5-Deazaalloxazine as photosensitizer of singlet oxygen and potential redox-sensitive agent

Flavins are a unique class of compounds that combine the features of singlet oxygen generators and redox-dependent fluorophores. From a broad family of flavin derivatives, deazaalloxazines are significantly underdeveloped from the point of view of photophysical properties. Herein, we report photophysics of 5-deazaalloxazine (1a) in water, acetonitrile, and some other solvents. In particular, triplet excited states of 1a in water and in acetonitrile were investigated using ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The measured triplet lifetimes for 1a were all on the microsecond time scale (≈ 60 μs) in deoxygenated solutions. The quantum yield of S₁ → T₁ intersystem crossing for 1a in water was 0.43 based on T₁ energy transfer from 1a to indicaxanthin (5) acting as acceptor and on comparative actinometric measurements using benzophenone (6). 1a was an efficient photosensitizer for singlet oxygen in aerated solutions, with quantum yields of singlet oxygen in methanol of about 0.76, compared to acetonitrile ~ 0.74, dichloromethane ~ 0.64 and 1,2-dichloroethane ~ 0.54. Significantly lower singlet oxygen quantum yields were obtained in water and deuterated water (Ф_Δ ~ 0.42 and 0.44, respectively). Human red blood cells (RBC) were used as a cell model to study the antioxidant capacity in vitro and cytotoxic activity of 1a. Fluorescence-lifetime imaging microscopy (FLIM) data were analyzed by fluorescence lifetime parameters and distribution for different parts of the emission spectrum. Comparison of multidimensional fluorescent properties of RBC under physiological-like and oxidative-stress conditions in the presence and absence of 1a suggests its dual activity as probe and singlet-oxygen generator and opens up a pathway for using FLIM to analyze complex intracellular behavior of flavin-like compounds. These new data on structure-property relationship contribute to the body of information required for a rational design of flavin-based tools for future biological and biochemical applications.

Environmental photochemistry of fenamate NSAIDs and their radical intermediates

Fenamates are a class of nonsteroidal anti-inflammatory drugs (NSAIDs) that are not fully removed during wastewater treatment and can be released to surface waters. Here, near-surface photochemical half-lives were evaluated to range from minutes to hours of four fenamates and the closely related diclofenac. While quantum yields for direct photochemical reactions at the water surface vary widely from 0.071 for diclofenac to <0.001 for mefenamic acid, all fenamates showed significant reactivity towards singlet oxygen and hydroxyl radical with bimolecular reaction rate constants of 1.3-2.8 × 10⁷ M^-1 s^-1 and 1.1-2.7 × 10¹⁰ M^-1 s^-1, respectively. Photodecay rates increased in the presence of dissolved organic matter (DOM) for diclofenac (+19%), tolfenamic acid (+9%), and mefenamic acid (+95%), but decreased for flufenamic acid (-2%) and meclofenamic acid (-14%) after accounting for light screening effects. Fast reaction rate constants of all NSAIDs with model triplet sensitizers were quantified by laser flash photolysis. Here, the direct observation of diphenylamine radical intermediates by transient absorption spectroscopy demonstrates one-electron oxidation of all fenamates. Quenching rate constants of these radical intermediates by ascorbic acid, a model antioxidant, were also quantified. These observations suggest that the balance of oxidation by photoexcited triplet DOM and quenching of the formed radical intermediates by antioxidant moieties determines whether net sensitization or net quenching by DOM occurs in the photochemical degradation of fenamates.