native Mb with cyanog en bromide (BrCN), and its electrochemical behavior was investigated at an In2O3 electrode. The CN-E7-Mb showed a well-defined redox wave at an In2O3 electrode with a highly hydrophilic surface. The redox potentials of CN-E7- Mbs from SW and HI-I were - 0.035 and - 0.038 V (vs. Ag \ AgCl), respective ly, which are ca. 100 mV more positive than those of corresponding native M bs. This positive shift in redox potential is reasonable because no water m olecule is present at the sixth coordination site of CN-E7-Mb. More interes tingly, the heterogeneous electron transfer rate constants (k degrees') of CN-E7-Mb from SW and HH were 2.3 (+/- 0.2) x 10(-3) and ca. 2.5 (+/- 0.2) x 10(-3) cm s(-1), respectively, which are much faster than those of native Mbs (1.0 (+/- 0.1) x 10(-4) for Mb(SW) and 8 (+/- 0.5) x 10(-4) cm s(-1) fo r Mb(HH) under the best conditions). These values are large for Mb having a five-coordinated high-spin heme iron. CN-E7-Mb is an excellent example whi ch indicates that the rapid electron transfer takes place when less displac ement of the heme iron is required during the electron transfer and the six th ligand of the heme iron is not always necessary for the rapid electron t ransfer. The k degrees' values obtained for CN-E7-Mbs were independent of t he pH of the solution in the pH region tested (pH 5.5-7.5). Modification of distal histidine of Mbs by tetrazolyl imidazole was also carried out to gi ve the modified Mb (Tet-E7-Mb) having a six-coordinated low-spin heme iron. Tet-E7-Mb showed a reduction peak around -0.5 V (vs. Ag \ AgCl) and slower electron transfer kinetics compared with those of native Mb and CN-E7-Mb. Also, voltammograms for Tet-E7-Mb suggested the de-coordination of the tetr azolyl imidazole moiety from the heme iron during electrochemical reduction . (C) 1999 Elsevier Science S.A. All rights reserved.