Conformation control through concurrent N–H⋯S and N–H⋯O=C hydrogen bonding and hyperconjugation effects
Résumé
In addition to the classical N-H/O]C 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 a-amino acid. 6-Membered ring inter-residue N-H/S interactions (C6 g), assisted by hyperconjugation between the thietane ring and the backbone, combine with 5-membered ring intra-residue backbone N-H/O]C interactions (C5) to provide a C5-C6 g feature that stabilizes a planar geometry in the monomer unit. Two contiguous C5-C6 g features in the planar dimer implicate an unprecedented three-centre H-bond of the type C]O/H(N)/SR 2 , while the trimer adopts two C5-C6 g features separated by a Ramachandran a-type backbone configuration. These low-energy conformers are fully characterized in the gas phase and support is presented for their existence in solution state.
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