A. Characterization of PUFA-rich Linseed oil (LSO) and Fish oil (FO) susceptibility to thermal autoxidation using 1H LF-NMR energy relaxation Time Domain (TD) graphical peak fingerprint material analysis.
To understand the LF 1H NMR analysis of emulsions we first characterized the pure linseed and fish oil. Linseed oil and fish oil are both PUFA-rich materials. Linseed oil is composed mainly of linolenic acid (18:3, n-3; ALA), linoleic acid (18:2, n-6; LA) and oleic acid (18:1, n-9; OA). Fish oil is rich in the omega-3 long chain PUFAs docosahexaenoic acid (22:6, n-3; DHA) and eicosapentaenoic acid (20:5, n3; EPA) (Azuma et al., 2009). PUFAs have biochemical and physiological effects on human health enhancement, but the presence of multiple double bonds and bisallylic carbons affects their oxidative susceptibility, and generation of toxic oxidative products, directly proportional to their degree of unsaturation (Miyashita et al., 1993; 2018).
In the liquid oil phase, fatty acids are self-associated as hydrogen-bonded head to head dimers that are further aggregate as quasi-liquid crystal clusters alternating head to tail arrangement. The cluster aggregates determine the fatty acids liquid properties (Iwahashi and Kasahara, 2011; Berman et al., 2015; Resende et al., 2019b). The dimer fatty acid assembly is characterized by four different segmental motions. The ionic head to head groups are the most rigid microstructure, followed by the double bonds, then aliphatic chain, and the tail chain is the most mobile segment. In the triacylglycerol(TAG) structure, a glycerol bridges 3 FA chains, and is also characterized by four different segmental motions (Resende et al., 2019a)
The 1H LF-NMR energy relaxation TD graphical peak fingerprinting analysis of LSO and FO are shown on Fig. 1 LSO and FO TAG structure is characterized by four different rigidity-mobility segments and each one of these segments have characteristic T1(spin-lattice) and T2 (spin-spin) energy relaxation times. Analysis of the one-dimensional T1 and T2 spectra of LSO and FO, shows the presence of fours characteristic peaks. From previous studies (Berman et al., 2015; Resende et al., 2019b), peak 1 is assigned to glycerol since the most rigid microstructure and therefore corresponds to the lowest energy relaxation times. Peak 2 is assigned as double bonds as the second most rigid microstructure. Peak 3 is assigned to the aliphatic chains that have less rigidity and peak 4 is assigned as the tail chain as the most mobile segment, corresponding to the longest energy relaxation time.
Thus as described in the above paragraph and shown in Fig. 1 and Table 1 the basic molecular segment pattern analysis by 1H LF-NMR is based on the glycerol segment as the most rigid moiety, followed by the double bonds of the FA chains, the aliphatic chains and aliphatic tails, respectively. In the cases of linseed oil (LSO) and fish oil (FO) of the present study a similar T1-T2 TD (59-55 ms) was assigned to the triacylglycerols head groups of the two oils. Additional three relatively rigid TDs with low mobility peaks were assigned to double bonds (mono-, di- and polyunsaturated fatty acids) with increasing mobility for the two oils (Table 1), in agreement with previous reports (Berman et al., 2015, Meiri at., al 2015). Two peaks assigned to the long and short aliphatic chains were also assigned as well as two peaks assigned to the longer tails of omega-6 and short tail of omega-3 were identified for the two oils.