Electron Spin-Dependent Electrocatalysis for the Oxygen Reduction Reaction in a Chiro-Self-Assembled Iron Phthalocyanine Device.

The chiral-induced spin selectivity effect (CISS) is a breakthrough phenomenon that has revolutionized the field of electrocatalysis. We report the first study on the electron spin-dependent electrocatalysis for the oxygen reduction reaction, ORR, using iron phthalocyanine, FePc, a well-known molecular catalyst for this reaction. The FePc complex belongs to the non-precious catalysts group, whose active site, FeN4, emulates catalytic centers of biocatalysts such as Cytochrome c. This study presents an experimental platform involving FePc self-assembled to a gold electrode surface using chiral peptides (L and D enantiomers), i.e., chiro-self-assembled FePc systems (CSAFePc). The chiral peptides behave as spin filters axial ligands of the FePc. One of the main findings is that the peptides' handedness and length in CSAFePc can optimize the kinetics and thermodynamic factors governing ORR. Moreover, the D-enantiomer promotes the highest electrocatalytic activity of FePc for ORR, shifting the onset potential up to 1.01 V vs. RHE in an alkaline medium, a potential close to the reversible potential of the O2 /H2 O couple. Therefore, this work has exciting implications for developing highly efficient and bioinspired catalysts, considering that, in biological organisms, biocatalysts that promote O2 reduction to water comprise L-enantiomers.

Labelling fish diets with 15 N -Leucine for monitoring feed consumption and bio-distribution in Atlantic salmon.

Feeding represents 50-70% of the cost of production in salmon farming, higher than any other animal farm. The improvement of this percentage is challenging as the food is thrown into the fish tank, there is no quantification of the amount of food that is consumed by the fish. In consequence, it is difficult to adjust the food composition making it more nutritive or promoting food consumption by fish. In this study, to investigate food consumption, bio-distribution and food residues, leucine containing 15 N (a stable isotope of nitrogen) was used to label the fish food. Atlantic salmon (Salmo salar) weighing 100-120 g were maintained in 30 L tanks at a density of 14 kg/m3 . Fishes were fed daily at 1% of the fish weight with pellet labelled with 15 N-leucine. The 15 N incorporation was determined 14 hours after the feeding in all the fish organs. Results showed that 14 hours after the administration of a single dose of labelled food to Atlantic salmon enables the detection of the tracer in the whole organism allowing determining the food consumption. Through the analysis of nitrogen use efficiency (NUE), we showed that the trunk, pyloric caeca and head incorporate the highest level of the marker (72.7, 8.7 and 5.7%, respectively). This methodology would permit monitoring feeding to minimize food loss, improve administration methodologies or select the preferred foods for the fish, among others to reduce production costs.

M. jannaschii FtsZ, a key protein in bacterial cell division, is inactivated by peroxyl radical-mediated methionine oxidation.

Oxidation and inactivation of FtsZ is of interest due to the key role of this protein in bacterial cell division. In the present work, we studied peroxyl radical (from AAPH, 2,2'-azobis(2-methylpropionamidine)dihydrochloride) mediated oxidation of the highly stable FtsZ protein (MjFtsZ) from M. jannaschii, a thermophilic microorganism. MjFtsZ contains eleven Met, and single Tyr and Trp residues which would be expected to be susceptible to oxidation. We hypothesized that exposure of MjFtsZ to AAPH-derived radicals would induce Met oxidation, and cross-linking (via di-Tyr and di-Trp formation), with concomitant loss of its functional polymerization and depolymerization (GTPase) activities. Solutions containing MjFtsZ and AAPH (10 or 100 mM) were incubated at 37 °C for 3 h. Polymerization/depolymerization were assessed by light scattering, while changes in mass were analyzed by SDS-PAGE. Amino acid consumption was quantified by HPLC with fluorescence detection, or direct fluorescence (Trp). Oxidation products and modifications at individual Met residues were quantified by UPLC with mass detection. Oxidation inhibited polymerization-depolymerization activity, and yielded low levels of irreversible protein dimers. With 10 mM AAPH only Trp and Met were consumed giving di-alcohols, kynurenine and di-Trp (from Trp) and the sulfoxide (from Met). With 100 mM AAPH low levels of Tyr oxidation (but not di-Tyr formation) were also observed. Correlation with the functional analyses indicates that Met oxidation, and particularly Met164 is the key driver of MjFtsZ inactivation, probably as a result of the position of this residue at the protein-protein interface of longitudinal interactions and in close proximity to the GTP binding site.

Free radicals derived from γ-radiolysis of water and AAPH thermolysis mediate oxidative crosslinking of eGFP involving Tyr-Tyr and Tyr-Cys bonds: the fluorescence of the protein is conserved only towards peroxyl radicals.

The enhanced green fluorescent protein (eGFP) is one of the most employed variants of fluorescent proteins. Nonetheless little is known about the oxidative modifications that this protein can undergo in the cellular milieu. The present work explored the consequences of the exposure of eGFP to free radicals derived from γ-radiolysis of water, and AAPH thermolysis. Results demonstrated that protein crosslinking was the major pathway of modification of eGFP towards these oxidants. As evidenced by HPLC-FLD and UPLC-MS, eGFP crosslinking would occur as consequence of a mixture of pathways including the recombination of two protein radicals, as well as secondary reactions between nucleophilic residues (e.g. lysine, Lys) with protein carbonyls. The first mechanism was supported by detection of dityrosine and cysteine-tyrosine bonds, whilst evidence of formation of protein carbonyls, along with Lys consumption, would suggest the formation and participation of Schiff bases in the crosslinking process. Despite of the degree of oxidative modifications elicited by peroxyl radicals (ROO•) generated from the thermolysis of AAPH, and free radicals generated from γ-radiolysis of water, that were evidenced at amino acidic level, only the highest dose of γ-irradiation (10 kGy) triggered significant changes in the secondary structure of eGFP. These results were accompanied by the complete loss of fluorescence arising from the chromophore unit of eGFP in γ-irradiation-treated samples, whereas it was conserved in ROO•-treated samples. These data have potential biological significance, as this fluorescent protein is widely employed to study interactions between cytosolic proteins; consequently, the formation of fluorescent eGFP dimers could act as artifacts in such experiments.

Theoretical rationalisation of the photophysics of a TICT excited state of cinnamoyl-coumarin derivatives in homogeneous and biological membrane models.

A new hybrid cinnamoyl-coumarin probe was synthesised to study the formation and dynamics of a twisted internal charge transfer (TICT) excited state in homogeneous and biological membrane models. This probe showed a large bathochromic shift of the fluorescence band with the solvent polarity, which is associated with the decrease in the fluorescence intensity due to fast non-radiative deactivation pathways, ascribed to TICT excited state formation in polar solvents. The calculated potential energy surfaces using density functional theory (DFT) and time dependent-DFT (TD-DFT) along with the energetic barriers calculated using the ABF methodology established the energy requirements for a rotational twisting of the cinnamoyl-coumarin bond for TICT excited state formation. This strategy has allowed estimating the role of the ground state conformation and excited state distribution that, concomitant with fluorescence lifetime measurements, describes in detail dual fluorescence emission from TICT and ICT excited states. Moreover, the high sensitivity of fluorescence lifetimes of the TICT excited state in liposomes allows us to propose the use of this type of probes as a powerful tool for the study of gel and crystalline liquid phases in lipid membrane models. The development of this new approach will allow rationalizing and understanding the photochemical behavior of fluorescent TICT-based probes in constrained biological environments.

Aggregation of α- and ß- caseins induced by peroxyl radicals involves secondary reactions of carbonyl compounds as well as di-tyrosine and di-tryptophan formation.

The present work examined the role of Tyr and Trp in oxidative modifications of caseins, the most abundant milk proteins, induced by peroxyl radicals (ROO•). We hypothesized that the selectivity of ROO• and the high flexibility of caseins (implying a high exposure of Tyr and Trp residues) would favor radical-radical reactions, and di-tyrosine (di-Tyr) and di-tryptophan (di-Trp) formation. Solutions of α- and ß-caseins were exposed to ROO• from thermolysis and photolysis of AAPH (2,2'-azobis(2-methylpropionamidine)dihydrochloride). Oxidative modifications were examined using electrophoresis, western blotting, fluorescence, and chromatographic methodologies with diode array, fluorescence and mass detection. Exposure of caseins to AAPH at 37 °C gave fragmentation, cross-linking and protein aggregation. Amino acid analysis showed consumption of Trp, Tyr, Met, His and Lys residues. Quantification of Trp and Tyr products, showed low levels of di-Tyr and di-Trp, together with an accumulation of carbonyls indicating that casein aggregation is, at least partly, associated with secondary reactions between carbonyls and Lys and His residues. AAPH photolysis, which generates a high flux of free radicals increased the extent of formation of di-Tyr in both model peptides and α- and ß- caseins; di-Trp was only detected in peptides and α-casein. Thus, in spite of the high flexibility of caseins, which would be expected to favor radical-radical reactions, the low flux of ROO• generated during AAPH thermolysis disfavours the formation of dimeric radical-radical cross-links such as di-Tyr and di-Trp, instead favoring other O2-dependent crosslinking pathways such as those involving secondary reactions of initial carbonyl products.

Reaction Kinetics of Phenolic Antioxidants toward Photoinduced Pyranine Free Radicals in Biological Models.

8-Hydroxy-1,3,6-pyrenetrisulfonic acid (pyranine, PyOH) free radicals were induced by laser excitation at visible wavelengths (470 nm). The photochemical process involves photoelectron ejection from PyO- to produce PyO• and PyO•- with maxima absorption at 450 and 510 nm, respectively. The kinetic rate constants for phenolic antioxidants with PyO•, determined by nanosecond time-resolved spectroscopy, were largely reliant on the ionic strength depending on the antioxidant phenol/phenolate dissociation constant. Further, the apparent rate constant measured in the presence of Triton X100 micelles was influenced by the antioxidant partition between the micelle and the dispersant aqueous media but limited by its exit rates from the micelle. Similarly, the rate reaction between ascorbic acid and PyO• was markedly affected by the presence of human serum albumin responding to the dynamic of the ascorbic acid binding to the protein.

Effect of dye localization and self-interactions on the photosensitized generation of singlet oxygen by rose bengal bound to bovine serum albumin.

The spectroscopic and photophysical properties of rose bengal (RB) encased in bovine serum albumin (BSA) have been examined to evaluate the photosensitized generation of singlet molecular oxygen ((1)O2). The results show that RB photophysical and photosensitizing properties are highly modulated by the average number of dye molecules per protein (n). At n ≪ 1, the dye molecule is tightly located into the hydrophobic nanocavity site I of the BSA molecule with a binding constant Kb = 0.15 ± 0.01 µM(-1). The interaction with surrounding amino acids induces heterogeneous decay of both singlet and triplet excited states of RB and partially reduce its triplet quantum yield as compared with that in buffer solution. However, despite of the diffusive barrier imposed by the protein nanocavity to (3)O2, the quenching of (3)RB(∗):BSA generates (1)O2 with quantum yield ΦΔ = 0.35 ± 0.05. In turns, the intraprotein generated (1)O2 is able to diffuse through the bulk solution, where is dynamically quenched by BSA itself with an overall quenching rate constant of 7.3 × 10(8) M(-1) s(-1). However, at n>1, nonspecific binding of up to ≈ 6RB molecules per BSA is produced, allowing efficient static quenching of excited states of RB preventing photosensitization of (1)O2. These results provide useful information for development of dye-protein adducts suitable for using as potential intracellular photosensitizers.

Use of pyrogallol red and pyranine as probes to evaluate antioxidant capacities towards hypochlorite.

Hypochlorite is a strong oxidant able to induce deleterious effects in biological systems. The goal of this work was to investigate the use of PGR and PYR as probes in assays aimed at evaluating antioxidant activities towards hypochorite and apply it to plant extracts employed in Chilean folk medicine. The consumption of PGR and PYR was evaluated from the decrease in the visible absorbance and fluorescence intensity, respectively. Total phenolic content was determined by the Folin Ciocalteau assay. PGR and PYR react with hypochlorite with different kinetics, being considerably faster the consumption of PGR. Different stoichiometric values were also determined: 0.7 molecules of PGR and 0.33 molecules of PYR were bleached per each molecule of added hypochlorite. Both probes were protected by antioxidants, but the rate of PGR bleaching was too fast to perform a kinetic analysis. For PYR, the protection took place without changes in its initial consumption rate, suggesting a competition between the dye and the antioxidant for hypochlorite. Plant extracts protected PYR giving a PYR-HOCl index that follows the order: Fuchsia magellanica ≈ Marrubium vulgare ≈ Tagetes minuta > Chenopodium ambrosoides ≈ Satureja montana > Thymus praecox. Based on both the kinetic data and the protection afforded by pure antioxidants, we selected PYR as the best probe. The proposed methodology allows evaluating an antioxidant capacity index of plant extracts related to the reactivity of the samples towards hypochlorite.

Special issue dedicated to the memory of Elsa Beatriz Abuin Saccomano (1942-2012).

In memoriam of Elsa Abuin (1942-2012), teacher, mentor and friend. This Special Issue presents a collection of review articles and papers dedicated to the memory of Elsa Abuin and most of them presented during the Meeting of the Latin-American photochemists (Encuentro Latino-Americano de Fotoquímica y Fotobiología, ELAFOT) held in October 2012 in Córdoba, Argentina.

Coumarin 314 free radical cation: formation, properties, and reactivity toward phenolic antioxidants.

We have explored the photogeneration of the coumarin 314 radical cation by using nanosecond laser excitation at wavelengths longer than 400 nm in benzene, acetonitrile, dichloromethane, and aqueous media. In addition, time-resolved absorption spectroscopy measurements allowed detection of the triplet excited state of coumarin 314 (C(314)) with a maximum absorption at 550 nm in benzene. The triplet excited state has a lifetime of 90 µs in benzene. It is readily quenched by oxygen (k(q) = 5.0 × 10(9) M(-1) s(-1)). From triplet-triplet energy transfer quenching experiments, it is shown that the energy of this triplet excited state is higher than 35 kcal/mol, in accord with the relatively large singlet oxygen quantum yield (Φ(Δ) = 0.25). However, in aqueous media, the coumarin triplet was no longer observed, and instead of that, a long-lived (160 µs in air-equilibrated solutions) free radical cation with a maximum absorbance at 370 nm was detected. The free radical cation generation, which has a quantum yield of 0.2, occurs by electron photoejection. Moreover, density functional theory (DFT) calculations indicate that at least 40% of the electronic density is placed on the nitrogen atom in aqueous media, which explains its lack of reactivity toward oxygen. On the other hand, rate constant values close to the diffusion rate limit in water (>10(9) M(-1) s(-1)) were found for the quenching of the C(314) free radical cation by phenolic antioxidants. The results have been interpreted by an electron-transfer reaction between the phenolic antioxidant and the radical cation where ion pair formation could be involved.

Electrochemical and spectroscopic study of pyranine fluorescent probe: role of intermediates in pyranine oxidation.

8-Hydroxy-1,3,6-pyrenetrisulfonic acid (pyranine) is a hydrophilic pyrene derivative, highly reactive toward free radicals, that has been widely used in methodologies for the evaluation of antioxidant capability and the monitoring of free-radical polymer processes. In this work, we studied and characterized the electrochemical oxidation of pyranine by cyclic voltammetry, chronocoulometry, and spectroelectrochemical techniques. The electrochemical oxidation of pyranine leads to the formation of a pyranine free radical (PyO•) that is easily detectable by spectroelectrochemistry measurements. This oxidation process takes place through a CEC (chemical-electrochemical-chemical) mechanism where the chemical step previous to the oxidation corresponds to an acid-base equilibrium. A reversible diffusion-controlled electron-transfer process observed at high pH suggests that the charge-transfer process is followed by a chemical step associated with secondary PyO• dimerization-disproportionation chemical reactions. Surprisingly, pyranine oxidation mediated by peroxyl radicals showed a change in the stoichiometry of the reaction at pH values close to the pK(a) of pyranine. This behavior was attributed to the different reactivities of the ionized (PyO(-)) and the nonionized (PyOH) phenolic forms of pyranine for peroxyl radicals, affecting the role of secondary reactions at different pH values and, therefore, the measured stoichiometry.

Photophysics and photochemistry of dyes bound to human serum albumin are determined by the dye localization.

The photophysics and photochemistry of rose bengal (RB) and methylene blue (MB) bound to human serum albumin (HSA) have been investigated under a variety of experimental conditions. Distribution of the dyes between the external solvent and the protein has been estimated by physical separation and fluorescence measurements. The main localization of protein-bound dye molecules was estimated by the intrinsic fluorescence quenching, displacement of fluorescent probes bound to specific protein sites, and by docking modelling. All the data indicate that, at low occupation numbers, RB binds strongly to the HSA site I, while MB localizes predominantly in the protein binding site II. This different localization explains the observed differences in the dyes' photochemical behaviour. In particular, the environment provided by site I is less polar and considerably less accessible to oxygen. The localization of RB in site I also leads to an efficient quenching of the intrinsic protein fluorescence (ascribed to the nearby Trp residue) and the generation of intra-protein singlet oxygen, whose behaviour is different to that observed in the external solvent or when it is generated by bound MB.

Oxidative damage in lymphocytes of copper smelter workers correlated to higher levels of excreted arsenic.

Arsenic has been associated with multiple harmful effects at the cellular level. Indirectly these defects could be related to impairment of the integrity of the immune system, in particular in lymphoid population. To characterize the effect of Arsenic on redox status on this population, copper smelter workers and arsenic unexposed donors were recruited for this study. We analyzed urine samples and lymphocyte enriched fractions from donors to determinate arsenic levels and lymphocyte proliferation. Moreover, we studied the presence of oxidative markers MDA, vitamin E and SOD activity in donor plasma. Here we demonstrated that in human beings exposed to high arsenic concentrations, lymphocyte MDA and arsenic urinary levels showed a positive correlation with SOD activity, and a negative correlation with vitamin E serum levels. Strikingly, lymphocytes from the arsenic exposed population respond to a polyclonal stimulator, phytohemaglutinin, with higher rates of thymidine incorporation than lymphocytes of a control population. As well, similar in vitro responses to arsenic were observed using a T cell line. Our results suggest that chronic human exposure to arsenic induces oxidative damage in lymphocytes and could be considered more relevant than evaluation of T cell surveillance.

Antioxidant reactivity toward nitroxide probes anchored into human serum albumin. A new model for studying antioxidant repairing capacity of protein radicals.

A new strategy to evaluate accessibility of antioxidants to radical proteins has been developed using nitroxide prefluorescent probes anchored into human serum albumin (HSA). Binding association constants for the nitroxide probes C(343)T and QT with HSA were 5 x 10(4) and 9 x 10(4)M(-1), respectively. Rate constants for the nitroxide reduction by antioxidants in HSA were determined finding k(HSA)/k(buffer) ratio of 0.8, 1.9, and 0.075 for ascorbic acid, Trolox, and caffeic acid, respectively, for the nitroxide C(343)T reduction.

Photophysics and photochemistry of rose bengal bound to human serum albumin.

Rose bengal (RB) readily binds to human serum albumin (HSA). At low RB concentrations, 90% of the dye is associated to the protein (5 microM), This association takes place in specific binding sites I and/or II. At higher RB concentrations, unspecific binding takes place with up to 10 RB molecules bound per protein molecule. The behavior of excited RB molecules bound to HSA is widely different to that observed in aqueous solution. Furthermore, the data also show that the behavior of bound RB molecules changes with the average number of dye molecules per protein (n). In particular, when n is large, the fluorescence yield is significantly reduced and no measurable long-lived triples and free singlet oxygen formation from bound dyes is detected. These results are related to self-quenching of the singlet and, most likely, excited triplets. All results point to the relevance of intra-protein generated singlet oxygen. However, when the dye is bound to the protein, at low oxygen concentrations such as those prevailing in vivo, trapping by oxygen of the triplet becomes inefficient and type I processes could contribute to the observed photoprocesses.

Photophysics and photochemistry of zinc phthalocyanine/bovine serum albumin adducts.

Zinc phthalocyanine (ZnPc) is a well known Type II (singlet oxygen mediated) hydrophobic photosensitizer with potential use in PDT. We have found that the presence of bovine serum albumin diminishes the aggregation degree of ZnPc in aqueous solution, indicating that albumins could be potentially useful carriers for this type of photosensitizer in PDT. In order to explore the photochemical and photophysical behavior of ZnPc associated to the protein, we have evaluated triplet excited state lifetime and yield, dye bleaching, oxygen consumption, formation of carbonyls and peroxides, and the spontaneous chemiluminescence emitted after photolysis. The results show that dye association to BSA modifies the photophysics and photochemistry of ZnPC. In particular the decreased yield of long lived triplets suggests singlet state and/or static triplet quenching of the bound dye by the host protein.

Photophysical properties of the prefluorescent nitroxide probes QT and C343T.

The photophysical properties of the nitroxide prefluorescent probes 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (QT) and 2,3,4,6,7,8-hexahydro-quinolizino [1,10,9-gh] coumarin-3-carboxylic acid (1-oxyl-2,2,6,6-tetramethyl-piperidin-4-yl) ester free radical (C343T) were evaluated as a function of pH and solvent properties. The absorbance of QT showed high pH sensitivity. The pKa values for the different ionization forms involved in the acid-base equilibrium of the quinoline chromophore were determined in the ground and excited states. The fluorescence lifetimes of QT, and N-hydroxylamine (QTH) and quinoline methyl ester (QMe) derivatives, showed that the intramolecular quenching efficiency by the nitroxide moiety is independent of the quinoline ionization form. The fluorescence and absorbance of C343T were highly sensitive to solvent polarity in agreement with a charged transfer excited state of the chromophore. However, we noted a decrease in the intramolecular fluorescence quenching efficiency by the nitroxide moiety when changing the polarity of the solvent from hexane to water. This behavior has been attributed to a suppression of an energy transfer mechanism in the nitroxide quenching process in very polar solvents. The results obtained in micelles allow us to propose QT and C343T as sensors for pH and micropolarity, respectively, in addition to their role as monitors for free radicals or hydrogen transfer from phenols.

Competitive kinetics in free radical reactions of cinnamic acid derivatives. Influence of phenoxyl radicals reactions.

Relative rates of consumption of caffeic, ferulic and sinapic acids by 2,2'-azobis(2-amidine propane) derived peroxyl radicals has been measured in parallel experiments employing a single substrate and in competitive experiments. Rates of consumption measured in independent experiments at low substrate concentrations (first order limit) follow the order: sinapic > ferulic > caffeic. In agreement with this, in competitive experiments employing simultaneously sinapic and caffeic acid the former compound is consumed considerably faster. On the other hand, in competitive experiments employing ferulic and caffeic acids over a wide range of experimental conditions, caffeic acid is consumed considerably faster than ferulic acid, a result that contrasts with that obtained when both compounds are reacted independently. These apparently anomalous results are interpreted in terms of secondary reactions of the phenol-derived radicals. In particular, hydrogen transfer among phenoxyl radicals and the phenols and fast reactions (disproportionation) of caffeic acid derived radicals could explain these discrepancies.

The ompW (porin) gene mediates methyl viologen (paraquat) efflux in Salmonella enterica serovar typhimurium.

Porins are channels that enable passive diffusion of hydrophilic solutes, nutrients and toxins through the outer bacterial membrane. This explains in part the ability of Gram-negative microorganisms to grow in several different environments, as well as their drug resistance. OmpD is an outer membrane channel that works with the inner membrane pump YddG to expel methyl viologen (MV) from Salmonella enterica serovar Typhimurium; this occurs independently of SmvA, also involved in MV resistance. On the other hand, DeltatolC strains show increased MV resistance when compared to wild-type cells, suggesting that there may be other porin(s) that could function with SmvA to pump MV out of S. typhimurium. A strong candidate is OmpW. Here we show that DeltaompW strains of S. typhimurium are 2.5-fold more sensitive to MV than the wild-type strain. Transcriptional fusions replacing ompW by lacZ show that ompW is induced at least 2-fold in the presence of MV. This result was observed both at the mRNA and protein levels, suggesting that ompW participates in MV resistance. In addition, DeltasmvADeltaompW strains are not fully complemented by smvA, suggesting that OmpW may function through an independent pathway different from that used by SmvA to move MV outside the cell.