A variable temperature 1H NMR and DFT study of procyanidin B2 conformational interchange
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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.
KeywordsProcyanidin B2 DFT Conformational isomerism 1H NMR
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
All ethical guidelines have been adhered to.
Conflict of interest
The authors declare that they have no conflict of interest.
- 11.Foo LY, Porter LJ (1983) Synthesis and conformation of procyanidin diastereoisomers. J Chem Soc Perkin Trans. https://doi.org/10.1039/p19830001535
- 12.Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF, Bloino J, Zheng G, Sonnenber DJ (2009) Gaussian 09. Gaussian, Inc, Wallingford, pp 2–3Google Scholar
- 14.Grimme S, Antony J, Ehrlich S, Krieg H (2010) A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys. https://doi.org/10.1063/1.3382344
- 17.Kalili KM, de Villiers A (2013) Systematic optimisation and evaluation of on-line, off-line and stopflow comprehensive hydrophilic interaction chromatography x reversed phase liquid chromatographic analysis of procyanidins. Part II: application to cocoa procyanidins. J Chromatogr A 1289:69–79CrossRefPubMedGoogle Scholar
- 21.Mathews JH, Fink KK (2004) Numerical methods using Matlab. Pearson, New JearseyGoogle Scholar
- 22.McConnell HM (1958) Reaction rates by nuclear magnetic resonance. J Chem Phys. https://doi.org/10.1063/1.1744152
- 25.Pianet I, Andre Y, Ducasse MA, Tarascou I, Lartigue JC, Pinaud N, Fouquet E, Dufourc EJ, Laguerre M (2008) Modeling procyanidin self-association processes and understanding their micellar organization: a study by diffusion NMR and molecular mechanics. Langmuir 24(19):11027–11035CrossRefPubMedGoogle Scholar
- 31.Tarascou I, Barathieu K, Simon C, Ducasse MA, Andre Y, Fouquet E, Dufourc EJ, de Freitas V, Laguerre M, Pianet I (2006) A 3D structural and conformational study of procyanidin dimers in water and hydro-alcoholic media as viewed by NMR and molecular modeling. Magn Reson Chem 44(9):868–880CrossRefPubMedGoogle Scholar
- 34.Zhang Y, Kong L, Yin C, Jiang D, Jiang J, He J, Xiao W (2013) Extraction optimization by response surface methodology, purification and principal antioxidant metabolites of red pigments extracted from bayberry (Myrica rubra) pomace. J Food Sci Technol 51(1):343–347Google Scholar