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

The cation ‐π interaction between a sodium cation and an indole moiety significantly decreases the total rate of removal of singlet oxygen (kT) in an indole ‐crown model system. A type I photooxidation of the indole group is also inhibited by the cation‐π interaction. These results imply that there may be a photoprotective effect by the cation‐π interaction for tryptophan. AbstractWe 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 cationvs (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 sys...

Source: Photochemistry and Photobiology - June 19, 2020 Category: Science Authors: Gary E. Arevalo, David A. Cagan, Charlotte G. Monsour, Arman C. Garcia, Alison McCurdy, Matthias Selke Tags: Research Article Source Type: research