ELECTROCHEMICAL STUDIES OF CYANOMETMYOGLOBIN AND METMYOGLOBIN - IMPLICATIONS FOR LONG-RANGE ELECTRON-TRANSFER IN PROTEINS

The electron-transfer reactions of high-spin metmyoglobin and low-spin cyanometmyoglobin have been studied electrochemically. The striking differences seen for the reactions of these complexes are apparent when the solutions studied contain both metmyoglobin and cyanometmyoglobin. The redox potential for cyanometmyoglobin has shifted negatively over 400 mV, from +0.046 to -0.385 (+/-0.015) V vs NHE, and the rate of electron transfer has increased by more than 1 order of magnitude compared with that for metmyoglobin, from 7 x 10(-6) to 5.4 (+/-0.9) x 10(-4) cm s-1. Only subtle effects are observed when the high-spin complexing ligand is fluoride. The reorganizational energy accompanying spin-state change of the heme iron is believed to be the primary factor controlling the rates of electron-transfer reactions described here. As expected, results from cyclic voltammetric experiments show that when cyanometmyoglobin undergoes electron transfer, Mb(III)CN- and Mb(II)CN- are the oxidized and reduced forms involved. Surprisingly, when metmyoglobin undergoes electron transfer, it also appears that it is the six-coordinate Mb(III)H2O and Mb(II)H2O that undergo electron-transfer reactions and that dissociation of water from Mb(II)H2O is unexpectedly slow, k = 1.0 (+/-0.5) s-1, and produces an electroinactive five-coordinate Mb(II) under these experimental conditions. Support for the conclusions given above comes from the comparison of experimental and calculated cyclic voltammograms.