Showing papers by "André M. Braun published in 2007"

Photosensitization of 2′-deoxyadenosine-5′-monophosphate by pterin

Abstract: UV-A radiation (320–400 nm) induces damages to the DNA molecule and its components through photosensitized reactions. Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have investigated the photosensitization of 2′-deoxyadenosine-5′-monophosphate (dAMP) by pterin (PT) in aqueous solution under UV-A radiation. The effect of pH was evaluated, the participation of oxygen was investigated and the products analyzed. Kinetic studies revealed that the reactivity of dAMP towards singlet oxygen (1O2) is very low and that this reactive oxygen species does not participate in the mechanism of photosensitization, although it is produced by PT upon UV-A excitation. In contrast, analysis of irradiated solutions by means of electrospray ionization mass spectrometry strongly suggested that 8-oxo-7,8-dihydro-2′-deoxyadenosine-5′-monophosphate (8-oxo-dAMP) was produced, indicating that the photosensitized oxidation takes place via a type I mechanism (electron transfer).

Reactivity of Conjugated and Unconjugated Pterins with Singlet Oxygen (O2(1Δg)): Physical Quenching and Chemical Reaction†

Abstract: Pterins (PTs) belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. We have investigated the reactivity of conjugated PTs (folic acid [FA], 10-methylfolic acid [MFA], pteroic acid [PA]) and unconjugated PTs (PT, 6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT]) with singlet oxygen ( 1 O 2 ) in aqueous solutions, and compared the efficiencies of chemical reaction and physical quenching. The chemical reactions between 1 O 2 , produced by photosensitization, and PT derivatives were followed by UV-visible spectrophotometry and high-performance liquid chromatography, and corresponding rate constants (k r ) were evaluated. Whenever possible, products were identified and quantified. Rate constants of 1 O 2 total quenching by the PT derivatives investigated were obtained from steady-state 1 O 2 luminescence measurements. Results show that the behavior of conjugated PTs differs considerably from that of unconjugated derivatives, and the mechanisms of 1 O 2 physical quenching by these compounds and of their chemical reaction with 1 O 2 are discussed in relation to their structural features.

Industrial Wastewater Treatment by Photochemical Processes Based on Solar Energy

Abstract: Background: The solar photo-Fenton process has enormous potential for becoming a viable alternative to conventional processes for the treatment of industrial wastewater. However the costs associated with the use of artificial irradiation have hindered many times industrial application of these processes. Method of Approach: In this work, the photo-Fenton remediation of various industrial wastewaters (containing silicones, pesticides, phenol and hydrocarbons, model, and real) in aqueous systems has been studied using Fe(II), H 2 O 2 , and UV-visible sunlight. Experiments were carried out using a concentrating parabolic trough reactor (PTR) and a nonconcentrating falling-film reactor. Results: In general, at low contaminant concentration, more than 90% of the total organic carbon content could be converted to inorganic carbon within about 2-3 h, using sunlight, in reactors of different geometry. Conclusions: Solar light can be used either as an effective complementary or alternative source of photons to the photo-Fenton degradation process of a diversity of chemical pollutants.

Characterization of Nanostructured Surfaces Generated by Reconstitution of the Porin MspA from Mycobacterium smegmatis

Abstract: Nanostructures with long-term stability at the surface of gold electrodes are generated by reconstituting the porin MspA from Mycobacterium smegmatis into a specially designed monolayer of long-chain lipid surfactant on gold. Tailored surface coverage of gold electrodes with long-chain surfactants is achieved by electrochemically assisted deposition of organic thiosulfates (Bunte salts). The subsequent reconstitution of the octameric-pore MspA is guided by its extraordinary self-assembling properties. Importantly, electrochemical reduction of copper(II) yields copper nanoparticles within the MspA nanopores. Electrochemical impedance spectroscopy, reflection electron microscopy, and atomic force microscopy (AFM) show that: 1) the MspA pores within the self-assembled monolayer (SAM) are monodisperse and electrochemically active, 2) MspA reconstitutes in SAMs and with a 10-nm thickness, 3) AFM is a suitable method to detect pores within SAMs, and 4) the electrochemical reduction of Cu2+ to Cu0 under overpotential conditions starts within the MspA pores.

Singlet oxygen (O2(1Deltag)) quenching by dihydropterins.

Abstract: Pterins belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. Dihydropterins are one of the biologically active forms of pterins. The photoinduced production and quenching of singlet oxygen ( 1 O 2 ) by a series of dihydropterins (7,8-dihydrobiopterin (DHBPT), 7,8-dihydroneopterin (DHNPT), 6-formyl-7,8-dihydropterin (FDHPT), sepiapterin (SPT), 7,8-dihydrofolic acid (DHFA), and 7,8-dihydroxanthopterin (DHXPT)) in aqueous solution at physiological pH (∼7) were investigated, and the quantum yields of 1 O 2 production (Φ Δ ) and rate constants of total quenching (k t ) of 1 O 2 were determined. Studied compounds do not produce 1 O 2 under UV-A irradiation and are very efficient 1 O 2 quenchers. The chemical reactions between 1 O 2 and dihydropterin derivatives were investigated, and the corresponding rate constants (k r ) were found to be particularly high. The oxidized pterin derivatives, biopterin (BPT), neopterin (NPT), 6-formylpterin (FPT), and folic acid (FA), were identified and quantified during the reaction of 1 O 2 with DHBPT, DHNPT, FDHPT, and DHFA, respectively. Besides the oxidation of the dihydropyrazine ring to yield the corresponding oxidized pterins, a second oxidation pathway, leading to fragmentation of the dihydropterin and formation of non-pterinic products, was identified. Mechanisms and biological implications are discussed.