Tin porphyrin immobilization significantly enhances visible-light-photosensitized degradation of Microcystins: Mechanistic implications

This study demonstrates that tin porphyrin (SnP) loading on a silica substrate (SnP/silica) markedly accelerates the degradation of Microcystins (MCs) under visible light irradiation, despite a reduction of photosensitized singlet oxygen (¹O₂) production. A comparative study using Rose Bengal, SnP, and C₆₀ aminofullerene suggested that the MC-RR decay rate was directly proportional to the photosensitizing activity for triplet state-induced oxidation, while it exhibited poor correlation to singlet oxygenation efficiency. This implies that electron transfer from MC to the triplet state of SnP (facilitated by favorable MC sorption on silica) contributes to the photosensitized MC oxidation. Experiments to examine sensitizers for the one-electron oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) showed the superiority of SnP/silica for photo-initiated electron transfer as a possible MC oxidation route. This was corroborated by the negligible effects of reagents that quench or facilitate singlet oxygenation (e.g., azide ion, D₂O) on the MC-RR degradation rate. Despite MC-RR removal below detection levels, residual toxicity (indicated by a significant decrease in protein phosphatase inhibition activity) was observed. Tandem mass spectrometric analysis suggests that this residual toxicity may be ascribed to byproducts resulting from addition of a single oxygen atom to the Adda moiety.