Chemistry of nitrated lipids: remarkable instability of 9-nitrolinoleic acid in neutral aqueous medium and a novel nitronitrate ester product by concurrent autoxidation/nitric oxide-release pathways

Despite the mounting interest in nitrolinoleic acids and related nitrated polyunsaturated fatty acids as a novel class of bioactive signaling lipids, their chemistry and metabolic fate have remained poorly elucidated. Herein, we report an expedient nitroselenenylation/oxidation route to 9-nitrolinoleic acid (1) and 10-nitrolinoleic acid (2), which enabled comparative product studies under physiologically relevant conditions. Under biomimetic conditions, 1 decayed at an unusually fast rate to give the hydroxy-, keto-, and nitronitrate ester derivatives 3, 4, and 5 as main products, identified by ESI-MS and 2D NMR spectroscopy, including (1)H, (15)N HMBC experiments on the (15)N-labeled derivatives. The 13-nitrato functionality in 5 suggested partitioning of 1 between concurrent peroxidation and nitric oxide (NO)-release pathways. Lipid 2 decayed at a much slower rate giving only the hydroxynitro derivative 6 as an isolable product. Diphenylpicrylhydrazide (DPPH) radical quenching experiments and DFT computations concurred to support a higher H-atom donating ability of 1 versus 2, due to more effective stabilization of the resulting pentadienyl radical by the terminal nitro group. The markedly different stability of isomeric nitrolinoleic acids disclosed in the present study may provide an explanation for the previous identification of 2, but not 1, in body fluids and offers a key for future insights into the biological activities of nitrated lipids.