Structural Studies of Monounsaturated and ω-3 Polyunsaturated Free Fatty Acids in Solution with the Combined Use οf NMR and DFT Calculations-Comparison with the Liquid State

Abstract

Molecular structures, in chloroform and DMSO solution, of the free fatty acids (FFAs) caproleic acid, oleic acid, α-linolenic acid, eicosapentanoic acid (EPA) and docosahexaenoic acid (DHA) are reported with the combined use of NMR and DFT calculations. Variable temperature and concentration chemical shifts of the COOH protons, transient 1D NOE experiments and DFT calculations demonstrate the major contribution of low molecular weight aggregates of dimerized fatty acids through intermolecular hydrogen bond interactions of the carboxylic groups, with parallel and antiparallel interdigitated structures even at the low concentration of 20 mM in CDCl₃. For the dimeric DHA, a structural model of an intermolecular hydrogen bond through carboxylic groups and an intermolecular hydrogen bond between the carboxylic group of one molecule and the ω-3 double bond of a second molecule is shown to play a role. In DMSO-d₆ solution, NMR and DFT studies show that the carboxylic groups form strong intermolecular hydrogen bond interactions with a single discrete solvation molecule of DMSO. These solvation species form parallel and antiparallel interdigitated structures of low molecular weight, as in chloroform solution. This structural motif, therefore, is an intrinsic property of the FFAs, which is not strongly affected by the length and degree of unsaturation of the chain and the hydrogen bond ability of the solvent.

Keywords: 1D 1H NOE; 1H NMR chemical shift; ALA; DFT; DHA; EPA.

Figure 1

The temperature dependence of the COOH ¹H NMR chemical shifts of caproleic acid (CA), oleic acid (OA), ALA, EPA and DHA in DMSO-d₆, c = 20 mM (A) and CDCl₃, c = 40 mM (B).

Figure 2

The 1D transient NOE NMR spectra of: (A) oleic acid (OA) and (B) α-linolenic acid (ALA), concentration = 20 mM in CDCl₃ solution (number of scans = 512, T = 298 K, T_acq = 4.09 s, relaxation delay = 4 s), using various τ_m values. The amplitude of the excited CH₃ group (denoted with the asterisk (*)) is reduced by a factor of 30 relative to the amplitude of the rest of the NOE signals.

Figure 3

The 1D transient NOE NMR spectra of: (A) EPA and (B) DHA, concentration = 20 mM in CDCl₃ at 298 K (number of scans = 512, T_acq = 4.09 s, relaxation delay = 4 s), using various τ_m values. The amplitude of the excited CH₃ group (denoted with the asterisk (*)) is reduced by a factor of 30 relative to the amplitude of the rest of the NOE signals.

Figure 4

The 1D transient NOE NMR spectra of: (A) oleic acid (OA) and (B) α-linolenic acid (ALA), concentration = 20 mM in DMSO-d₆ solution (number of scans = 512, T = 298 K, T_acq = 4.09 s, relaxation delay = 4 s), using various τ_m values. The amplitude of the excited CH₃ group (denoted with the asterisk (*)) is reduced by a factor of 30 relative to the amplitude of the rest of the NOE signals.

Figure 5

Optimized structures of caproleic acid: (a) dimeric structure forming OH^…OC centro-symmetric hydrogen bonds. (b) Cyclic trimeric structure and (c) linear trimeric structure in implicit solvation (IEFPCM chloroform).

Figure 6

Optimized structures of caproleic acid (CA) with a discrete solvation molecule of DMSO on the carboxylic group: single molecule of CA (a); dimeric structures of CA in parallel (b) and antiparallel configuration (c).