Conformation control through concurrent N–H⋯S and N–H⋯OC hydrogen bonding and hyperconjugation effects

Abstract

In addition to the classical N–H⋯OC non-covalent interaction, less conventional types of hydrogen bonding, such as N–H⋯S, may play a key role in determining the molecular structure. In this work, using theoretical calculations in combination with spectroscopic analysis in both gas phase and solution phase, we demonstrate that both these H-bonding modes exist simultaneously in low-energy conformers of capped derivatives of Attc, a thietane α-amino acid. 6-Membered ring inter-residue N–H⋯S interactions (C6^γ), assisted by hyperconjugation between the thietane ring and the backbone, combine with 5-membered ring intra-residue backbone N–H⋯OC interactions (C5) to provide a C5–C6^γ feature that stabilizes a planar geometry in the monomer unit. Two contiguous C5–C6^γ features in the planar dimer implicate an unprecedented three-centre H-bond of the type CO⋯H(N)⋯SR₂, while the trimer adopts two C5–C6^γ features separated by a Ramachandran α-type backbone configuration. These low-energy conformers are fully characterized in the gas phase and support is presented for their existence in solution state.