Electron-transfer processes induced by the triplet state of pterins in aqueous solutions

Synthesis, Characterization and Photocleavage of Bis-decyl Pteroic Acid: A Folate Derivative with Affinity to Biomembranes†

Here, we provide mechanistic insight to the photocleavage of a compound in the folate family, namely pteroic acid. A bis-decyl chain derivative of pteroic acid was synthesized, structurally characterized and photochemically investigated. We showed that, like folic acid, pteroic acid and the decylated derivative undergo a photocleavage reaction in the presence of H₂ O, while no reaction was observed in methanol solution. Furthermore, density functional theory calculations were carried out to predict relative stabilities of hypothetical mono-, bis- and tris-decylated pteroic acid derivatives to help rationalize the regioselectivity of the bis-decyl pteroic acid product. Additionally, the lipophilicity of the bis-decyl pteroic acid appears to confer a hydrophobic property enabling an interaction with biomembranes.

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.

Effects of Folic Acid and Caffeic Acid on Indirect Photo-oxidation of Proteins and Their Costabilization under Irradiation

Proteins, vitamins, and phenols are often present together in foods, but they are sensitive to environmental factors. Folic acid (FA), a synthetic form of folate, decomposes under light, leading to protein oxidation. Caffeic acid (CA), a phenolic acid, exhibits remarkable activity for scavenging reactive molecules. The exploitation of their interactions offers opportunities for designing the stabilizing system. In this study, FA-photodecomposition-induced protein (β-lactoglobulin, α-lactalbumin, bovine serum albumin, and β-casein) damage and its inhibition by CA were investigated in terms of protein composition and structural change. The results indicated that FA photoproducts oxidized the proteins via the electron transfer pathway, leading to degradation, aggregation, and unfolding. At the same time, photostability of FA, CA, and proteins in the tertiary mixture was better than that of any individual components. The antioxidant activity of the proteins contributed to their protection for FA. CA and its products inhibited FA photodecomposition and the photodecomposition-induced protein oxidation by trapping excited states.

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.

Theoretical determination of two-photon absorption in biologically relevant pterin derivatives

Given the prevalence of fluorescence spectroscopy in biological systems, and the prevalence of pterin derivatives throughout biological systems, presented here is an assessment of the two-photon absorption spectroscopy as it applies to a range of the most commonly studied pterin derivatives. QR-CAMB3LYP//ccpVTZ calculations suggest that the use of two-photon spectroscopic methods would enable a more capable differentiation between closely related derivatives in comparison to the one-photon spectra, which show minimal qualitative deviation. Study of short tail derivatives shows that, in most cases, two-photon accessible states solely involve the π* LUMO as the particle orbital, with biopterin, neopterin, and 6-(hydroxymethyl)pterin presenting exceptional potential for targetting. Investigation of derivatives in which the tail contains an aromatic ring resulted in the observation of a series of two-photon accessible states involving charge transfer from the tail to the pterin moiety, the cross sections of which are highly dependent on the adoption of a planar geometry. The observation of these states presents a novel method for tracking the substitution of biologically important molecules such as folic acid and 5-methenyltetrahydrofolylpolyglutamate.

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.

Thymidine radical formation via one-electron transfer oxidation photoinduced by pterin: Mechanism and products characterization

UV-A radiation (320-400nm), recognized as a class I carcinogen, induces damage to the DNA molecule and its components through different mechanisms. Pterin derivatives are involved in various biological functions, including enzymatic processes, and it has been demonstrated that oxidized pterins may act as photosensitizers. In particular, they accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder. We have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the degradation of the pyrimidine nucleotide thymidine 5'-monophosphate (dTMP) in aqueous solutions under UV-A irradiation. Although thymine is less reactive than purine nucleobases, our results showed that Ptr is able to photoinduce the degradation of dTMP and that the process is initiated by an electron transfer from the nucleotide to the triplet excited state of Ptr. In the presence of molecular oxygen, the photochemical process leads to the oxidation of dTMP, whereas Ptr is not consumed. In the absence of oxygen, both compounds are consumed to yield a product in which the pterin moiety is covalently linked to the thymine. This compound retains some of the spectroscopic properties of Ptr, such as absorbance in the UV-A region and fluorescence properties.

A quantitative structure–property relationship (QSPR) study of singlet oxygen generation by pteridines

Singlet oxygen production quantum yields of pteridine photosensitizers were analyzed with the QSPR method. The ability of pterins and flavins to generate¹O₂in D₂O correlated withE_HOMOand electronegativity, as well as with the dipole moment and some other parameters.

Photooxidation of tetrahydrobiopterin under UV irradiation: possible pathways and mechanisms

Tetrahydrobiopterin (H4 Bip) is a cofactor for several key enzymes, including NO synthases and aromatic amino acid hydroxylases (AAHs). Normal functioning of the H4 Bip regeneration cycle is extremely important for the work of AAHs. Oxidized pterins may accumulate if the H4 Bip regeneration cycle is disrupted or if H4 Bip autoxidation occurs. These oxidized pterins can photosensitize the production of singlet molecular oxygen (1)O2 and thus cause oxidative stress. In this context, we studied the photooxidation of H4 Bip in phosphate buffer at pH 7.2. We found that UV irradiation of H4 Bip affected its oxidation rate (quantum yield Φ300 = (2.7 ± 0.4) × 10(-3)). The effect of UV irradiation at λ = 350 nm on H4 Bip oxidation was stronger, especially in the presence of biopterin (Bip) (Φ350 = (9.7 ± 1.5) × 10(-3)). We showed that the rate of H4 Bip oxidation linearly depends on Bip concentration. Experiments with KI, a selective quencher of triplet pterins at micromolar concentrations, demonstrated that the oxidation is sensitized by the triplet state biopterin (3) Bip. Apparently, electron transfer sensitization (Type-I mechanism) is dominant. Energy transfer (Type-II mechanism) and singlet oxygen generation play only a secondary role. The mechanisms of H4 Bip photooxidation and their biological meaning are discussed.

Tryptophan oxidation photosensitized by pterin

Pterins are normal components of cells and they have been previously identified as good photosensitizers under UV-A irradiation, inducing DNA damage and oxidation of nucleotides. In this work, we have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the oxidation of another class of biomolecules, amino acids, using tryptophan (Trp) as a model compound. Irradiation of Ptr in the UV-A spectral range (350 nm) in aerated aqueous solutions containing Trp led to the consumption of the latter, whereas the Ptr concentration remained unchanged. Concomitantly, hydrogen peroxide (H₂O₂) was produced. Although Ptr is a singlet oxygen ((1)O₂) sensitizer, the degradation of Trp was inhibited in O₂-saturated solutions, indicating that a (1)O₂-mediated process (type II oxidation) was not an important pathway leading to Trp oxidation. By combining different analytical techniques, we could establish that a type I photooxidation was the prevailing mechanism, initiated by an electron transfer from the Trp molecule to the Ptr triplet excited state, yielding the corresponding radical ions (Trp(·+)/Trp(-H)· and Ptr(·-)). The Trp reaction products that could be identified by UPLC-mass spectrometry are in agreement with this conclusion.

Type I photosensitization of 2'-deoxyadenosine 5'-monophosphate (5'-dAMP) by biopterin and its photoproduct formylpterin

Biopterin (Bip) and its photoproducts 6-formylpterin (Fop) and 6-carboxypterin (Cap) accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder where the protection against UV radiation fails because of the lack of melanin. These compounds absorb in the UV-A inducing a potential photosensitizing action that can cause damage to DNA and other biomolecules. In this work, we have investigated the capability of these pterin derivatives (Pt) to act as photosensitizers under UV-A irradiation for the degradation of 2'-deoxyadenosine 5'-monophosphate (5'-dAMP) in aqueous solutions, as model DNA target. Steady-state and time-resolved experiments were performed and the effect of pH was evaluated. The results showed that photosensitized degradation of 5'-dAMP was only observed under acidic conditions, and a mechanistic analysis revealed the participation of the triplet excited state of the pterin derivatives ((3)Pt*) by electron transfer yielding the corresponding pair of radical ions (Pt(•-) and 5'-dAMP(•+)), with successive photosensitizer recovery by electron transfer from Pt(•-) to O2. Finally, 5'-dAMP(•+) participates in subsequent reactions to yield degradation products.

Photosensitizing properties of biopterin and its photoproducts using 2'-deoxyguanosine 5'-monophosphate as an oxidizable target

UV-A radiation (320-400 nm) induces damage to the DNA molecule and its components through photosensitized reactions. Biopterin (Bip) and its photoproducts 6-formylpterin (Fop) and 6-carboxypterin (Cap) accumulate in the skin of human beings suffering from vitiligo, a depigmentation disorder where the protection against UV radiation fails because of the lack of melanin. This study was aimed to evaluate the photosensitizing properties of oxidized pterins present in the skin and to elucidate the mechanisms involved in the photosensitized oxidation of purine nucleotides by pterins in vitro. For this purpose, steady-state and time-resolved experiments in acidic (pH 5.0-5.8) aqueous solution were performed using Bip, Fop and Cap as photosensitizers and the nucleotide 2'-deoxyguanosine 5'-monophosphate (dGMP) as an oxidizable target. The three pterin derivatives are able to photosensitize dGMP, being Fop the most efficient sensitizer. The reactions proceed through two competing pathways: (1) electron transfer from dGMP to triplet excited-state of pterins (type I mechanism) and (2) reaction of dGMP with (1)O(2) produced by pterins (type II mechanism). Kinetic analysis revealed that the electron transfer pathway is the main mechanism and the interaction of dGMP with the triplet excited-state of pterins and the formation of the corresponding dGMP radicals were demonstrated by laser flash photolysis experiments. The biological implications of the results obtained are also discussed.

Characterization and reactivity of photodimers of dihydroneopterin and dihydrobiopterin

7,8-Dihydrobiopterin (H(2)Bip) and 7,8-dihydroneopterin (H(2)Nep) belong to a class of heterocyclic compounds present in a wide range of living systems. H(2)Bip accumulates in the skin of patients suffering from vitiligo, whereas H(2)Nep is secreted by human macrophages when the cellular immune system is activated. We have investigated the photochemical reactivity of both compounds upon UV-A irradiation (320-400 nm), the chemical structures of the products and their thermal stability. The study was performed in neutral aqueous solutions. The reactions were followed by UV/Visible spectrophotometry and HPLC and the products were analyzed by means of electrospray ionization mass spectrometry and (1)H-NMR. Excitation of H(2)Bip and H(2)Nep leads to the formation, in each case, of two main isomeric dimers. The latter compounds undergo a thermal process that may consist in a retro [2 + 2]-cycloaddition and hydrolysis to yield the reactant (H(2)Bip or H(2)Nep) and a product that has incorporated a molecule of H(2)O.

Mechanism of electron transfer processes photoinduced by lumazine

UV-A (320-400 nm) and UV-B (280-320 nm) radiation causes damage to DNA and other biomolecules through reactions induced by different endogenous or exogenous photosensitizers. Lumazines are heterocyclic compounds present in biological systems as biosynthetic precursors and/or products of metabolic degradation. The parent and unsubstituted compound called lumazine (pteridine-2,4(1,3H)-dione; Lum) is able to act as photosensitizer through electron transfer-initiated oxidations. To get further insight into the mechanisms involved, we have studied in detail the oxidation of 2'-deoxyadenosine 5'-monophosphate (dAMP) photosensitized by Lum in aqueous solution. After UV-A or UV-B excitation of Lum and formation of its triplet excited state ((3)Lum*), three reaction pathways compete for the deactivation of the latter: intersystem crossing to singlet ground state, energy transfer to O(2), and electron transfer between dAMP and (3)Lum* yielding the corresponding pair of radical ions (Lum˙(-) and dAMP˙(+)). In the following step, the electron transfer from Lum˙(-) to O(2) regenerates Lum and forms the superoxide anion (O(2)˙(-)), which undergoes disproportionation into H(2)O(2) and O(2). Finally dAMP˙(+) participates in subsequent reactions to yield products.

Emission properties of dihydropterins in aqueous solutions

Pterins belong to a class of heterocyclic compounds present in a wide range of living systems and accumulate in the skin of patients affected by vitiligo, a depigmentation disorder. The study of the emission of 7,8-dihydropterins is difficult because these compounds are more or less unstable in the presence of O(2) and their solutions are contaminated with oxidized pterins which have much higher fluorescence quantum yields (Φ(F)). In this work, the emission properties of six compounds of the dihydropterin family (6-formyl-7,8-dihydropterin (H(2)Fop), sepiapterin (Sep), 7,8-dihydrobiopterin (H(2)Bip), 7,8-dihydroneopterin (H(2)Nep), 6-hydroxymethyl-7,8-dihydropterin (H(2)Hmp), and 6-methyl-7,8-dihydropterin (H(2)Mep)) have been studied in aqueous solution. The fluorescence characteristics (spectra, Φ(F), lifetimes (τ(F))) of the neutral form of these compounds have been investigated using the single-photon-counting technique. Φ(F) and τ(F) values obtained lie in the ranges 3-9 × 10(-3) and 0.18-0.34 ns, respectively. The results are compared to those previously reported for oxidized pterins.

Oxidation of 2'-deoxyadenosine 5'-monophosphate photoinduced by lumazine

UV radiation induces damages to the DNA molecule and its components through photosensitized reactions. Among these processes, photosensitized oxidations may occur through electron transfer or hydrogen abstraction (type I mechanism) and/or the production of singlet molecular oxygen ((1)O(2)) (type II mechanism). Lumazines are an important family of heterocyclic compounds present in biological systems as biosynthetic precursors and/or products of metabolic degradation. To evaluate the capability of lumazines to act as photosensitizers through type I mechanism, we have investigated the oxidation of 2'-deoxyadenosine 5'-monophosphate (dAMP) photosensitized by the specific compound called lumazine (pteridine-2,4(1,3H)-dione; Lum) in aqueous solutions under UV irradiation. The photochemical reactions were followed by UV/vis spectrophotometry, HPLC, electrochemical measurement of dissolved O(2), and an enzymatic method for H(2)O(2) determination. The effect of pH was evaluated and the participation of oxygen was investigated. In aerated solutions, oxidation of dAMP photoinduced by the acid form of Lum (pH 5.5) takes place through a type I mechanism, in which the excitation of Lum is followed by an electron transfer from dAMP molecule to the Lum triplet excited state. During the process, O(2) is consumed and H(2)O(2) is generated, whereas the photosensitizer is not consumed. In contrast, no evidence of a photochemical reaction induced by the basic form of Lum (pH 10.5) was observed.

Mechanism of photooxidation of folic acid sensitized by unconjugated pterins

Folic acid, or pteroyl-l-glutamic acid (PteGlu), is a precursor of coenzymes involved in the metabolism of nucleotides and amino acids. PteGlu is composed of three moieties: a 6-methylpterin (Mep) residue, a p-aminobenzoic acid (PABA) residue, and a glutamic acid (Glu) residue. Accumulated evidence indicates that photolysis of PteGlu leads to increased risk of several pathologies. Thus, a study of PteGlu photodegradation can have significant ramifications. When an air-equilibrated aqueous solution of PteGlu is exposed to UV-A radiation, the rate of the degradation increases with irradiation time. The mechanism involved in this "auto-photo-catalytic" effect was investigated in aqueous solutions using a variety of tools. Whereas PteGlu is photostable under anaerobic conditions, it is converted into 6-formylpterin (Fop) and p-aminobenzoyl-l-glutamic acid (PABA-Glu) in the presence of oxygen. As the reaction proceeds and enough Fop accumulates in the solution, a photosensitized electron-transfer process starts, where Fop photoinduces the oxidation of PteGlu to Fop, and H(2)O(2) is formed. This process also takes place with other pterins as photosensitizers. The results are discussed with the context of previous mechanisms for processes photosensitized by pterins, and their biological implications are evaluated.