Molecular models of three ω-3 fatty acids based on NMR and DFT calculations of 1H NMR chemical shifts

Structures of low-energy conformers of alpha-linolenic acid (ALA;18:3-cis-9,12,15 omega-3), eicosapentaenoic acid (EPA;20:5-cis 5,8,11,14,17 omega-3) and docosahexaenoic acid (DHA;22:6-cis 4,7,10,13,16,19 omega-3) in the liquid state, based on NMR and density functional theory calculations of H-1 NMR chemical shifts, are provided. A critical discussion of the J. Mol. Liq. 314 (2020) 113,376 article is also presented on a number of issues related to: (i) the method of assignment of H-1 and C-13 chemical shifts of ALA, EPA and DHA; (ii) attribution of NMR signals of impurities to those of EPA and (iii) the conclusion of an unusually strong electron C-13 and H-1 shielding of =HC-(CH2) under bar -CH= protons and carbons, due to an asymmetry in a twist between the two ends, which results in torque at the methylene site near the geometric center. The present data demonstrate that the folding effects of the chain as a function of the fragment number (CH=CH-CH2)(n), were n = 3, 5 and 6 for ALA, EPA and DHA, do not result in unusually strong electron H-1 and C-13 shielding. (C) 2021 Elsevier B.V. All rights reserved.