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In this work, attention is focused on the non-essential amino acid L-Tyrosine (TYR) hydroxylated to L-DOPA, which is the precursor to the neurotransmitters dopamine, noradrenaline (norepinephrine; NE) and adrenaline (epinephrine; EP) known as catecholamines and their interactions with redox-active Cu(II). Catecholamines have multiple functions in biological systems, including the regulation of the central nervous system, and free (unbound) redox metal ions are present in many diseases with disturbed metal homeostasis. The interaction between catecholamines and Cu(II) has been studied by means of Electron Paramagnetic Resonance spectroscopy (EPR), EPR spin trapping and UV-vis spectroscopy. The obtained spectroscopic results are supported by Density Functional Theory calculations. Only minor qualitative and quantitative changes in the UV-vis spectra of all the studied compounds have been observed following their interactions with Cu(II) ions. The low-temperature EPR spectra were more convincing and confirmed the interaction between Cu(II) ions and all the studied compounds, involving hydroxyl groups and amino nitrogens. The use of an ABTS assay revealed that the compounds under study possessed radical-scavenging activities against ABTS•+ in the order TYR < EP < DA < NE~L-DOPA. The neurotransmitters DA, NE and EP, following their interaction with Cu(II), exhibit the ability to (partially) reduce Cu(II) to Cu(I) species which was confirmed using the Cu(I) specific chelator neocuproine. EPR spin-trapping experiments revealed the suppressed formation of hydroxyl radicals (•OH) in a copper(II) catalyzed Fenton-like system in the presence of catecholamines. Only in the case of EP was autooxidation in a stock solution observed. Furthermore, the oxidation of EP is enhanced in the presence of Cu(II) ions. In conclusion, it has been confirmed that the oxidation of catecholamines in the presence of copper promotes the redox cycling process, resulting in the formation of ROS, which may, in turn, cause damage to neuronal systems. [ABSTRACT FROM AUTHOR]

Sodium hexatitanate (Na2Ti6O13) nanoparticles have been synthesized by the hydrothermal method with microwave and conventional heating, after which their photocatalytic properties toward an azo dye pollutant degradation have been investigated. Insights into the dynamics and reactivity of the species involved in the photocatalytic mechanism of the (Na2Ti6O13) samples were precisely investigated, for the first time, by X-band electron paramagnetic resonance (EPR) spectroscopy under different experimental conditions. X-ray diffraction structural analysis revealed that all samples crystallized in a monoclinic C2/m structure, with different short-range structural order according to the employed heating, as indicated by Raman. Field-emission scanning electron microscopy and transmission electron microscopy results revealed the formation of rod- and fiber-like nanoparticles with different diameters and lengths. EPR measurements indicated the presence of different Ti3+ point defects and F centers in the samples. X-ray photoelectron spectroscopy analysis proved the presence of oxygen-related defects, but no Ti3+ was detected on the surface. Spin trapping experiments monitored the generation of hydroxyl (OH·) radicals over UV-irradiation time. Various parameters contribute to the photocatalytic activity of the samples; however, the type of defect and particle morphology appeared as key factors for enhanced efficiency. Our study provides significant information about paramagnetic defects in Na2Ti6O13 materials and their role in photocatalysis to design other Ti-based photocatalysts. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) spectroscopy using sterically hindered amine is extensively applied to detect singlet oxygen ( 1 O 2 ) possibly generated in advanced oxidation processes. However, EPR-detectable 1 O 2 signals were observed in not only the 1 O 2 -dominated hydrogen peroxide (H 2 O 2 )/hypochlorite (NaClO) reaction but surprisingly also the 1 O 2 -absent Fe(II)/H 2 O 2 , UV/H 2 O 2 , and ferrate [Fe(VI)] process with even stronger intensities. By taking advantage of the characteristic reaction between 1 O 2 and 9,10-diphenyl-anthracene and near-infrared phosphorescent emission of 1 O 2 , 1 O 2 was excluded in the Fe(II)/H 2 O 2 , UV/H 2 O 2 , and Fe(VI) process. The false detection of 1 O 2 was ascribed to the direct oxidation of hindered amine to piperidyl radical by reactive species [e.g., • OH and Fe(VI)/Fe(V)/Fe(IV)] via hydrogen transfer, followed by molecular oxygen addition (forming a piperidylperoxyl radical) and back reaction with piperidyl radical to generate a nitroxide radical, as evidenced by the successful identification of a piperidyl radical intermediate at 100 K and theoretical calculations. Moreover, compared to the highly oxidative species (e.g., • OH and high-valence Fe), the much lower reactivity of 1 O 2 and the profound nonradiative relaxation of 1 O 2 in H 2 O resulted it too selective and inefficient in organic contaminant destruction. This study demonstrated that EPR-based 1 O 2 detection could be remarkably misled by common oxidative species and thereby jeopardize the understandings on 1 O 2 .

[Display omitted] • The hyperfine splitting constants (aN, aH) observed by ESR were adopted to identify free radical. • This study presents the range of hyperfine splitting constants (aN, aH) of alkyl, alkoxyl, oxygen-centered free radicals. • The transition pathway of alkyl free radical of camellia oil under heating process was revealed. In this study, the radicals formed in camellia oil upon a heating process were identified and quantified using Electron Spin Resonance (ESR) spectroscopy coupled with spin-trapping technique. PBN and DMPO were served as spin traps. The total amounts of free radicals of heated camellia oil showed an increasing trend with the extension of heating time at 140 °C, 150 °C and 160 °C. In accordance with hyperfine splitting constants (aN, aH) ─ the crucial parameter for identifying free radical species ─ of free radical, it was definitely confirmed that alkyl, alkoxyl, oxygen-centered and DMPO oxidate free radicals were present in heated camellia oil. A free radical transition pathway was proposed that alkyl free radical is initially generated, then, alkyl peroxy free radical is subsequently generated in the presence of oxygen which eventually shifts into alkoxyl free radical by means of an intermediate formed from H-capture reaction of alkyl peroxy free radical. [ABSTRACT FROM AUTHOR]

The linear-density (number of molecules on an arbitrary distance) of X-ray-induced markedly dense hydroxyl radicals (•OH) in water was estimated based on EPR spin-trapping measurement. A lower (0.13 mM–2.3 M) concentration series of DMPO water solutions and higher (1.7–6.0 M) concentration series of DMPO water solutions plus neat DMPO liquid (8.8 M as DMPO) were irradiated with 32 Gy of X-rays. Then, the yield of DMPO-OH in DMPO water solutions and the total spin-adduct of DMPO in neat DMPO were quantified. For the higher concentration DMPO series, the EPR peak area was estimated by double integration, and the baseline correction of the integral spectrum is necessary for accurate estimation of the peak area. The preparation of a suitable standard sample corresponding to the electric permittivity according to DMPO concentration was quite important for quantification of DMPO-OH, especially in DMPO concentration beyond 2 M. The linear-density of •OH generation in water by X-ray irradiation was estimated from the inflection point on the plot of the DMPO-OH yield versus DMPO linear-density. The linear-density of X-ray-induced markedly dense •OH was estimated as 1168 μm−1, which was converted to 0.86 nm as the intermolecular distance and 2.6 M as the local concentration. [ABSTRACT FROM AUTHOR]