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Polyunsaturated fatty acid (PUFA) oxidation by thiyl radicals (RS • ) is believed to be responsible for some of the biological radiation damage. At the same time, RS • can cause isomerization of PUFA double bonds with the formation of trans isomers. The aim of this study was to better understand the competition between lipid peroxidation and geometrical isomerization processes in biomimetic model system of linoleic acid in the presence of 2-mercaptoethanol using irradiation as a method for free radicals generation. In air-equilibrated conditions the propagation of lipid peroxidation was dominant up to the dose of 400 Gy, after which at higher doses up to 10 kGy the termination occurred with the predominance of geometrical isomerization. This study revealed that undesirable and permanent lipid modifications are possible at higher irradiation doses which should be considered in the planning of irradiation treatment of foods and feeds with high content of lipids and sulfur compounds. [ABSTRACT FROM AUTHOR]

The biomimetic model of micelles of linoleic acid containing 2-mercaptoethanol and the antioxidant was examined under gamma irradiation up to 400 Gy in aerobic or deoxygenated conditions where thiyl radicals are the main reactive species. Lipid peroxidation was retarded by ascorbic acid and α-tocopherol, whereas this process was strongly inhibited by resveratrol as effectively as the ascorbic acid/α-tocopherol mixture. Furthermore, antioxidants have a much stronger inhibitory effect on the peroxidation in the presence of 2-mercaptoethanol, and at the same time show protective properties of the double bond, decreasing thecis–transisomerization. Under anaerobic conditions,cis–transisomerization occurred and antioxidants efficiency increased along the series: resveratrol < α-tocopherol < ascorbic acid. This result is explained taking into account the double bond localization in the hydrophobic core of the micelle and the need of co-localization of the antioxidant in order to get an anti-isomerizing activity and protection of the natural lipid geometry. [ABSTRACT FROM AUTHOR]

Thiols are well known for their role in cellular redox homeostasis, while aromatic amines and phenols are the best known classes of chain-breaking antioxidants. On the other hand, thiyl radicals are known to catalyse the double bond isomerization in PUFA. We investigated the role and interplay of 2-mercaptoethanol and diphenylamine in the parallel processes of peroxidation and cis–trans isomerization of linoleic acid (LA) during gamma radiolysis, both in solution and micelles. Both compounds, used alone were able to protect LA from oxidation; however pro-oxidant activity and enhanced isomerization was observed when they were used together, depending on the experimental settings. Instead, α-tocopherol protected LA from both oxidation and isomerization in the presence of thiols under any tested settings. The mechanistic scenario is discussed highlighting the role of diphenylaminyl radicals in promoting thiyl-radical-induced cis–trans isomerization in the presence of oxygen. [ABSTRACT FROM AUTHOR]

[Display omitted] • Diarylethene triazoles undergo E-Z photoisomerization via distinctly different pathways. • Elucidation of substituent dependent reaction mechanism. • Adiabatic singlet excited state reaction or triplet excited state driven by the nitro group. • Experiments are in an excellent agreement with quantum-chemical calculations. Photochemical reactivity in the Z-E isomerization for two heterostilbene derivatives containing 1,2,3-triazole unit were investigated theoretically and experimentally by irradiation experiments, fluorescence and laser flash photolysis (LFP). The molecules were designed to probe the effect of the para -nitro group in 1 on the photochemical E-Z pathways, as well as to investigate the steric effect of the ortho -methyl group in 2. The quantum yield for the Z → E isomerization for both cis -isomers is 0.42, and for the E → Z is somewhat lower 0.16 and 0.12, respectively. Furthermore, fluorescence measurements for the ortho -methyl derivative indicated that the Z → E isomerization takes place in an adiabatic reaction on the potential energy surface of the S 1 state. On the contrary, the para -nitro derivative undergoes the Z → E isomerization via a triplet excited state, which was detected by LFP. For both cis - and trans -isomers of the nitro derivative a transient was detected absorbing with a maximum at 520 nm, which was assigned to the triplet excited state of the trans -isomer. All experimental observations were corroborated by computations. The stationary points were computed at the PBE50/6-31++G** level of theory, whereas potential energy surfaces were obtained by linear interpolation and computations at the SF-TDDFT/PBE50/6-31++G** level of theory. The mechanistic investigation presented gives insight in the fundamental and simple Z → E isomerization and provides new findings which are important in the rational design of different photoreactive diarylethene derivatives used in different fields of science. [ABSTRACT FROM AUTHOR]