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A Photoprotective Effect by Cation‐π‐Interaction? Quenching of Singlet Oxygen by an Indole Cation‐π Model System
Author(s) -
Arevalo Gary E.,
Cagan David A.,
Monsour Charlotte G.,
Garcia Arman C.,
McCurdy Alison,
Selke Matthias
Publication year - 2020
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/php.13287
Subject(s) - indole test , chemistry , singlet oxygen , photochemistry , tryptophan , quenching (fluorescence) , methylene blue , oxygen , crown ether , ether , singlet state , excited state , sodium , ion , stereochemistry , organic chemistry , photocatalysis , catalysis , fluorescence , amino acid , biochemistry , physics , quantum mechanics , nuclear physics
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 ( k T ) for the model system, that is, ( k T = 2.4 ± 0.2) × 10 8 m −1 s −1 without sodium cation vs ( k T = 6.9 ± 0.9) × 10 7 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.