Rate constants of quenching of the fluorescence of pterins by the iodide anion in aqueous solution

Photosensitized isomerization of resveratrol: Evaluation of energy and electron transfer pathways

Resveratrol (3,5,4'-trihydroxystilbene, RSV) is a natural stilbene synthetized as trans-isomer in plants exposed to oxidative stress. In order to understand the mechanism involved during photosensitized degradation of trans-resveratrol, steady-state and time-resolved experiments were performed and compared with quantum-chemical calculations using density functional theory (DFT). Pterin (Ptr), a well-known photosensitizer, under UV-A radiation induces the oxidation of several biomolecules mainly through electron-transfer mechanisms. On the one hand, it was observed that trans-RSV participates in an energy-transfer pathway with Ptr triplet excited state (3Ptr*) forming 3trans-RSV*, which dissipates the energy by isomerization to cis-RSV. On the other hand, RSV neutral radical (trans-RSV(-H)•) was detected in laser flash photolysis experiments, evidencing an electron-transfer mechanism. The electron-transfer from 3Ptr* to trans-RSV is a barely feasible reaction, however, more favorable is the formation of trans-RSV(-H)• in a reaction between trans-RSV and Ptr radical cation (Ptr•+), which is produced during irradiation. The combination of experimental and theoretical approaches evidences the capability of trans-RSV to undergo energy-transfer (feasible by DFT calculations) and/or one-electron transfer pathways with 3Ptr*. These findings reveal the mechanisms involved in the interaction of trans-RSV and pterin excited states and provide information on the antioxidant action of resveratrol during photosensitized oxidation of biomolecules.

Insights into Molecular Structure of Pterins Suitable for Biomedical Applications

Pterins are an inseparable part of living organisms. Pterins participate in metabolic reactions mostly as tetrahydropterins. Dihydropterins are usually intermediates of these reactions, whereas oxidized pterins can be biomarkers of diseases. In this review, we analyze the available data on the quantum chemistry of unconjugated pterins as well as their photonics. This gives a comprehensive overview about the electronic structure of pterins and offers some benefits for biomedicine applications: (1) one can affect the enzymatic reactions of aromatic amino acid hydroxylases, NO synthases, and alkylglycerol monooxygenase through UV irradiation of H₄pterins since UV provokes electron donor reactions of H₄pterins; (2) the emission properties of H₂pterins and oxidized pterins can be used in fluorescence diagnostics; (3) two-photon absorption (TPA) should be used in such pterin-related infrared therapy because single-photon absorption in the UV range is inefficient and scatters in vivo; (4) one can affect pathogen organisms through TPA excitation of H₄pterin cofactors, such as the molybdenum cofactor, leading to its detachment from proteins and subsequent oxidation; (5) metal nanostructures can be used for the UV-vis, fluorescence, and Raman spectroscopy detection of pterin biomarkers. Therefore, we investigated both the biochemistry and physical chemistry of pterins and suggested some potential prospects for pterin-related biomedicine.

Autoxidation and photooxidation of tetrahydrobiopterin: a theoretical study

Pterins are naturally occurring pigments and enzyme cofactors widespread in living organisms. Tetrahydrobiopterin (H4Bip) is a coenzyme of aromatic amino acid hydroxylases, NO-synthases, and alkylglycerol monooxygenases. This coenzyme is prone to oxidation in the presence of molecular oxygen, a so-called autoxidation. The reactions participating in H4Bip autoxidation are well known. However, our study is an attempt to evaluate theoretically the feasibility of reactions participating in autoxidation. To do so, we have calculated the Gibbs free energy of elementary reactions between H4Bip, its derivatives, molecular oxygen, and reactive oxygen species (ROS). In the last few years, we have established the photosensitized oxidation of H4Bip experimentally. Thus, we have also evaluated the feasibility of H4Bip photooxidation reactions, which may occur according to both type-I and type-II photosensitized oxidation. We calculated Fukui indices for H4Bip and found particular atoms in the molecule that interact with nucleophiles (for example, singlet oxygen 1O2) and radicals (in particular, molecular oxygen 3O2). Therefore, we evaluated the probability of H4Bip autoxidation reactions, photooxidation reactions, and the reactivity of particular atoms in H4Bip molecule using the theoretical methods of quantum chemistry.

Type I Photosensitized Oxidation of Methionine†

Methionine (Met) is an essential sulfur-containing amino acid, sensitive to oxidation. The oxidation of Met can occur by numerous pathways, including enzymatic modifications and oxidative stress, being able to cause relevant alterations in protein functionality. Under UV radiation, Met may be oxidized by direct absorption (below 250 nm) or by photosensitized reactions. Herein, kinetics of the reaction and identification of products during photosensitized oxidation were analyzed to elucidate the mechanism for the degradation of Met under UV-A irradiation using pterins, pterin (Ptr) and 6-methylpterin (Mep), as sensitizers. The process begins with an electron transfer from Met to the triplet-excited state of the photosensitizer (Ptr or Mep), to yield the corresponding pair of radicals, Met radical cation (Met^•+ ) and the radical anion of the sensitizer (Sens^•- ). In air-equilibrated solutions, Met^•+ incorporates one or two atoms of oxygen to yield methionine sulfoxide (MetO) and methionine sulfone (MetO₂ ), whereas Sens^•- reacts with O₂ to recover the photosensitizer and generate superoxide anion (O₂ ^•- ). In anaerobic conditions, further free-radical reactions lead to the formation of the corresponding dihydropterin derivatives (H₂ Ptr or H₂ Mep).

Electronic relaxation pathways of the biologically relevant pterin chromophore

Femtosecond-to-microsecond transient absorption spectroscopy is used to report the ultrafast relaxation mechanism of 2-amino-1H-pteridin-4-one (pterin) for the first time.

Photosensitized reactions initiated by 6-carboxypterin: singlet and triplet reactivity

The photophysical and photochemical properties of 6-carboxypterin, a model compound for the pterin derivatives present at the sea surface microlayer, were studied and its singlet and triplet reactivity toward halides and selected organics demonstrated.