Studies of the Transient Intermediates of a Thiopyronine Aqueous Solution under Flash Excitation

Abstract

Flash photolytic studies of the transient intermediates produced in the flash excitation of an aqueous buffered solution of the photosensitizing dye “thiopyronine” were undertaken in order to obtain direct insight into the reaction mechanism of the photosensitizing action of the dye. As had been expected from the structural similarity to a well-known photosensitizer, methylene blue, previously reported on, the flash excitation of thiopyronine, in a deaerated buffered solution at pH 7.4 without any substrate, produced three transient intermediates: a triplet-state (Dt), a half-oxidized form (Oxd), and a half-reduced form or semiquinone (Red) of the dye. Red and Oxd were generated simultaneously through electron tansfer reactions, i.e., Dt+Dt→Red+Oxd and Dt+D→Red+Oxd, where D is the dye in the ground state. Thiopyronine solutions containing some reducing and oxidizing agents were studied in order to identify the intermediates. The absorption spectra of these intermediates were identified, and their reaction mechanism studied. The rate constants of some elementary processes obtained are: Dt→k1=2.36×103sec−1 Dt+Dt→k′=7.69×108m−1sec−1 Dt+D→k″=7.69×108m−1sec−1 Red+Red→k6=(1–2)×109m−1sec−1 Oxd→k7=(1–3)×103sec−1 Dt+EDTA→k8=1.6×107m−1sec−1 Dt+ATU→k9≈small Oxd+ATU→k10(2–5)×106m−1small The high efficieny of thiopyrinine in its Photodynamic effect was discussed in connection with the high quantum yield of triplet-state production and the high reversibility of the high reversibility of the photochemical redox reaction of the dye.

Mechanism of light-induced reduction of biological redox centers by amino acids. A flash photolysis study of flavin photoreduction by EDTA and nitrilotriacetate

“…This fast process changes the redox properties of the radical and makes a further donation of an electron equivalent possible. In the case of EDTA the electron acceptor of the second electron is flavin, which was left unexcited by the flash or is formed by dismutation from the flavosemiquinone, generated in the primary one-electron F'rom the early periods of flavin photochemistry, EDTA1 *was used as a photoreductant and now has seen a steadily widening application as donor of redox equivalents in the photochemistry of dyes [see, for example, Morita & Kato (1969), Bonneau et al (1973), Usui et al (1980), and Krasna (1980)] and in the study of redox-enzyme mechanisms ; .…”