Towards liquid EPR dosimetry using nitroxides in aqueous solution

Objective. Water-equivalent dosimeters are desirable for dosimetry in radiotherapy. The present work investigates basic characteristics of novel aqueous detector materials and presents a signal loss approach for electron paramagnetic resonance (EPR) dosimetry.Approach. The proposed principle is based on the radiation dose dependent annihilation of EPR active nitroxides (NO·) in aqueous solutions. Stable nitroxide radicals (3-Maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (MmP), 3-Carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (CmP)) in aqueous solutions containing dimethyl sulfoxide (DMSO) as an additive were filled in glass capillaries for irradiation and EPR readout. Radiation doses ranging from 1 to 64 Gy were applied with a clinical 6 MV flattening filter free photon beam. EPR readout was then performed with a X-band benchtop spectrometer. The dose response, temporal stability and reproducibility of the samples' EPR signal amplitudes as well as the influence of the nitroxide concentration between 10 and 160μM on the absolute signal loss were investigated using MmP. CmP was used to examine the dependence of the dose response on DMSO concentration between 0 and 10 vol%. An indirect effect model was fitted to the experimental data assuming irradiation induced radical reactions as the underlying mechanism.Main results. For an initial MmP concentration of 20μM, absolute EPR signal loss is linear up to a dose of 16 Gy with a yield G(-NO·) of approximately 0.4μmol J-1. Within five weeks upon sample irradiation to doses between 0 and 32 Gy relative EPR signal fluctuations were on average (126 readouts) below 1% (1σ). For c(MmP) ≥ 20μM, absolute signal loss is only weakly dependent on c(MmP), whereas it increases strongly with increasing c(DMSO) in the range 0-5 vol%. An indirect effect model is applicable to describe the reaction mechanism resulting in the observed dose response curve.Significance. Liquids consisting of nitroxides in aqueous solution and small amounts of DMSO (2 vol%) show promising basic characteristics for application as water-equivalent EPR dosimeter materials in radiotherapy. The EPR signal loss is based on an indirect effect mediated by diffusing radicals originating from the radiolysis of the water/DMSO mixture.

Elimination of oxacillin, its toxicity and antibacterial activity by using ionizing radiation

The chemical changes caused by electron beam and γ irradiations and the biochemical characteristics of degradation products of a frequently used antibiotic oxacillin were investigated and compared with those of cloxacillin by applying pulse radiolysis, chemical and biochemical oxygen demand, total organic carbon content, oxygen uptake rate, toxicity and antibacterial activity measurements. Oxacillin was found to be non-toxic, but poorly biodegradable by the mixed microbial population of the activated sludge of a wastewater treatment plant. Therefore, it can significantly contribute to the spread of β-lactam antibiotic resistant bacteria. However, the products formed by γ-irradiation were more easily biodegradable as they were utilized as nutrient source by the microbes of the activated sludge and the products did not show antibacterial activity. During irradiation treatment of aerated aqueous solutions mainly hydroxyl radicals induce the elimination of antimicrobial activity by making alterations at the bicyclic β-lactam part of these antibiotics. Since the β-lactam part is the same in oxacillin and cloxacillin, the biochemical characteristics of products of the two antibiotics are similar. The attack of hydrated electron takes place on the carbonyl groups. When the irradiation is made under anoxic conditions these reactions may also contribute considerably to alterations at the β-lactam part and thereby to the loss of antibacterial activity.

Hydroxyl Radical Generation by the H2O2/CuII/Phenanthroline System under Both Neutral and Alkaline Conditions: An EPR/Spin-Trapping Investigation

The copper–phenanthroline complex CuI(Phen)2 was the first artificial nuclease studied in biology. The mechanism responsible for this activity involves CuII(Phen)2 and H2O2. Even if H2O2/Cu systems have been extensively studied in biology and oxidative chemistry, most of these studies were carried out at physiological pH only, and little information is available on the generation of radicals by the H2O2/CuII-Phen system. In the context of paper pulp bleaching to improve the bleaching ability of H2O2, this system has been investigated, mostly at alkaline pH, and more recently at near-neutral pH in the case of dyed cellulosic fibers. Hence, this paper aims at studying the production of radicals with the H2O2/CuII-Phen system at near-neutral and alkaline pHs. Using the EPR/spin-trapping method, HO• formation was monitored to understand the mechanisms involved. DMPO was used as a spin-trap to form DMPO–OH in the presence of HO•, and two HO• scavengers were compared to identify the origin of the observed DMPO–OH adduct, as nucleophilic addition of water onto DMPO leads to the same adduct. H2O2 decomposition was enhanced by the addition of CuII–Phen (and only slightly by addition of CuSO4), reaching a level similar to the Fenton reagent at near-neutral pH. This evidences the role of Phen, which improves the effect of CuII by tuning the electronic structure and structural properties of the corresponding CuII complexes.

Contribution of indirect action to radiation-induced mammalian cell inactivation: dependence on photon energy and heavy-ion LET

The contribution of indirect action mediated by OH radicals to cell inactivation by ionizing radiations was evaluated for photons over the energy range from 12.4 keV to 1.25 MeV and for heavy ions over the linear energy transfer (LET) range from 20 keV/microm to 440 keV/microm by applying competition kinetics analysis using the OH radical scavenger DMSO. The maximum level of protection provided by DMSO (the protectable fraction) decreased with decreasing photon energy down to 63% at 12.4 keV. For heavy ions, a protectable fraction of 65% was found for an LET of around 200 keV/microm; above that LET, the value stayed the same. The reaction rate of OH radicals with intracellular molecules responsible for cell inactivation was nearly constant for photon inactivation, while for the heavy ions, the rate increased with increasing LET, suggesting a reaction with the densely produced OH radicals by high-LET ions. Using the protectable fraction, the cell killing was separated into two components, one due to indirect action and the other due to direct action. The inactivation efficiency for indirect action was greater than that for direct action over the photon energy range and the ion LET range tested. A significant contribution of direct action was also found for the increased RBE in the low photon energy region.

Kinetics Studies of Aqueous Phase Reactions of Cl Atoms and Cl2- Radicals with Organic Sulfur Compounds of Atmospheric Interest

A laser flash photolysis-long path UV-visible absorption technique has been employed to investigate the kinetics of aqueous phase reactions of chlorine atoms (Cl) and dichloride radicals (Cl2(-)) with four organic sulfur compounds of atmospheric interest, dimethyl sulfoxide (DMSO; CH3S(O)CH3), dimethyl sulfone (DMSO2; CH3(O)S(O)CH3), methanesulfinate (MSI; CH3S(O)O-), and methanesulfonate (MS; CH3(O)S(O)O-). Measured rate coefficients at T = 295 +/- 1 K (in units of M(-1) s(-1)) are as follows: Cl + DMSO, (6.3 +/- 0.6) x 10(9); Cl2(-) + DMSO, (1.6 +/- 0.8) x 10(7); Cl + DMSO2, (8.2 +/- 1.6) x 10(5); Cl2(-) + DMSO2, (8.2 +/- 5.5) x 10(3); Cl2(-) + MSI, (8.0 +/- 1.0) x 10(8); Cl + MS, (4.9 +/- 0.6) x 10(5); Cl2(-) + MS, (3.9 +/- 0.7) x 10(3). Reported uncertainties are estimates of accuracy at the 95% confidence level and the rate coefficients for MSI and MS reactions with Cl2(-) are corrected to the zero ionic strength limit. The absorption spectrum of the DMSO-Cl adduct is reported; peak absorbance is observed at 390 nm and the peak extinction coefficient is found to be 5760 M(-1) cm(-1) with a 2sigma uncertainty of +/-30%. Some implications of the new kinetics results for understanding the atmospheric sulfur cycle are discussed.