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. 2020 Aug 11;11(34):9191-9197.
doi: 10.1039/d0sc03339a.

Conformation control through concurrent N-H⋯S and N-H⋯O[double bond, length as m-dash]C hydrogen bonding and hyperconjugation effects

Zeynab Imani  1 , Venkateswara Rao Mundlapati  2 , Gildas Goldsztejn  2 , Valérie Brenner  2 , Eric Gloaguen  2 , Régis Guillot  1 , Jean-Pierre Baltaze  1 , Katia Le Barbu-Debus  3 , Sylvie Robin  1   4 , Anne Zehnacker  3 , Michel Mons  2 , David J Aitken  1
Affiliations

Conformation control through concurrent N-H⋯S and N-H⋯O[double bond, length as m-dash]C hydrogen bonding and hyperconjugation effects

Zeynab Imani et al. Chem Sci. .

Abstract

In addition to the classical N-H⋯O[double bond, length as m-dash]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 α-amino acid. 6-Membered ring inter-residue N-H⋯S interactions (C6γ), assisted by hyperconjugation between the thietane ring and the backbone, combine with 5-membered ring intra-residue backbone N-H⋯O[double bond, length as m-dash]C interactions (C5) to provide a C5-C6γ feature that stabilizes a planar geometry in the monomer unit. Two contiguous C5-C6γ features in the planar dimer implicate an unprecedented three-centre H-bond of the type C[double bond, length as m-dash]O⋯H(N)⋯SR2, while the trimer adopts two C5-C6γ 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.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Background for this study.
Fig. 2
Fig. 2. Structures of the compounds 1–3 used in this study.
Fig. 3
Fig. 3. (left) Gas phase UV spectra of jet-cooled compounds 1–3. (centre) Gas phase IR spectra (black), obtained on UV bands A (1 and 2) or A′ (3), and solution phase spectra (red) of compounds 1–3, compared to the theoretical IR spectrum (stick) of the Cbz-cap trans rotamer of the lowest energy conformations. (right) Corresponding calculated lowest energy conformations of 1, 2, 3 (half-extended conformer) and 3 (extended conformer).
Fig. 4
Fig. 4. Significant NOESY correlations observed for compound 2 and their assignments.
Fig. 5
Fig. 5. X-ray crystal structures of 1–3 (left to right). Only one of the three molecules in the unit cell is shown for 2. Intramolecular C10 H-bonding interactions are shown for 2 and 3. Only NH hydrogen atoms are shown for clarity.
See this image and copyright information in PMC

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