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

AbstractWe investigated the effect of the cation ‐π interaction on the susceptibility of a tryptophan model system towards interaction with singlet oxygen, i.e. 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 pr esence 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, i.e. (kT = 2.4 ± 0.2) × 108 M‐1sec‐1 without sodium cation vs. (kT = 6.9 ± 0.7) × 107 M‐1sec‐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.
Source: Photochemistry and Photobiology - Category: Science Authors: Tags: RESEARCH ARTICLE Source Type: research
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