Conformational equilibria in selected A-type trimeric procyanidinsElectronic supplementary information (ESI) available: The 1H NMR spectra of 1-4 in four different solvents (MeOD, DMSO, (CD3)2OD and pyridine-d5) together with the signal assignments and Table 1SI with energies obtained for the lowest energy conformers of 1-4 are gathered in the ESI. In addition, a detailed description on the ΔG calculations from the temperature NMR spectra is included. See DOI: 10.1039/c4ob02086c

A-type procyanidin trimers cinnamtannin B-1, cinnamtannin D-1, lindetannin, and aesculitannin B were studied in terms of their conformation and interaction with four solvents: methanol, acetone, DMSO and pyridine. The experiments demonstrated that for each trimer there are two principal conformers observable in the NMR. The ratio of the conformers (rotamers) depends on the structure of a given trimer as well as on the solvent used for NMR measurements. The DFT calculations (B3LYP/6-31G(d,p)) proved the presence of two main conformers to be the result of a steric hindrance that prevents free rotation along the B-type interflavan bond. An analysis of the solvent-procyanidin interactions showed that the strong electron donating solvents, pyridine and DMSO, favor different conformers from methanol and acetone, which prefer the lowest-energy gas phase conformer. These findings are in line with predictions of DFT/M06-2X calculations with the inclusion of the thermal corrections. The variations in the rotamer ratios in the studied solvents correlate with the solvent's capacity to induce local changes in the electron density of the particular procyanidin trimer. There are two favored conformations of A-type procyanidin trimers: the (+ )-rotamer and the (− ) one. Their ratio depends on the procyanidin structure and solvent - different solvents favor different conformers.