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Structural Chemistry

, Volume 29, Issue 5, pp 1551–1564 | Cite as

A variable temperature 1H NMR and DFT study of procyanidin B2 conformational interchange

  • S. J. O’Kennedy
  • A. de Villiers
  • D. J. Brand
  • W. J. Gerber
Original Research

Abstract

Two procyanidin B2 conformers were identified in a relative abundance ratio of approximately 3:1 at 298 K by 1H NMR experiments in acetonitrile. The conformational interchange reactions between these two conformers are 1st order in both reactions, with ∆G for forward and reverse of 57.12 ± 5.62 and 54.56 ± 5.48 kJ mol−1, respectively. The experimentally obtained standard thermodynamic energies for this reaction are ΔH0rxn (3.67 ± 0.22 kJ mol−1), ΔS0rxn (4.05 ± 1.57 kJ mol−1 K−1), and ΔH0rxn (2.96 ± 0.33 kJ.mol−1). Conformational search results at the DFT (PBE, PBE-D2, and B3LYP with 6-311++g**) level of theory yielded four novel conformations, named fully compact (FC), partially compact (PC), partially extended (PE), and fully extended (FE). Although the FC conformer is electronically the most stable of the four as a result of extensive intramolecular non-covalent interactions, the PC and FE conformers are thermodynamically favored in a 5:1 ratio (B3LYP), with the FC and PE conformers present in negligible amounts at equilibrium. The DFT computed standard reaction energies using the B3LYP functional for the PCmajor to FEminor conformational interchange reaction compare exceptionally well with experimental data at 298 K: ∆G0rxn (3.86 kJ mol−1), ΔH0rxn (5.34 kJ mol−1), and ∆S0rxn (4.97 kJ mol−1 K−1). It was found that inclusion of solvation energies is crucial to obtain accurate thermodynamic energies. The secondary equilibria found in chromatographic separations are predicted to be highly dependent on solvent polarity and temperature. Similar conformational diversity and hierarchies should exist for all non-rigid procyanidin oligomers and the unique chromatographic behavior of these compounds may be a result of conformational interchange.

Keywords

Procyanidin B2 DFT Conformational isomerism 1H NMR 

Notes

Funding information

The authors received financial support from SASOL (Collaborative grant to AdV), and the National Research Foundation (NRF, grant 98897 to AdV).

Compliance with ethical standards

Ethical statement

All ethical guidelines have been adhered to.

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

11224_2018_1153_MOESM1_ESM.docx (1.4 mb)
ESM 1 (DOCX 1416 kb)

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Chemistry and Polymer ScienceStellenbosch UniversityStellenboschSouth Africa
  2. 2.Central Analytical FacilityStellenbosch UniversityStellenboschSouth Africa

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