Conformational Dependence of Electron Transfer across de novo Designed Metalloproteins

Flash photolysis and pulse radiolysis measurements demonstrate a conformational dependence of electron transfer rates across a 16-mer helical bundle (three-helix metalloprotein) modified with a capping CoIII(bipyridine)3 electron acceptor at the N terminus and a 1-ethyl-1′-ethyl-4,4′-bip yridinium donor at the C terminus. For the CoIII(peptide)3-1-ethyl-1$^{\prime}$ -ethyl-4,4′-bipyridinium maquettes, the observed transfer is a first order, intramolecular process, independent of peptide concentration or laser pulse energy. In the presence of 6 M urea, the random coil bundle (≈ 0% helicity) has an observed electron transfer rate constant of kobs = 900 ± 100 s-1. In the presence of 25% trifluoroethanol (TFE), the helicity of the peptide is 80% and the kobs 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 electron transfer in proteins and peptides.