THE ROLE OF THE MEDIUM IN LONG-RANGE ELECTRON-TRANSFER

The role of the polypeptide matrix in electron transfer processes in p roteins has been studied in two distinct systems: first in a protein w here the induced ET is artificial, and second as part of the catalytic cycle of an enzyme. Azurins are structurally well-characterized blue single-copper proteins consisting of a rigid beta-sheet polypeptide ma trix. We have determined rate constants and activation parameters for intramolecular long-range electron transfer between the disulfide radi cal anions (generated by pulse radiolysis) and the copper(II) centre a s a function of driving force and nature of the intervening medium in a large number of wild-type and single-site-mutated proteins. In ascor bate oxidase, for which the three-dimensional structure is equally wel l characterized, the internal ET from the type-I Cu(I) to the trinucle ar Cu(II) centre has been studied. We find that the results correlate well with distance through well-defined pathways using a through-bond electron tunnelling mechanism.