Multidimensional Proton Nuclear Magnetic Resonance Relaxation Morphological and Chemical Spectrum Graphics for Monitoring and Characterization of Polyunsaturated Fatty‐Acid Oxidation

Abstract

Polyunsaturated fatty acids (PUFA) are components of many commercial products such as edible oils, foods, cosmetics, medication, and in biological systems such as phospholipids of cellular membranes. Although PUFA aggregates are important functional components, they are also related to system degradation, because PUFA are susceptible to oxidation via their multiple double bonds and allylic carbons. Current technologies are not effective in characterizing the morphological and chemical structural domains of saturated, monounsaturated fatty acids (MUFA) and PUFA materials, or how the morphological structures of fatty acids, at the mesomolecular, nanomolecular, and molecular levels, affect their oxidation mechanisms. In this article, the 1 H low-field (LF) NMR energy relaxation time technology is proposed as a tool to analyze PUFA oils undergoing thermal oxidation. This technology generates two-dimensional (2D) chemical and morphological spectra using a primal-dual interior method for the convex objectives (PDCO) optimization solver for computational processing of the energy relaxation time signals T 1 (spinlattice) and T 2 (spin-spin). The 2D graphical maps of T 1 vs. T 2 generated for butter, rapeseed oil, soybean oil, and linseed oil show that the different degrees of unsaturation of fatty-acid oils affect their chemical and morphological domains, which influences their oxidative propensity. The technology of the 1 H LF-NMR energy relaxation time proved to be an effective tool to characterize and monitor PUFA oxidation.

1H LF‐NMR Energy Relaxation Time Characterization of the Chemical and Morphological Structure of PUFA‐Rich Linseed Oil During Oxidation With and Without Antioxidants

“…To characterize antioxidant efficacy by 1 H LF‐NMR, it is important to understand the mechanism of oxidation and the chemical morphological characterization of the oils at different stages of oxidation. In our previous paper on the autoxidation of different fatty acid oils, we described in detail the correlation between the molecular structure of linseed oil and its liquid phase morphological chemical domains characterized by 1 H LF‐NMR Linseed oil is primarily PUFA linolenic acid (53.21%), oleic acid (18.51%), and linoleic acid (17.25%), that are self‐associated as hydrogen‐bonded dimers in their liquid phase, that further aggregate to form clusters possessing the structure of a quasi‐smectic liquid crystal, of an interdigitated alternating head to tail arrangement . The clusters aggregate formation is responsible for the fatty acid liquid properties, such as fluidity and viscosity …”

“…Autoxidation experimental design was based on previous studies . The samples were autoxidized by heating on an a 80 °C hot plate, 100 mL samples in a 250 mL beaker.…”

“…Before each measurement, the samples were stabilized at 40 °C for 40 min. The 2D cross‐correlation experiments were performed by a T 1 versus T 2 sequence, where the inversion recovery step [180°–t 1 ] is inserted prior to the Carr–Purcell–Meiboom–Gill (CPMG) sequence …”

“…This technology is non‐destructive and does not require the use of solvents or sample preparation . Furthermore, we recently reported that 1 H LF‐NMR may evaluate both the primary and the secondary oxidation products in the same graph as separate peaks . Thus, it is an effective tool that can provide global chemical and structural information relative to the degree of oxidation of the tested sample.…”

“…1 H LF‐NMR equipment is widely used in the food industry for its easily used, and of relatively low cost and compact size . For complex material analysis, however, the generated NMR energy relaxation time signals contain important structural information that for access requires a unique reconstruction into 2D and 3D graphics.…”