CONFORMATIONAL DEPENDENCE OF ELECTRON-TRANSFER ACROSS DE-NOVO DESIGNED METALLOPROTEINS

Flash photolysis and pulse radiolysis measurements demonstrate a confo rmational dependence of electron transfer rates across a 16-mer helica l bundle (three-helix metalloprotein) modified with a capping Co-III(b ipyridine)(3) electron acceptor at the N terminus and a 1-ethyl-1'-eth yl-4,4'-bipyridinium donor at the C terminus. For the III(peptide)(3)- 1-ethyl-1'-ethyl-4,4'-bipyridinium maquettes, the observed transfer is a first order, intramolecular process, independent of peptide concent ration or laser pulse energy. In the presence of 6 M urea, the random coil bundle (approximate to 0% helicity) has an observed electron tran sfer rate constant of k(obs) = 900 +/- 100 s(-1). In the presence of 2 5% trifluoroethanol (TFE), the helicity of the peptide is 80% and the k(obs) increases to 2000 +/- 200 s(-1). Moreover, the increase in the rate constant in TFE is consistent with the observed decrease in donor -acceptor distance in this solvent, Such bifunctional systems provide a class of molecules for testing the effects of conformation on electr on transfer in proteins and peptides.