Singlet Oxygen Quenching by Resveratrol Derivatives.

We investigated the singlet oxygen quenching ability of several derivatives of trans-resveratrol which have been reported to have significant antioxidant ability, including photoprotective activity. We measured the total rate constants of singlet oxygen removal (kT ) by the methylated resveratrol derivative 1,3-dimethoxy-5-[(E)-2-(4-methoxyphenyl)ethenyl]benzene, and the partially methylated resveratrol derivatives 4-((E)-2-(3,5-dimethoxyphenyl)ethenyl)phenol (pterostilbene), 5-[(E)-2-(4-methoxyphenyl)ethenyl]benzene-1,3-diol and (2R,3R)-3,5,7-trihydroxy-2-(3,4,5-trihydroxyphenyl)-2,3-dihydrochromen-4-one (dihydromyricetin). A protic solvent system results in higher kT values, except for the completely methylated derivative. We also investigated the ability of trans-resveratrol to directly act as a photosensitizer (rather than via secondary photoproducts resulting from other primary photochemical reactions) for the production of singlet oxygen but found that neither resveratrol nor any of its derivatives are able to do so. We then studied the chemical reactions of the methylated derivative with singlet oxygen. The main pathway consists of a [4 + 2] cycloaddition reaction involving the trans-double bond and the para-substituted benzene ring similar to what has been observed for trans-resveratrol. Unlike trans-resveratrol, the primary singlet oxygen product undergoes a second [4 + 2] cycloaddition with singlet oxygen leading to the formation of diendoperoxides. A second reactivity pathway for both trans-resveratrol and the methylated derivative leads to the formation of aldehydes via cleavage of a transient dioxetane.

Singlet Oxygen Generation, Quenching and Reactivity with Metal Thiolates.

Metal thiolate complexes can act as photosensitizers for the generation of singlet oxygen, quenchers of singlet oxygen, and they may undergo chemical reactions with singlet oxygen leading to oxidized thiolate ligands. This review covers all of the chemical reactions of thiolate ligands with singlet oxygen (through early 2021). Since some of these reactions are self-sensitized photooxidations, singlet oxygen generation by metal complexes is also discussed. Mechanistic features such as the effects of protic vs. aprotic conditions are presented and compared with the comparatively well-understood photooxidation of organic sulfides. In general, the total rate of singlet oxygen removal correlates with the nucleophilicity of the thiolate ligand which in turn can be influenced by the metal. Some interesting patterns of reactivity have been noted as a result of this survey: Metal thiolate complexes bearing arylthiolate ligands appear to exclusively produce sulfinate (metal-bound sulfone) products upon reaction with singlet oxygen. In contrast, metal thiolate complexes bearing alkylthiolate ligands may produce sulfinate and/or sulfenate (metal-bound sulfoxide) products. Several mechanistic pathways have been proposed for these reactions, but the exact nature of any intermediates remains unknown at this time.

A Photoprotective Effect by Cation-π-Interaction? Quenching of Singlet Oxygen by an Indole Cation-π Model System.

We investigated the effect of the cation-π interaction on the susceptibility of a tryptophan model system toward interaction with singlet oxygen, that is, type II photooxidation. The model system consists of two indole units linked to a lariat crown ether to measure the total rate of removal of singlet oxygen by the indole units in the presence of sodium cations (i.e. indole units subject to a cation-π interaction) and in the absence of this interaction. We found that the cation-π interaction significantly decreases the total rate of removal of singlet oxygen (kT ) for the model system, that is, (kT  = 2.4 ± 0.2) × 108  m-1  s-1 without sodium cation vs (kT  = 6.9 ± 0.9) × 107  m-1  s-1 upon complexation of sodium cation to the crown ether. Furthermore, we found that the indole moieties undergo type I photooxidation processes with triplet excited methylene blue; this effect is also inhibited by the cation-π interaction. The chemical rate of reaction of the indole groups with singlet oxygen is also slower upon complexation of sodium cation in our model system, although we were unable to obtain an exact ratio due to differences of the chemical reaction rates of the two indole moieties.