Investigation of the coordination capabilities of epinephrine towards Fe2+ and Fe3+ cations and their redox activity

Epinefrin je kateholamin sa značajnom fiziološkom ulogom. Konformacija ovog molekula utiče na njegovu interakciju sa drugim molekulima i na njegove biološke efekte. Na fiziološkim pH vrednostima, koordinativne sposobnosti epinefrina prema gvožđu i redoks interakcije epinefrina sa gvožđem su od suštinske važnosti za razumevanje dve veoma različite pojave. Prva pojava je štetno dejstvo koje hronični psihološki/sredinski stres izaziva na nivou kardiovaskularnog sistema. Druga pojava je umrežavanje kateholaminima bogatih biopolimera i struktura. Kako bi se rasvetlile uloge rastvarača i vodoničnih veza u interakcijama epinefrina sa gvožđem, proučavana je konformacija epinefrina u vodi i polarnom rastvaraču dimetil sulfoksidu (DMSO). U ovoj disertaciji su predstavljeni rezultati proučavanja mehanizama interakcije epinefrina sa Fe2+ i Fe3+ jonima pri različitim koncentracionim odnosima na pH 7,4, odnosno na pH vrednosti koja odgovara fiziološkim uslovima. U svrhu istraživanja bioloških efekata epinefrina, u ovoj disertaciji su predstavljena ispitivanja efekta Epi-Fe3+ kompleksa na ćelije koje konstitutivno eksprimiraju adrenergičke receptore. Konformacije epinefrina u polarnim rastvaračima, vodi i DMSO, su proučavane metodama H nuklearne magnetne rezonance (1H NMR) kao i dvodimenzionalnim metodama nuklearne magnetne rezonance, i to: 1H - 1H COSY, 1H - 15N HSQC i NOESY. Na NH2 i CH2 grupama epinefrina su uočeni hemijski neekvivalentni protoni prilikom korišćenja DMSO kao rastvarača. Ove pojave nisu uočene kada je rastvarač bila voda. Analizom uticaja korišćenog rastvarača na NMR spektar, i analizom uticaja povećanja temperature uzorka na NMR spektar, dolazi se do zaključka da jedan od protona amino grupe formira jaku intramolekulsku vezu sa alifatičnom hidroksilnom grupom, koja je pak H donor drugoj intermolekularnoj vezi sa DMSO. Pomoću NOESY metode su prikupljeni podaci o prostornoj poziciji protona u bočnom lancu. Na taj način je formiran 3D model konformacije epinefrina u DMSO. Ukratko, epinefrin formira dodatni petočlani prsten koji sadrži bifurkovane intramolekulske/intermolekulske vodonične veze i zauzima strukturu oblika škorpiona, gde kateholni prsten predstavlja telo škorpiona, a bočni lanac predstavlja rep koji je povijen u smeru glave škorpiona. U vodi, kao rastvaraču, konformacija epinefrina ne poseduje intramolekulske vodonične veze pa je tada struktura ovog molekula najverovatnije definisana vodoničnim vezama sa molekulima vode. U okviru ove disertacije ispitivanjima je ustanovljeno da epinefrin sa Fe3+ jonima gradi stabilne visokospinske komplekse čija je stehiometrija 1:1 ili 3:1. Stehiometrija ovog kompleksa zavisi od odnosa koncentracija epinefrina i Fe3+ jona. Na kateholnom prstenu epinefrina atomi kiseonika predstavljaju mesto za formiranje koordinacione veze unutar fiziološki relevantnog bidentatnog 1:1 kompleksa. Fe3+ katjon ima slab uticaj na redoks osobine epinefrina. Međutim, epinefrin i Fe2+ joni grade kompleks koji je jak redukcioni agens. Posledica je redukcija O2, proizvodnja vodonik peroksida i formiranje Epi-Fe3+ kompleksa. U ovom procesu epinefrin se ne oksiduje, odnosno Fe2+ jon nije prenosilac, već donor elektrona. Oksidacija Fe2+ jona koja je katalizovana epinefrinom predstavlja moguće hemijsko objašnjenje za stresom izazvana oštećenja ćelija srca. Takođe, rezultati ovih ispitivanja su u skladu sa prethodnim istraživanjima kateholamina u polimerima i njihovih interakcija sa gvožđem, i upućuju na nove strategije za poboljšavanje efikasnosti umrežavanja kateholaminima bogatih biopolimera i struktura. U stresnim situacijama epinefrin se luči i može interagovati sa labilnim gvožđem koje se nalazi u ljudskoj plazmi. Te interakcije mogu prouzrokovati značajne patofiziološke posledice. Uiv ovoj disertaciji su prikazani rezultati istraživanja prema kojima epinefrin i Fe3+ joni na fiziološkom pH grade stabilni 1:1 bidentatni kompleks. Takođe je pokazano da na fiziološkom pH epinefrin ne degradira u prisustvu gvožđa. Utvrđeno je i da epinefrin i Fe2+ joni grade bezbojni kompleks i da je taj kompleks stabilan pri anaerobnim uslovima. Uočeno je i da epinefrin u prisustvu O2 značajno promoviše oksidaciju Fe2+ jona i formiranje Epi-Fe3+ kompleksa. Pri eksperimentima rađenim metodom ciklične voltametrije, Epi-Fe2+ kompleks je pokazao potencijal E1/2 = -582 mV (u odnosu na standardnu vodoničnu elektrodu). Ovakva vrednost E1/2 potencijala objašnjava katalizovanu oksidaciju. Interakcije sa gvožđem mogu uticati na biološke efekte/efikasnost epinefrina. Uticaj vezivanja gvožđa na biološko ponašanje epinefrina je ispitivan metodom nametnute voltaže na deliću membrane u konfiguraciji cela ćelija, u kulturi ćelija koje konstitutivno eksprimiraju adrenergičke receptore. Epinefrin je samostalno, bez značajnog prisustva gvožđa, uzrokovao povećanje amplitude struja usmerenih ka spoljašnosti ćelija, tj. povećanje izlaznih struja. Kompleks epinefrina sa Fe3+ nije izazivao slične posledice. Ovim se nameće zaključak da formiranje kompleksa sa gvožđem sprečava vezivanje epinefrina za adrenergičke receptore i njihovu posledičnu aktivaciju. Prooksidativna aktivnost Epi-Fe2+ kompleksa možda predstavlja vezu između hroničnog stresa i kardiovaskularnih problema, a labilno gvožđe u plazmi je potencijalni modulator bioloških aktivnosti liganda.Epinephrine (Epi) is a catecholamine with important physiological roles. Interactions with other molecules and associated biological effects of Epi are controlled by its molecular conformation. Coordinate interactions of epinephrine with iron at physiological pH and their redox activity are crucial for understanding two distinct phenomena. First, the adverse effects that chronic stress causes to cardiovascular system. Second, the cross-linking of biopolymers and frameworks which are rich in catecholamines. Conformation of epinephrine in polar solvents, namely in dimethyl sulfoxide (DMSO) and water, was investigated in order to shed light on effects solvents and hydrogen bonds exert on interactions of epinephrine with iron. Mechanism of epinephrine interactions with Fe2+ and Fe3+ ions was studied at different concentration ratios, at physiological pH (pH 7.4), and the results are presented in this dissertation. For the sake of exploration of biological effects of epinephrine, this dissertation also contains the results of examination of effects Epi-Fe3+ complex has on cell culture with constitutive expression of adrenergic receptors. Conformation of epinephrine in polar solvents, namely in dimethyl sulfoxide (DMSO) and water, was investigated using 1H NMR, 1H - 1H COSY, NOESY and 1H - 15N HSQC methods. When DMSO was used as a solvent, chemical and magnetic nonequivalence of protons was spotted at NH2 and CH2 groups on epinephrine. Characteristics of hydrogen bonds in DMSO were determined by studying effect which temperature rise has on NMR spectra and also analyzing influences of solvent substitution on NMR spectra. Results have shown that epinephrine induces strong intramolecular bond between one of the protons of NH2 group and the OH group on the side chain. On the other hand, the OH group on the side chain, i.e. the aliphatic OH group, presents a proton donor for intermolecular bond between epinephrine and DMSO. This phenomenon was not noticed when water was used as a solvent. 3D modelling of epinephrine molecule structure was based on information about spatial arrangement of protons, which in turn was obtained using NOESY method. Obtained 3D model shows that epinephrine in DMSO has a rigid structure that resembles the shape of a scorpion, in which the catechol ring presents the body of the scorpion and the side chain presents the tail of the scorpion. This structure is a consequence of formation of an additional five–membered ring limited by inter/intra–molecular bonds. If water is used as a solvent (instead of DMSO), epinephrine takes different and non-rigid conformation which does not possess the aforementioned intramolecular hydrogen bond. In this case, conformation of epinephrine is determined by hydrogen bonds with solvent molecules. Examinations conducted in the scope of this dissertation showed that epinephrine and Fe3+ form stable high-spin complexes in 1:1 and 3:1 stoichiometry. Stoichiometry of these depends on concentration ratio of epinephrine and Fe3+. Results acquired using Raman spectroscopy have shown that 1:1 bidentate Epi–Fe3+ complex is formed by coordinative bonding of Fe3+ ions to epinephrine molecule through O atoms on the catechol ring. Effect of Fe3+ and Fe2+ ions on redox properties of epinephrine was studied using method of cyclic voltammetry. It was observed that Fe3+ ions do not significantly affect redox properties of epinephrine, but epinephrine with Fe2+ ions forms strong reducing agent. Fe2+ ion presents electron donor that in the presence of epinephrine reduces O2 and causes production of H2O2. Specific hemism of epinephrine, which includes oxidation of Fe2+ ions, may present a mechanism that explains stress-induced cardiotoxicity and heart diseases. Also, these results can be used for improvement of synthesis and development of biopolymers.vi In stressful situations epinephrine is released and it may interact with labile iron in human blood plasma. These interactions can have potentially important (patho)physiological effects. In this dissertation, it is shown that at physiological pH epinephrine and Fe3+ build stable 1:1 high-spin bidentate complex. It is also shown that in presence of iron, at physiological pH, epinephrine does not degrade. It was observed that epinephrine and Fe2+ build colorless complex, which was stable under anaerobic conditions. In presence of O2, epinephrine significantly catalyzed oxidation of Fe2+ ions and formation of Epi-Fe3+ complex. Cyclic voltammetry results showed that the mid-point potential of Epi-Fe2+ complex equals -582 mV (vs. standard hydrogen electrode). This value of mid-point potential explains the oxidation promotion. Biological effects/efficiency of epinephrine are influenced by its interaction with iron. Iron binding effects on biological performance of epinephrine were examined using patch clamping in cell culture with constitutive expression of adrenergic receptors. Epinephrine, on its own, induced an increase of outward currents, whereas Epi-Fe3+ complex did not evoke similar phenomenon. These imply that the binding of epinephrine to adrenergic receptors and their activation is inhibited by the formation of the complex of Epi with iron. Oxidative promoting activity of Fe2+ in the presence epinephrine may represent a basis for cardiovascular problems caused by chronic stress. The results obtained in this dissertation indicate that the labile iron pool may have a new function that represents a modulation of the activity of biologically significant ligands/molecules.Epinephrine (Epi) is a catecholamine with important physiological roles. Interactions with other molecules and associated biological effects of Epi are controlled by its molecular conformation. Coordinate interactions of epinephrine with iron at physiological pH and their redox activity are crucial for understanding two distinct phenomena. First, the adverse effects that chronic stress causes to cardiovascular system. Second, the cross-linking of biopolymers and frameworks which are rich in catecholamines. Conformation of epinephrine in polar solvents, namely in dimethyl sulfoxide (DMSO) and water, was investigated in order to shed light on effects solvents and hydrogen bonds exert on interactions of epinephrine with iron. Mechanism of epinephrine interactions with Fe2+ and Fe3+ ions was studied at different concentration ratios, at physiological pH (pH 7.4), and the results are presented in this dissertation. For the sake of exploration of biological effects of epinephrine, this dissertation also contains the results of examination of effects Epi-Fe3+ complex has on cell culture with constitutive expression of adrenergic receptors. Conformation of epinephrine in polar solvents, namely in dimethyl sulfoxide (DMSO) and water, was investigated using 1H NMR, 1H - 1H COSY, NOESY and 1H - 15N HSQC methods. When DMSO was used as a solvent, chemical and magnetic nonequivalence of protons was spotted at NH2 and CH2 groups on epinephrine. Characteristics of hydrogen bonds in DMSO were determined by studying effect which temperature rise has on NMR spectra and also analyzing influences of solvent substitution on NMR spectra. Results have shown that epinephrine induces strong intramolecular bond between one of the protons of NH2 group and the OH group on the side chain. On the other hand, the OH group on the side chain, i.e. the aliphatic OH group, presents a proton donor for intermolecular bond between epinephrine and DMSO. This phenomenon was not noticed when water was used as a solvent. 3D modelling of epinephrine molecule structure was based on information about spatial arrangement of protons, which in turn was obtained using NOESY method. Obtained 3D model shows that epinephrine in DMSO has a rigid structure that resembles the shape of a scorpion, in which the catechol ring presents the body of the scorpion and the side chain presents the tail of the scorpion. This structure is a consequence of formation of an additional five–membered ring limited by inter/intra–molecular bonds. If water is used as a solvent (instead of DMSO), epinephrine takes different and non-rigid conformation which does not possess the aforementioned intramolecular hydrogen bond. In this case, conformation of epinephrine is determined by hydrogen bonds with solvent molecules. Examinations conducted in the scope of this dissertation showed that epinephrine and Fe3+ form stable high-spin complexes in 1:1 and 3:1 stoichiometry. Stoichiometry of these depends on concentration ratio of epinephrine and Fe3+. Results acquired using Raman spectroscopy have shown that 1:1 bidentate Epi–Fe3+ complex is formed by coordinative bonding of Fe3+ ions to epinephrine molecule through O atoms on the catechol ring. Effect of Fe3+ and Fe2+ ions on redox properties of epinephrine was studied using method of cyclic voltammetry. It was observed that Fe3+ ions do not significantly affect redox properties of epinephrine, but epinephrine with Fe2+ ions forms strong reducing agent. Fe2+ ion presents electron donor that in the presence of epinephrine reduces O2 and causes production of H2O2. Specific hemism of epinephrine, which includes oxidation of Fe2+ ions, may present a mechanism that explains stress-induced cardiotoxicity and heart diseases. Also, these results can be used for improvement of synthesis and development of biopolymers.vi In stressful situations epinephrine is released and it may interact with labile iron in human blood plasma. These interactions can have potentially important (patho)physiological effects. In this dissertation, it is shown that at physiological pH epinephrine and Fe3+ build stable 1:1 high-spin bidentate complex. It is also shown that in presence of iron, at physiological pH, epinephrine does not degrade. It was observed that epinephrine and Fe2+ build colorless complex, which was stable under anaerobic conditions. In presence of O2, epinephrine significantly catalyzed oxidation of Fe2+ ions and formation of Epi-Fe3+ complex. Cyclic voltammetry results showed that the mid-point potential of Epi-Fe2+ complex equals -582 mV (vs. standard hydrogen electrode). This value of mid-point potential explains the oxidation promotion. Biological effects/efficiency of epinephrine are influenced by its interaction with iron. Iron binding effects on biological performance of epinephrine were examined using patch clamping in cell culture with constitutive expression of adrenergic receptors. Epinephrine, on its own, induced an increase of outward currents, whereas Epi-Fe3+ complex did not evoke similar phenomenon. These imply that the binding of epinephrine to adrenergic receptors and their activation is inhibited by the formation of the complex of Epi with iron. Oxidative promoting activity of Fe2+ in the presence epinephrine may represent a basis for cardiovascular problems caused by chronic stress. The results obtained in this dissertation indicate that the labile iron pool may have a new function that represents a modulation of the activity of biologically significant ligands/molecules

Redox interactions of epinephrine with iron at physiological pH

Epinephrine ((R)-4-(1-hydroxy-2-(methylamino)ethyl)-benzene-1,2-diol (Epi) is catecholamine that is released by the sympathetic nervous system and adrenal medulla. It is a physiologically important molecule that acts as a hormone, neurotransmitter, and medication with a broad range of effects 1-3 . Coordinate and redox interaction of Epi with iron affects the interactions with other molecules and its biological effects 4 . In this study, we reported details of redox interactions of Epi with Fe 2+ at pH 7.4, which correspond to the pH value of human plasma Epi and Fe 2+ form a complex that acts as a strong reducing agent. Cyclic voltammetry showed that the positions of E pa and E pc potentials were at approximately -480 and -1100 mV. This implies that Epi and Fe 2+ build a complex with unique redox properties. E1/2 was significantly lower compared to E0' for O 2 /O 2•- (-350 mV). It is important to point out this because superoxide radical anion is produced via spontaneous Fe 2+ reaction with O 2. In other words, Epi-Fe 2+ complex should be capable of reducing transition metals in (patho)physiologicaly relevant complexes that are not susceptible to reduction by O 2. Our results confirmed that Epi-Fe 2+ is capable of reducing the S-S group of glutathione disulfide. On the other hand, Epi acted in a catalyst-like fashion to promote Fe 2+ oxidation by molecular oxygen, and to a facilitated formation of the Epi–Fe 3+ complexes, at physiological pH. In addition, we examined the effects of epinepfrine and Epi/Fe3+ system on glioma cells. Epinephrine alone evokes changes in the membrane currents of glioma cells, but such effects were not observed for the complex with Fe 3+ . This implies that Epi-Fe 3+ might modulate neural activity of Epi in CNS

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

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

Supplementary information for the article: Korać Jačić, J.; Milenković, M. R.; Bajuk-Bogdanović, D.; Stanković, D.; Dimitrijević, M.; Spasojević, I. The Impact of Ferric Iron and PH on Photo-Degradation of Tetracycline in Water. Journal of Photochemistry and Photobiology A: Chemistry 2022, 433, 114155. https://doi.org/10.1016/j.jphotochem.2022.114155.

There is a significant interest in understanding coordination and photo-chemistry of tetracycline antibiotics, primarily in relation to the development of advanced oxidation processes for degradation of these pollutants in water processing. Herein we analyzed the pH-dependence of interactions of tetracycline with ferric iron and photosensitivity of tetracycline to UV-A and UV-B, using a set of methods – UV–vis, Raman and electron paramagnetic resonance spectroscopy, MS spectrometry, HPLC, and cyclic voltammetry. Tetracycline and Fe3+ mainly bind through amide and OH groups in tricarbonylamide moiety to form a stable complex with 1:1 stoichiometry at pH ≤ 5. The interaction is reversible and tetracycline is released from the complex with pH increase. Tetracycline in the complex is stabilized and less susceptible than free tetracycline to oxidation by hydroxyl radical that is produced by UV-induced photolysis of Fe3+–OH- complexes. Redox properties of tetracycline were altered with increasing pH and it showed increased susceptibility to UV-induced degradation. In close, the system composed of tetracycline and ferric iron shows coordination and photo-redox chemistry that is dependent of pH in relation to the solubility of Fe3+ species and protonation of tetracycline. The development and optimization of advanced oxidation processes should take into account that iron may bind and stabilize pollutants and that the redox landscape of water changes drastically with pH.Supplementary material for: [https://doi.org/10.1016/j.jphotochem.2022.114155]Related to published version: [https://cherry.chem.bg.ac.rs/handle/123456789/5511

The impact of ferric iron and pH on photo-degradation of tetracycline in water

There is a significant interest in understanding coordination and photo-chemistry of tetracycline antibiotics, primarily in relation to the development of advanced oxidation processes for degradation of these pollutants in water processing. Herein we analyzed the pH-dependence of interactions of tetracycline with ferric iron and photosensitivity of tetracycline to UV-A and UV-B, using a set of methods – UV–vis, Raman and electron paramagnetic resonance spectroscopy, MS spectrometry, HPLC, and cyclic voltammetry. Tetracycline and Fe3+ mainly bind through amide and OH groups in tricarbonylamide moiety to form a stable complex with 1:1 stoichiometry at pH ≤ 5. The interaction is reversible and tetracycline is released from the complex with pH increase. Tetracycline in the complex is stabilized and less susceptible than free tetracycline to oxidation by hydroxyl radical that is produced by UV-induced photolysis of Fe3+–OH- complexes. Redox properties of tetracycline were altered with increasing pH and it showed increased susceptibility to UV-induced degradation. In close, the system composed of tetracycline and ferric iron shows coordination and photo-redox chemistry that is dependent of pH in relation to the solubility of Fe3+ species and protonation of tetracycline. The development and optimization of advanced oxidation processes should take into account that iron may bind and stabilize pollutants and that the redox landscape of water changes drastically with pH.Supplementary material: [https://cherry.chem.bg.ac.rs/handle/123456789/5512

Coordination of hydralazine with Cu2+ at acidic pH promotes its oxidative degradation at neutral pH

Hydralazine (HL), a frequently prescribed oral antihypertensive drug, shows redox interactions with transition metals such as copper that are not fully understood. Copper may be present at high concentrations in the digestive tract and can affect oral drugs. An important parameter for such interactions is pH, which changes from acidic in the gastric juice to neutral pH in intestines. In this study, we examined interactions of HL with Cu2+ ions in conditions that mimic pH shift in the digestive tract using UV–Vis, Raman and EPR spectroscopy, cyclic voltammetry and oximetry. In the acidic solution, Cu2+ formed a stable mononuclear complex with two bidentate coordinated HL molecules. On the other hand, at neutral pH, Cu2+ initiated oxidation and degradation of HL. The degradation was more rapid in the HL-Cu2+ system that was initially prepared at acidic pH and then shifted to neutral pH. The formation of the complex at acidic pH increases the availability of Cu2+ for redox reactions after the shift to neutral pH at which Cu2+ is poorly soluble. These results imply that the change of pH along the digestive tract may promote HL degradation by allowing the formation of the complex at gastric pH which makes Cu2+ available for subsequent oxidation of HL at neutral pH.Peer-reviewed manuscript: [https://cherry.chem.bg.ac.rs/handle/123456789/5882

Ferrous iron binding to epinephrine promotes the oxidation of iron and impedes activation of adrenergic receptors

Upon release in response to stress, epinephrine (Epi) may interact with labile iron pool in human plasma with potentially important (patho)physiological consequences. We have shown that Epi and Fe3+ build stable 1:1 high-spin bidentate complex at physiological pH, and that Epi does not undergo degradation in the presence of iron. However, the interactions of Epi with the more soluble Fe2+, and the impact of iron on biological activity of Epi are still not known. Herein we showed that Epi and Fe2+ build colorless complex which is stable under anaerobic conditions. In the presence of O2, Epi promoted the oxidation of Fe2+ and the formation of Epi-Fe3+ complex. Cyclic voltammetry showed that mid-point potential of Epi-Fe2+ complex is very low (−582 mV vs. standard hydrogen electrode), which explains catalyzed oxidation of Fe2+. Next, we examined the impact of iron binding on biological performance of Epi using patch clamping in cell culture with constitutive expression of adrenergic receptors. Epi alone evoked an increase of outward currents, whereas Epi in the complex with Fe3+ did not. This implies that the binding of Epi to adrenergic receptors and their activation is prevented by the formation of complex with iron. Pro-oxidative activity of Epi-Fe2+ complex may represent a link between chronic stress and cardiovascular problems. On the other hand, labile iron could serve as a modulator of biological activity of ligands. Such interactions may be important in human pathologies that are related to iron overload or deficiency

The formation of Fe3+-doxycycline complex is pH dependent: implications to doxycycline bioavailability

The interactions of drugs with iron are of interest in relation to the potential effects of iron-rich foods and iron supplements on sorption and bioavailability. Doxycycline (DOX), a member of the tetracycline class of broad-spectrum antibiotics, is frequently administered by oral route. In the digestive tract, DOX can be exposed to iron at different pH values (stomach pH 1.5–4, duodenum pH 5–6, distal jejunum and ileum pH 7–8). In relation to this, we analyzed the impact of pH on Fe3+-DOX complex formation. The optimal conditions for Fe3+-DOX complex formation are pH = 4 and [Fe3+]/[DOX] = 6 molar ratio. HESI-MS showed that Fe3+-DOX complex has 1:1 stoichiometry. Raman spectra of Fe3+-DOX complex indicate the presence of two Fe3+-binding sites in DOX structure: tricarbonylamide group of ring A and phenolic-diketone oxygens of BCD rings. The Fe3+-DOX complex formed at pH = 4 is less susceptible to oxidation than DOX at this pH. The increase of pH induces the decomposition of Fe3+-DOX complex without oxidative degradation of DOX. The pH dependence of Fe3+-DOX complex formation may promote unwanted effects of DOX, impeding the absorption that mainly takes place in duodenum. This could further result in higher concentrations in the digestive tract and to pronounced impact on gut microbiota.This is the peer-reviewed version of the article: Korać Jačić J, Dimitrijević MS, Bajuk-Bogdanović DV, Stanković DM, Savić SD, Spasojević IB, MIlenković MR. The formation of Fe3+-doxycycline complex is pH dependent: implications to doxycycline bioavailability. in Journal of Biological Inorganic Chemistry. 2023;28:679-687. doi: [10.1007/s00775-023-02018-w]