The inhibitory effect of water on the Co²⁺ and Cu²⁺ catalyzed decomposition of methyl linoleate hydroperoxides

Abstract

The inhibitory effect of water on the decomposition of methyl linoleate hydroperoxides (MLHP) catalyzed by Co2+ and Cu2+ was studied in a model system using proton nuclear magnetic resonance (NMR) spectroscopy. MLHP were prepared by photoxidation and purified by chromatographic methods. Proton NMR spectroscopy was used to measure reaction rates by monitoring changes in the intensity of the OOH signal. The rate constant of the reaction was obtained by plotting the natural logarithm of MLHP concentration vs time. In the first part of the study, no transition metals were added to the model system, so that the effect of water could be attributed to the interaction between water and MLHP only. The rate constant of the reaction (K) was found inversely proportional to the concentration of water. There was a downfield chemical shift of both hydroperoxide and water peaks in the NMR spectra when water was added. As temperature increased to 40 degrees C, the difference in K between the systems with 0% and 2% water disappeared. It is proposed that the hydroperoxides were solvated with water which retarded their decomposition. When Co2+ was added to the model system, K decreased as the concentration of water increased from 0% to 1.5%. As temperature increased from 18 degrees C to 40 degrees C, differences between the K for 0% and 2% water disappeared. A similar phenomenon was observed in reactions catalyzed with Cu2+. These findings would support a mechanism in which the protective effect of water involves both the solvation of OOH and hydration of the metal catalyst.

Hydroperoxide formation during autoxidation of conjugated linoleic acid methyl ester

“…In deuterochloroform these appear as singlets at δ 8.05 for the 9-hydroperoxy isomer and at δ 7.97 for the 13-hydroperoxy isomer. The value for the latter is reported in the literature as δ 10.40 [14]. The latter value agrees with that reported by Dussault et al [13].…”

Impact of Surface Active Compounds on Iron Catalyzed Oxidation of Methyl Linolenate in AOT–Water–Hexadecane Systems

“…Incorporation of additional water could lead to the formation of water with different physical properties, as it would not be as strongly associated with the surface polar head groups. As water can interact with lipid hydroperoxides and inhibit their decomposition, 40 it is possible that the physical properties of the water in the reverse micelles could impact lipid oxidation rates.…”

Hydroperoxide formation during autoxidation of conjugated linoleic acid methyl ester

“…In deuteroacetone the hydroperoxide proton signals appeared at δ 10.47 for the 13-hydroperoxy isomer and at δ 10.46 for the 9-hydroperoxide isomer. The value for the latter is reported in the literature as δ 10.40 [14].…”