NMR and Computational Studies as Analytical and High-Resolution Structural Tool for Complex Hydroperoxides and Diastereomeric Endo-Hydroperoxides of Fatty Acids in Solution-Exemplified by Methyl Linolenate

Abstract

A combination of selective 1D Total Correlation Spectroscopy (TOCSY) and 1H-13C Heteronuclear Multiple Bond Correlation (HMBC) NMR techniques has been employed for the identification of methyl linolenate primary oxidation products without the need for laborious isolation of the individual compounds. Complex hydroperoxides and diastereomeric endo-hydroperoxides were identified and quantified. Strongly deshielded C–O–O–H 1H-NMR resonances of diastereomeric endo-hydroperoxides in the region of 8.8 to 9.6 ppm were shown to be due to intramolecular hydrogen bonding interactions of the hydroperoxide proton with an oxygen atom of the five-member endo-peroxide ring. These strongly deshielded resonances were utilized as a new method to derive, for the first time, three-dimensional structures with an assignment of pairs of diastereomers in solution with the combined use of 1H-NMR chemical shifts, Density Functional Theory (DFT), and Our N-layered Integrated molecular Orbital and molecular Mechanics (ONIOM) calculations.

Towards Elucidating Structure–Spectra Relationships in Rhamnogalacturonan II: Computational Protocols for Accurate 13C and 1H Shifts for Apiose and Its Borate Esters

“…Additionally, while the literature suggested that very large basis sets are needed to obtain accurate NMR predictions ( Taubert et al, 2005 ), we did not find this to be the case for our system as our top-performing NMR FBS combinations did not have particularly large basis sets. These findings, in agreement with similar observations by Kupka et al (2010) and Ahmed et al (2020) , highlighted the fact that generalized rules of using large basis sets do not inherently lead to accurate chemical shift predictions and thereby necessitate the evaluation of the efficacy of each combination.…”

“…As a result, these methods are often unable to distinguish between diastereomers or account for interspatial coupling ( Willoughby et al, 2014 ; Benassi, 2017 ), greatly hindering the elucidation of stereochemistry within a polysaccharide. On the other hand, quantum mechanical (QM) approaches to predicting NMR shifts have been more successful in aiding experimentalists to improve the reliability of NMR and correlate the spectra to the structure by predicting the chemical shift values of complex compounds ( Pierens, 2014 ; Tarazona et al, 2018 ; Ahmed et al, 2020 ). This approach has even been used to correct previously misassigned spectra that relied on experimental databases and empirical-based predictions ( Pierens, 2014 ).…”

Analytical and Structural Tools of Lipid Hydroperoxides: Present State and Future Perspectives

“…Oxidation of methyl linolenate in atmospheric oxygen in a glass vial for 48 h at 40 °C resulted in a significant number of sharp (Δν 1/2 ≤ 2.0 Hz) of the hydroperoxide protons in the region of 7.7 to 9.6 ppm [ 138 ] ( Figure 42 ). Addition of 1 to 2 microdrops of D 2 O resulted in the elimination of several of these signals, including those in the strongly deshielded region of the aldehyde protons ( Figure 43 ).…”

Analysis and Authentication of Avocado Oil Using High Resolution NMR Spectroscopy

“…it was also attributed to an oxidation product. The broad signals at δ 7.88 and 7.79 belong to OO H protons of hydroperoxides such as 16-OOH and 9-OOH; however, these chemical shifts are sensitive to hydrogen bonds and other sample conditions [ 21 ]; thus, comparisons with other samples and literature should be done with caution. TOCSY also shows the connections between the glyceridic protons of 1,2-DG, which share a common coupling pathway.…”