Photooxidation of Tryptophan: O₂(1Δ_g) versus Electron‐Transfer Pathway

Abstract

Tris (2,2'-bipyridyl)ruthenium(II)chloride hexahydrate (Ru[bpy]3(2+)) free in solution and adsorbed onto antimony-doped SnO2 colloidal particles was used as a photosensitizer for a comparison of the O2(1 delta g) and electron-transfer-mediated photooxidation of tryptophan (TRP), respectively. Quenching of excited Ru(bpy)3(2+) by O2(3 sigma g-) in an aerated aqueous solution leads only to the formation of O2(1 delta g) (phi delta = 0.18) and this compound was used as a type II photosensitizer. Excitation of Ru(bpy)3(2+) adsorbed onto Sb/SnO2 results in a fast injection of an electron into the conduction band of the semiconductor and accordingly to the formation of Ru(bpy)3(2+) and was used for the sensitization of the electron-transfer-mediated photooxidation. The Ru(bpy)3(3+) is reduced by TRP with a bimolecular rate constant kQ = 5.9 x 10(8) M-1 s-1, while O2(1 delta g) is quenched by TRP with kt = 7.1 x 10(7) M-1 s-1 (chemical + physical quenching). Relative rate constants for the photooxidation of TRP (kc) via both pathways were determined using fluorescence emission spectroscopy. With Np, the rate of photons absorbed, being constant for both pathways we obtained kc = (372/Np) M-1 s-1 for the O2(1 delta g) pathway and kc > or = (25,013/Np) M-1 s-1 for the electron-transfer pathway, respectively. Thus the photooxidation of Trp is more than two orders of magnitude more efficient when it is initiated by electron transfer than when initiated by O2(1 delta g).

Photodynamic Studies and Photoinactivation of Escherichia coli Using meso‐Substituted Cationic Porphyrin Derivatives with Asymmetric Charge Distribution¶

“…Thus, these results are not in agreement with those obtained for the DMA photooxidation, indicating that other photoreaction process, probably an electron transfer process, is also involved in the decomposition of Trp sensitized by Pd(II) porphyrin 3 . In addition, the photosensitized oxidation of Trp was found to be more than two orders of magnitude more efficient via electron transfer than via O 2 ( 1 Δ g ) (49).…”

The role of reactive oxygen species in Staphylococcus aureus photoinactivation by methylene blue

“…The ascending branch decrease of the dimer absorbance peak observed in the presence of Trp beforeand after 10 min irradiation time could be associated to a decrease in dye dimerisation. Trp, which was non-specific singlet oxygen quencher(Wessels et al 1997), did not protect the S. aureus bacteria, after 10 min of MB photosensitization. Probably the difference between the dye spectra profile in the presence of S. aureus bacteria without or with Trp may be attributed to our bacterial system and Trp molecules.…”

Porphyrin‐fullerene C60 Dyads with High Ability to Form Photoinduced Charge‐separated State as Novel Sensitizers for Photodynamic Therapy¶

“…From these plots, the values of the k Trp obs were calculated for P and dyads (Table 1). As observed in Table 1, higher photo‐oxidation rates were obtained using dyads as sensitizers than with P. In previous studies, the photosensitized oxidation of Trp was found to be more than two orders of magnitude more efficient via electron transfer than via O 2 ( 1 Δ g ) (32). Comparing the kinetic results of Trp and DMA in DMF, it can be observed that the ratio K Trp obs /k DMA obs takes values of 0.44, 7.7 and 25.3 for P, P‐C 60 and ZnP‐C 60 , respectively.…”