Photochemical and photophysical properties of lumazine in aqueous solutions

Luminescence enhancement by deuterium

Created literally at the dawn of time, deuterium has been extremely valuable in so many chemistry roles. The subject of this review focuses on one deuterium application in particular: its enhancement of luminescence in many substances. After providing general overviews of both deuterium and luminescence, the early exploration of deuterium's effect on luminescence is described, followed by a number of specific topics. These sections include a discussion of deuterium-influenced luminescence for dyes, proteins, singlet oxygen, and the lanthanide elements, as well as anomalous inverse deuterium luminescence effects. Future directions for this important research topic are also proposed, as well as a summary conclusion.

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.

Thiolumazines as Heavy-Atom-Free Photosensitizers for Applications in Daylight Photodynamic Therapy: Insights from Ultrafast Excited-State Dynamics

Finding appropriate photosensitizers (PSs) for daylight photodynamic therapy (dPDT) applications is extremely challenging, even though heavy-atom-free photosensitizers (HAFPSs) such as thiocarbonyl-modified nucleobases have shown a ray of hope. Few attempts have been made to find alternative natural products for dPDT applications. Pteridine heterocycles consisting of a pyrazine ring and a pyrimidine ring, such as lumazine, which exhibit many structural similarities to the alloxazine ring of the flavin molecule, could be an option for HAFPSs. The photophysical and quantum mechanical studies of the thio-modified lumazines revealed that sequential thiomodifications in lumazine result in a bathochromic shift. Additionally, higher tissue penetration depths were observed for thiolumazines. The fluorescence quenching in the case of thiomodified lumazines was explained using triplet state formation, whereas the contribution from the photoinduced electron transfer process cannot be ignored. It was also noticed that a strong one-photon absorption influenced the two-photon absorption (TPA) process, leading to a self-focusing effect in the visible spectral region. The higher tissue penetration and larger TPA cross section are the hallmark characteristics of the thiolumazines to be considered as potential HAFPSs for dPDT applications.

Simultaneous Determination of the Chemical (kr ) and the Physical (kq ) Quenching Rate Constants of Singlet Oxygen in Aqueous Solution by the Chemiluminescence-quenching Method†

This work reports a novel and visual method for the simultaneous determination of the chemical (k_r ) and the physical (k_q ) quenching rate constants of singlet oxygen (¹ O₂ ,¹ ∆_g ) in aqueous media. It is based on the disruption, by a water-soluble substrate S, of the ¹ O₂ chemiluminescence (CL) generated by the H₂ O₂ /Na₂ MoO₄ catalytic system. A mathematical analysis of the CL signal at 1270 nm vs time provides separately the overall (k_r  + k_q ) and the chemical (k_r ) quenching rate constants. In ordinary water (H₂ O), ¹ O₂ lifetime is short and the CL intensity is weak allowing solely the investigation of very reactive substrates for which (k_r  + k_q ) > 3 × 10⁶  m^-1  s^-1 while, in D₂ O, ¹ O₂ lifetime is significantly longer lifetime and the CL signal is much stronger allowing the study of poorly reactive substrates for which (k_r  + k_q ) > 4 × 10⁵  m^-1  s^-1 . The method has been successfully tested on a series of anionic and nonionic water-soluble naphthalene derivatives commonly used as bio-compatible ¹ O₂ carriers. The obtained k_r and k_q values are in good agreement with the values determined by conventional techniques, namely, flash photolysis and competitive kinetics with a reference quencher.

Mono- and Bis-Alkylated Lumazine Sensitizers: Synthetic, Molecular Orbital Theory, Nucleophilic Index and Photochemical Studies

Mono- and bis-decylated lumazines have been synthesized and characterized. Namely, mono-decyl chain [1-decylpteridine-2,4(1,3H)-dione] 6a and bis-decyl chain [1,3-didecylpteridine-2,4(1,3H)-dione] 7a conjugates were synthesized by nucleophilic substitution (S_N 2) reactions of lumazine with 1-iododecane in N,N-dimethylformamide (DMF) solvent. Decyl chain coupling occurred at the N¹ site and then the N³ site in a sequential manner, without DMF condensation. Molecular orbital (MO) calculations show a p-orbital at N¹ but not N³ , which along with a nucleophilicity parameter (N) analysis predict alkylation at N¹ in lumazine. Only after the alkylation at N¹ in 6a, does a p-orbital on N³ emerge thereby reacting with a second equivalent of 1-iododecane to reach the dialkylated product 7a. Data from NMR (¹ H, ¹³ C, HSQC, HMBC), HPLC, TLC, UV-vis, fluorescence and density functional theory (DFT) provide evidence for the existence of mono-decyl chain 6a and bis-decyl chain 7a. These results differ to pterin O-alkylations (kinetic control), where N-alkylation of lumazine is preferred and then to dialkylation (thermodynamic control), with an avoidance of DMF solvent condensation. These findings add to the list of alkylation strategies for increasing sensitizer lipophilicity for use in photodynamic therapy.

Naturally Occurring Lumazines

Natural products containing a lumazine motif were first isolated from natural sources in 1940. These natural products are relatively rare, with fewer than 100 lumazines known to occur in Nature. This review discusses the isolation of lumazines, their biological activity, and their biosynthesis, where known.

Photoprotolytic Processes of Lumazine

Steady-state and time-resolved UV-vis spectroscopies were used to study the photoprotolytic properties of lumazine, which belongs to a class of biologically important compounds-the petridines. We found that in water an excited-state proton transfer occurs with a time constant of ∼70 ps and competes with a nonradiative rate of about the same value. The nonradiative rate of the protonated form of lumazine in polar and nonpolar solvents is large k_nr ≥ 1.5 × 10¹⁰s^-1. The fluorescence properties indicate that in water, the ground-state neutral form of lumazine is already stable in two tautomeric forms. The fluorescence of the deprotonated form is quenched by protons in acidic solutions with a diffusion-controlled reaction rate. We conclude that the neutral form of lumazine is an irreversible mild photoacid.

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.

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.

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.

1H-pteridine-2,4-dione (lumazine): a new MALDI matrix for complex (phospho)lipid mixtures analysis

Nowadays, matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry represents an emerging and versatile tool for analysis of lipids. However, direct (i.e., with no previous separation of lipid classes) analysis of crude extracts containing a complex mixture of lipids (a problem typically encountered in shotgun lipidomics) is still a quite challenging task using a conventional MALDI matrix such as 2,5-dihydroxybenzoic acid (DHB). Indeed, in the presence of phospholipids containing quaternary ammonium groups, such as phosphatidylcholines and sphingomyelins, strong ionization-suppression effects are experienced especially in positive ion mode. To overcome this limitation, lumazine (1H-pteridine-2,4-dione) was evaluated as an alternative matrix. Lumazine in the solid state showed an absorption maximum at 350 nm, ionizes/desorbs without appreciable decomposition and extensive cluster formation, and can be used in both ion modes. In positive ion mode, the main species were M(*+) and 2M(*+) radical cations and cationized species ([M+H](+), [M+Na](+), [M+2Na+2Li-3H](+)). In negative ion mode, the main signals observed were the deprotonated molecular ion and the radical anion. The signal-to-noise ratio for phosphatidylglycerols and phosphatidylethanolamines using lumazine was almost 1 order of magnitude higher than that observed for DHB. Lumazine was successfully used for MALDI analysis (positive and negative ion modes) of crude lipid extracts of milk, soymilk, and hen egg, where phosphatidylethanolamines, phosphatidylserines, and phosphatidylinositols could additionally be detected.

The photosensitizing activity of lumazine using 2'-deoxyguanosine 5'-monophosphate and HeLa cells as targets

Lumazines are an important family of heterocyclic compounds present in biological systems as biosynthetic precursors and/or products of metabolic degradation. Upon UV irradiation, the specific compound called lumazine (pteridine-2,4(1,3H)-dione) is able to generate singlet oxygen (1O2), which is one of the main chemical species responsible for photodynamic effects. To further assess the photosensitizing capability of lumazine (Lum) experiments were performed using the nucleotide 2'-deoxyguanosine 5'-monophosphate (dGMP) and, independently, cervical cancer cells (HeLa cell line) as targets. In the dGMP experiments, the data revealed that dGMP indeed undergoes oxidation/oxygenation photoinduced by Lum. Moreover, dGMP disappearance proceeds through two competing pathways: (1) electron transfer between dGMP and excited-state Lum (Type I process) and (2) reaction of dGMP with 1O2 produced by Lum (Type II process). The multistep processes involved are convoluted and susceptible to changes in experimental conditions. The independent studies with HeLa cells included fluorescence analysis of cell extracts and phototoxicity experiments performed at the single-cell level. Results showed that, upon Lum uptake and irradiation, photodynamic effects occur. In particular, the mitochondria and cell membrane were perturbed, both of which reflect key stages in cell death. The data reported herein illustrate how the irradiation of an endogenous biological compound can have various effects which, depending on the system, can be manifested in different ways.