Role of intramolecular hydrogen bonds in promoting electron flow through amino acid and oligopeptide conjugates

Abstract

Significance Long-range electron transfer pervades biology, chemistry, and engineering, as it is critical for life-sustaining processes, chemical transformations, energy conversion, as well as electronic and photonic technologies. Elucidating the factors that control the rates of long-range electron transfer remains an outstanding challenge, owing in part to the complexity of proteins and other macromolecular structures that mediate such processes. We have found that short peptides linking electron donors and acceptors can assume folds with intramolecular hydrogen bond interactions that provide electronic-coupling pathways for ultrafast charge transfer. Our work will assist designs of donor–acceptor systems for efficient energy conversion and storage. Elucidating the factors that control charge transfer rates in relatively flexible conjugates is of importance for understanding energy flows in biology as well as assisting the design and construction of electronic devices. Here, we report ultrafast electron transfer (ET) and hole transfer (HT) between a corrole (Cor) donor linked to a perylene-diimide (PDI) acceptor by a tetrameric alanine (Ala)4. Selective photoexcitation of the donor and acceptor triggers subpicosecond and picosecond ET and HT. Replacement of the (Ala)4 linker with either a single alanine or phenylalanine does not substantially affect the ET and HT kinetics. We infer that electronic coupling in these reactions is not mediated by tetrapeptide backbone nor by direct donor–acceptor interactions. Employing a combination of NMR, circular dichroism, and computational studies, we show that intramolecular hydrogen bonding brings the donor and the acceptor into proximity in a “scorpion-shaped” molecular architecture, thereby accounting for the unusually high ET and HT rates. Photoinduced charge transfer relies on a (Cor)NH…O=C–NH…O=C(PDI) electronic-coupling pathway involving two pivotal hydrogen bonds and a central amide group as a mediator. Our work provides guidelines for construction of effective donor–acceptor assemblies linked by long flexible bridges as well as insights into structural motifs for mediating ET and HT in proteins.