Ligand and redox - interactions of adrenaline with iron at physiological pH

Adrenaline (Adr) is catecholamine that is released by the sympathetic nervous system and adrenal medulla. It is involved in several physiological functions, including regulation of blood pressure, vasoconstriction, cardiac stimulation, and regulation of the blood glucose levels 1 . Transients of high levels of Adr in the bloodstream have been recognized for a long time as a cause of cardiovascular problems that develop under chronic exposure to psychosocial and physical stress 2,3. A number of studies have found a connection between the excess of Adr, cardiotoxic effects, and oxidative stress, that is irrespective of adrenergic receptors stimulation 2-4. The mechanism behind this involves Adr (coordinate and redox) interactions with iron, which are still not clear. Two main concepts have been proposed - Adr autooxidation and redox interactions with iron, the most abundant transition metal in human plasma 5 . Fe3+ is known to build complexes with catechols 6 , but data on Fe3+ coordinate interactions with Adr at physiological pH are missing. In addition to its (patho)physiological role, Adr is of interest from the aspect of development of catecholamine-rich biopolymers with adhesive properties and metelloorganic frameworks 7,8. The adhesion and other properties materials are based on the cross-linking via coordinate bonds with Fe3+ at pH > 7. Finally, ligands might dramatically alter the redox potential of Fe3+/Fe2+ couple 9 . It has been shown that specific ligands with high affinity for Fe3+, including some catechols, might promote the oxidation and increase the reactivity of Fe2+ with molecular oxygen 10. The aim of our study was to examine the nature of Adr interactions with Fe3+ and Fe2+: stoichiometry, sites of coordinate bonds formation and structure of complex(es), and redox activity, at pH 7.4 and different concentration ratios. The coordinate and redox interactions were investigated using UV/Vis spectrophotometry, low temperature EPR, Raman 143 spectroscopy, cyclic voltammetry, and oximetry. The stability of Adr in the studied reactions was monitored by HPLC. At pH 7.4, Adr forms complexes with Fe3+, in the 1:1, and 3:1 stoichiometry, depending on (high or low) Adr/Fe3+ concentration ratio. The high-spin Fe3+ 1:1 and 3:1 complexes show different symmetries, with the 3:1 complex displaying higher EPR spectral anisotropy. Raman spectroscopy showed that oxygen atoms on the catechol ring represent the sites of coordinate bond formation in the bidentate Adr-Fe3+ complex. The bonds appear to be stronger in the 1:1 complex, and not to share the same plane with the ring. On the other hand, Adr and Fe2+ build a complex that acts as a strong reducing agent. In the presence of O2, this leads to the production of H2O2, and to a facilitated formation of Adr/Fe3+ complexes. Adr is not oxidized in this process, i.e. iron is not an electron shuttle but electron donor. Catalyzed oxidation of Fe2+ in the presence of Adr represents a plausible chemical basis of stress-related damage of heart cells. In addition, our results imply that the application/pre-binding of Fe2+ followed by oxidation at pH > 7 might be a simple alternative strategy for promotion of cross-linking in catecholamine-rich biopolymers frameworks

Penicillamine prevents damaging redox in vitro interactions of bilirubin and copper

Toxic effects of unconjugated bilirubin (BR) in neonatal hyperbilirubinemia have been related to redox and/or coordinate interactions with Cu2+. However, the development and mechanisms of such interactions at physiological pH have not been resolved. This study shows that BR reduces Cu2+ to Cu1+ in 1:1 stoichiometry. Apparently, BR undergoes degradation, i.e. BR and Cu2+ do not form stable complexes. The binding of Cu2+ to inorganic phosphates, liposomal phosphate groups, or to chelating drug penicillamine, impedes redox interactions with BR. Cu1+ undergoes spontaneous oxidation by O2 resulting in hydrogen peroxide accumulation and hydroxyl radical production. In relation to this, copper and BR induced synergistic oxidative/damaging effects on erythrocytes membrane, which were alleviated by penicillamine. The production of reactive oxygen species by BR and copper represents a plausible cause of BR toxic effects and cell damage in hyperbilirubinemia. Further examination of therapeutic potentials of copper chelators in the treatment of severe neonatal hyperbilirubinemia is needed

A Potential Source of Free Radicals in Iodine-Based Chemical Oscillators

The iodide-peroxide system in an acidic medium was investigated as a potential source of free radicals in iodine-based chemical oscillators. The radicals were detected by EPR spintrapping using spin-trap 5-(tert-butoxycarbonyl)-5-methyl-1-pyrroline N-oxide (BMPO), which forms stable spin-adducts with oxygen-centered radicals. The iodide-peroxide system is introduced as an easily available laboratory source of free radicals

Formation of stable radicals in catechin/nitrous acid systems: Participation of dinitrosocatechin

Catechins are transformed into dinitrosocatechins (diNOcats) and then oxidized to the quinones by salivary nitrite under conditions simulating the stomach. This manuscript deals with formation of stable radicals in the NO group of diNOcat during nitrite-induced oxidation of (+)-catechin and diNOcat at pH 2. We postulated two mechanisms for the stable radical formation; one is nitrous acid-induced oxidation of diNOcat in the A-ring, and the other intermolecular charge transfer from the A-ring of diNOcat and/or diNOcat quinone to the quinone moiety of the B-ring of diNOcat quinone. In addition, an unstable phenoxyl radical, which might be transformed into quinone, was also produced, accompanying the formation of the stable radical on the NO group. Taking the above results into account, we mainly focus on the adverse effects of the radicals and quinone, which may be produced from (+)-catechin in the stomach under the conditions of high salivary nitrite concentrations

Rapid X-ray photoreduction of dimetal-oxygen cofactors in ribonucleotide reductase.

Prototypic dinuclear metal cofactors with varying metallation constitute a class of O2-activating catalysts in numerous enzymes such as ribonucleotide reductase (RNR1). Reliable structures are required to unravel the reaction mechanisms. However, protein crystallography data may be compromised by X-ray photoreduction (XPR). We studied XPR of Fe(III)Fe(III) and Mn(III)Fe(III) sites in the R2 subunit of Chlamydia trachomatis RNR using X-ray absorption spectroscopy. Rapid and biphasic XPR kinetics at 20 K and 80 K for both cofactor types suggested sequential formation of (III,II) and (II,II) species and similar redox potentials of Fe and Mn sites. Comparing with typical X-ray doses in crystallography implies that (II,II) states are reached in <1 s in such studies. First-sphere metal coordinations and metal-metal distances differed after chemical reduction at room temperature and after XPR at cryogenic temperatures, as corroborated by model structures from density functional theory calculations. The inter-metal distances in the (II,II) states, however, are similar to R2 crystal structures. Therefore, crystal data of initially oxidized R2-type proteins mostly contain photoreduced (II,II) cofactors, which deviate from the native structures functional in O2-activation, explaining observed variable metal ligation motifs. This situation may be remedied by novel femtosecond free-electron-laser protein crystallography techniques

Reactions of superoxide dismutases with HS-/H2S and superoxide radical anion: An in vitro EPR study

Interactions of hydrogen sulfide (HS-/H2S), a reducing signaling species, with superoxide dimutases (SOD) are poorly understood. We applied low-T EPR spectroscopy to examine the effects of HS-/H2S and superoxide radical anion (O-2(-)) on metallocenters of FeSOD, MnSOD, and CuZnSOD. HS-/H2S did not affect FeSOD, whereas active centers of MnSOD and CuZnSOD were open to this agent. Cu2+ was reduced to Cu1+, while manganese appears to be released from MnSOD active center. Untreated and O-2(-) treated FeSOD and MnSOD predominantly show 5 d-electron systems, i.e. Fe3+ and Mn2+. Our study provides new details on the mechanisms of (patho)physiological effects of HS-/H2S. (C) 2015 Elsevier Inc. All rights reserved.Ministry for Education, Science and Technological Development of The Republic of Serbia {[}173014, III41005

Photo-redox reactions of indole and ferric iron in water

Iron-organic interactions are involved in a variety of environmental phenomena, including photo-redox reactions, iron cycling and bioavailability, as well as contaminant fate. In this study we examined UV-induced redox reactions of iron and indole in water. The presence of one indole in the irradiated system resulted in the presence of eight reduced ferric ions, not counting direct photolysis of Fe3+ complexes with OH-, which gives Fe2+ and hydroxyl radical (HO center dot) as products. The main mechanisms that contribute to indole-related Fe3+ reduction i.e. Fe2+ accumulation are: (i) HO center dot scavenging, which prevents oxidation of Fe2+ by HO center dot; (ii) oxidation of indole and its derivatives by excited ferric iron; (iii) reduction of ferric iron by excited indole (not present under UV-A). Hydrated electrons released by UV-B-excited indole play only a minor role in the reduction of iron. Indole-derived radicals emerged as byproducts of indole/iron photochemistry. H-1 NMR and low-T EPR spectroscopy showed that indole forms a weak low-symmetry complex with Fe3+. The strongest interactions between iron and pi-cloud in the indole ring are at positions 2, 3, and 7. The formation of complex promotes electron transfer from excited indole to Fe3+. Our findings are important for understanding the catalysis of photo-reduction of iron by heterocyclic aromatic pollutants, and for the development of protocols for indole processing in wastewaters