> π* Interaction in Designed Peptides

Abstract

An n‐‐>π* interaction is a non‐covalent interaction that orders two consecutive residues in a peptide/protein by delocalizing the lone pair of electrons from the oxygen of a donor carbonyl to the antibonding orbital of an acceptor carbonyl carbon. This interaction gives insight into the inherent biases that drive protein folding, as it stabilizes secondary structures such as α‐helices and polyproline helices.We synthesized a series of hydroxyproline‐based peptides with variable N‐terminal substituents, modulating the electron‐donating capabilities on the donor carbonyl. By changing the electronics of each donor carbonyl we can tune the strength of this n‐‐>π* interaction and explore its structural implications. The n‐‐>π* interaction stabilizes the trans conformation of a peptide bond. Because proline can readily populate both trans and cis conformations, the ratio of the two species (Ktrans/cis) indicates the strength of the n‐‐>π* interaction.The series of derivatives synthesized were selected to compare the electron‐donating capabilities of their donor carbonyl to its n‐‐>π* interaction strength. The derivatives synthesized for analysis were, X‐Hyp(4‐NO2Bz)‐OMe where X = Boc, pivaloyl, formyl, acetyl, propionyl, monochloroacetyl, monofluoroacetyl, dichloroacetyl, difluoroacetyl, trichloroacetyl and trifluoroacetyl. These derivatives provide a range of electronic properties to be analyzed for the strength of the nàπ* interaction in each compound.Combined analysis via 1H and 13C NMR and X‐ray crystallography reveals the characteristics of the n‐‐>π* interaction. Cis and trans species can be differentiated by 1H NMR, providing the Ktrans/cis for each derivative. Electron donation involved in the n‐‐>π* interaction is seen via the carbonyl chemical shifts using 13C NMR. The chemical shift of the acceptor carbonyl moves downfield with stronger electron‐donating substituents, while carbonyls uninvolved in the interaction remain constant. X‐ray crystallography reveals the distances and angles of the n‐‐>π* interaction, including torsion angles seen in protein secondary structures. Specifically, the propionyl donor carbonyl results in the crystallographic observation of an α‐helical conformation, inducing compact torsion angles (φ = −58.6° and ψ = −37.8°), without the presence of a hydrogen bond. An electron rich donor results in the strongest interaction. Our comparison of this series of donor‐acceptor carbonyl pairs reveals the energetics and geometry of the n‐‐>π* interaction, granting new and unique insights into the problem of protein folding.Support or Funding InformationI'd like to extend my gratitude towards David Plastino and the NSF for providing funding for this research.