Sequential Proton Coupled Electron Transfer (PCET)

Dynamics Observed over 8 Orders of Magnitude in Time

Charge transfer mechanisms lay at the heart of chemistry and biochemistry. Proton coupled electron transfers (PCET) are central in biological processes such as photosynthesis and in the respiratory chain, where they mediate long-range charge transfers. These mechanisms are normally difficult to harness experimentally due to the intrinsic complexity of the associated biological systems. Metal-peptide cations experience both electron and proton transfers upon photoexcitation, proving an amenable model system to study PCET.

The authors report on a time-resolved experiment designed to follow this dual charge transfer kinetics in [HG3W+Ag]+ (H = histidine, G = glycine, W = tryptophan) on time scales ranging from femtoseconds to milliseconds. While electron transfer completes in less than 4 ps, it triggers a proton transfer lasting over hundreds of microseconds. Molecular dynamics simulations conducted in the group of Markus Meuwly show that the rate of formation of a PT-reactive structure (H-bond between indoleNH on tryptophan and imidazoleN on histidine) strongly depends on the initial peptide structure. In particular, while this rate is in the nanosecond range when starting from an extended conformation, it is considerably longer when starting from compact peptide conformations as in the initial metal complex. In other words, molecular dynamics simulations show that conformational dynamic plays an important role in slowing down this reaction. The experimental study covers 8 orders of magnitude in time and shows that the 4 ps electron transfer induces a proton transfer hundreds of microseconds later. The combined experimental and computational approach provides a view of PCET as a single phenomenon despite its very wide time-domain span. Reference: MacAleese, L., S. Hermelin, K. El Hage, P. Chouzenoux, A. Kulesza, R. Antoine, L. Bonacina, M. Meuwly, J.-P. Wolf and P. Dugourd (2016). Sequential Proton Coupled Electron Transfer (PCET): Dynamics Observed over 8 Orders of Magnitude in Time. J. Am. Chem. Soc. (10.1021/jacs.5b12587) MacAleese-2016 (807 KB).