Acid-induced structural modifications of unsaturated fatty acids and phenolic olive oil constituents by nitrite ions: A chemical assessment

The structural modifications of the unsatd. fatty acid components of triglycerides in extra virgin olive oil (EVOO) following exposure to nitrite ions in acidic media were detd. by two-dimensional (2D) NMR spectroscopy, aided by 15N labeling and GC anal., allowing investigation of the matrix without fractionation steps. In the presence of excess nitrite ions in a 1% sulfuric acid/oil biphasic system, extensive double bond isomerization of the oleic/linoleic acid components of triglycerides was obsd. assocd. with nitration/oxidn. processes. Structurally modified species were identified as E/Z-nitroalkene, 1,2-nitrohydroxy, and 3-nitro-1-alkene(1,5-diene) derivs. based on 1H, 13C, and 15N 2D NMR anal. in comparison with model compds. Minor constituents of EVOO, including phenolic compds. and tocopherols, were also substantially modified by nitrite-derived nitrating species, even under milder reaction conditions relevant to those occurring in the gastric compartments. Novel nitrated derivs. of tyrosol, hydroxytyrosol, and oleuropein (6-8) were identified by LC/MS anal. of the polar fraction of EVOO and by comparison with synthetic samples. Overall, these results provide the first systematic description at the chem. level of the consequences of exposing EVOO to nitrite ions at acidic pH and offer an improved basis for further investigations in the field of toxic nitrosation/nitration reactions and dietary antinitrosating agents.