The mechanism of photooxidation of methionine (N-Ac-Met-NH-CH 3 , 1 ) and methyl-cysteine (N-Ac-MeCys-NH-CH 3 , 2 ) analogues by 3-carboxybenzophenone triplet (3CB*) in neutral aqueous solution was studied using techniques of nanosecond laser flash photolysis and steady-state photolysis. The short-lived transients derived from 3CB and sulfur-containing amino acids were identified, and their quantum yields and kinetics of formation and decay were determined. The stable photoproducts were analyzed using liquid chromatography coupled with high-resolution mass spectrometry. Substantial differences in the mechanisms were found for methionine and S -methyl-cysteine analogues for both primary and secondary photoreactions. A new secondary reaction channel (back hydrogen atom transfer from the ketyl radical to the carbon-centered α-thioalkyl radical yielding reactants in the ground states) was suggested. The detailed mechanisms of 3CB* sensitized photooxidation of 1 and 2 are proposed and discussed.

Within the reactive oxygen species (ROS) generated by cellular metabolisms, hydroxyl radicals (HO•) play an important role, being the most aggressive towards biomolecules. The reactions of HO• with methionine residues (Met) in peptides and proteins have been intensively studied, but some fundamental aspects remain unsolved. In the present study we examined the biomimetic model made of Ac-Met-OMe, as the simplest model peptide backbone, and of HO• generated by ionizing radiation in aqueous solutions under anoxic conditions. We performed the identification and quantification of transient species by pulse radiolysis and of final products by LC-MS and high-resolution MS/MS after γ-radiolysis. By parallel photochemical experiments, using 3-carboxybenzophenone (CB) triplet with the model peptide, we compared the outcomes in terms of short-lived intermediates and stable product identification. The result is a detailed mechanistic scheme of Met oxidation by HO•, and by CB triplets allowed for assigning transient species to the pathways of products formation.

This work presents the synthesis and characterization of metal-free, zinc (II), and cobalt (II) porphyrins substituted with short PEG chains. The synthesized compounds were characterized by UV-Vis, 1H and 13C NMR spectroscopy, and MALDI-TOF mass spectrometry. The origin of the absorption bands for tested compounds in the UV-Vis range was determined using a computational model based on the electron density functional theory (DFT) and its time-dependent variant (TD-DFT). The photosensitizing activity was evaluated by measuring the ability to generate singlet oxygen (ΦΔ), which reached values up to 0.54. The photodynamic activity was tested using bladder (5637), prostate (LNCaP), and melanoma (A375) cancer cell lines. In vitro experiments clearly showed the structure–activity relationship regarding types of substituents, their positions in the phenyl ring, and the variety of central metal ions on the porphyrin core. Notably, the metal-free derivative 3 and its zinc derivative 6 exerted strong cytotoxic activity toward 5637 cells, with IC50 values of 8 and 15 nM, respectively. None of the tested compounds induced a cytotoxic effect without irradiation. In conclusion, these results highlight the potential value of the tested compounds for PDT application.

In the present study, we investigated the photooxidation of the biomimetic model of C-terminal methionine, N-Acetyl-Methionine (N-Ac-Met), sensitized by a 3-Carboxybenzophenone (3CB) excited triplet in neutral and basic aqueous solutions. The short-lived transient species that formed in the reaction were identified and quantified by laser flash photolysis and the final stable products were analyzed using liquid chromatography coupled with high-resolution mass spectrometry (LC-MS) and tandem mass spectrometry (MSMS). Based on these complementary methods, it was possible to calculate the quantum yields of both competing reactions, and the deprotonation was found to be favored over decarboxylation (for neutral pH: ϕ-H = 0.23 vs. ϕ-CO2 = 0.09, for basic pH: ϕ-H = 0.23 vs. ϕ-CO2 = 0.05). Findings on such a model system, which can possibly mimic the complex protein environment, are important in understanding complicated biological systems, for example, the studied compound, N-Ac-Met, can, to some extent, mimic the methionine in the C-terminal domain of β-amyloid, which is thought to be connected with the pathogenesis of Alzheimer’s disease.

No abstract