Electrochemical and computational studies on intramolecular dissociative electron transfer in beta-peptides

The preparation of the β-peptides PCB-β3Val- β3Ala-β3Leu-NHC(CH3) 2OOtBu and PCB-(β3Val-β3Ala- β3Leu)2-NHC(CH3)2OOtBu, with a specific donor and acceptor at each terminus, is described. Circular dichroism, 2D NMR, and density functional theory calculations confirmed that PCB-(β3Val-β3Ala-β3Leu) 2-NHC(CH3)2OOtBu adopts a 14-helix conformation, whereas PCB-β3Val-β3Ala- β3Leu-NHC(CH3)2OOtBu has an ill-defined secondary structure. The electron-transfer rate constants in the two peptides were found to be 2580 and 9.8 s-1 respectively. Computational simulations based on Marcus theory coupled to constrained density functional theory provide clear theoretical evidence that different electron-transport pathways occur in the two peptides due to their different conformations: sequential hopping within PCB-(β3Val-β3Ala- β3Leu)2-NHC(CH3)2OOtBu and superexchange within PCB-β3Val-β3Ala- β3Leu-NHC(CH3)2OOtBu. Electron population analysis provides the first clear theoretical evidence that amide groups can act as hopping sites in long-range electron transfer. © 2012 American Chemical Society.Jingxian Yu, David M. Huang, Joe G. Shapter, and Andrew D. Abel