Experimental and theoretical insights into kinetics and mechanisms of hydroxyl and sulfate radicals-mediated degradation of sulfamethoxazole: Similarities and differences.

Hydroxyl radical (•OH)- and sulfate radical ()-based advanced oxidation technologies (AOTs) have been proven an effective method to remove antibiotics in wastewater treatment plants (WWTPs). This study aims to gain insights into kinetics and mechanisms of neutral sulfamethoxazole (SMX) degradation, a representative antibiotic, by •OH and using an experimental and theoretical approach. First, the second-order rate constants (k) of SMX with •OH and were determined to be (7.27 ± 0.43) × 109 and (2.98 ± 0.32) × 109 M-1 s-1 in UV/H2O2 and UV/persulfate (UV/PS) systems, respectively. The following theoretical calculations at the M06-2X level of theory revealed that addition of radicals to the benzene ring is the most favorable first-step reaction for both •OH and , but that exhibits higher energy barriers and selectivity than •OH due to steric hindrance. We further analyzed subsequent reactions and, interestingly, our findings closely corroborated HOMO/LUMO distributions of SMX to the oxidation pathways. Finally, the estimation of energy consumption for UV alone, •OH-, and -mediated oxidation processes was compared. These comparative results, for the first time, provide insights into the similarities and differences of degradation of SMX by •OH/ at the molecular level and can help improve antibiotics removal using radical based AOTs in WWTPs.

Kinetics and selectivity of permanganate chemiluminescence: a study of hydroxyl and amino disubstituted benzene positional isomers.

Examination of the chemiluminescence reactions of dihydroxybenzenes, aminophenols and phenylenediamines with acidic potassium permanganate has provided a new understanding of the relationships between analyte structure, reaction conditions, kinetics of the light-producing pathway and emission intensity, with broad implications for this widely utilised chemiluminescence detection system. Using a permanganate reagent prepared in a polyphosphate solution and adjusted to pH 2.5, large differences in the rate of reaction with different positional isomers were observed, with the meta-substituted forms reacting far slower and therefore exhibiting much lower chemiluminescence intensities in flow analysis systems. The preliminary partial reduction of permanganate to form significant concentrations of Mn(III) increased the rate of reaction with all analytes tested, resulting in comparable or (in the case of aminophenol and phenylenediamine) even greater emission intensities for the meta-isomers, demonstrating the opportunity to tune the selectivity of the reagent towards certain classes of compound or even specific positional isomers of the same compound. Using more acidic permanganate reagents, in which polyphosphates are not required, the discrepancy between the chemiluminescence intensities was still observed, but was less prominent due to the generally faster rates of reaction. The enhancement of these chemiluminescence reactions by on-line addition of formic acid or formaldehyde can in part also be attributed to the generation of significant pools of the key Mn(III) precursor to the emitting species.

Detailed FT near-infrared study of the behaviour of water and hydroxyl in sedimentary length-fast chalcedony, SiO2, upon heat treatment.

Chalcedony is a porous spatial arrangement of hydroxylated nanometre sized α-quartz (SiO(2)) crystallites. Due to micro-structural transformations upon heat treatment, the optical and mechanical properties of the rock are modified. We investigated these transformations in sedimentary length-fast chalcedony through Fourier Transform near- and mid-infrared spectroscopy using direct transmission and the reflectivity. Chemical adsorption potential and absorption of H(2)O by pores was studied after heat treatment. We found that water held in open porosity is reduced upon heat treatment to temperatures above 150°C. Silanole is noticeably lost from 250 to 300°C upwards and new bridging Si-O-Si further reduces the surface of open pores, creating a less porous material. Molecular water, resulting from the reaction Si-OH HO-Si→Si-O-Si+H(2)O creates new isolated pores within the material. At temperatures above 500°C, the samples start internal fracturing, permitting water held in isolated pores to be evacuated. These results shed light on thermal transformations in chalcedony and allow for a better understanding of mechanical transformations after heat treatment.

Evolution of algal toxicity during (photo)oxidative degradation of diuron.

In the aquatic environment and in engineered water treatment systems, organic contaminants can undergo oxidative and photochemical transformations. For an overall risk assessment, the toxicity of the resulting transformation products has to be investigated. In this study, the toxicity of degradation products of diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) formed during its degradation by four (photo)oxidative processes (direct phototransformation, triplet-induced photosensitized oxidation, oxidation by hydroxyl radicals and ozone) was investigated in buffered aqueous solution. The toxicity was evaluated using the combined algae test with Pseudokirchneriella subcapitata that determines both, specific inhibition of photosynthesis and inhibition of the growth rate. The comparison between evolution of toxicity and degradation kinetics indicated that the toxicity during all studied processes was caused predominantly by diuron whereas the formation of degradation products did not contribute to the mixture toxicity. This implies that, if any more toxic transformation products than diuron were formed, their concentration was not sufficiently high to affect the mixture toxicity, which was dominated by the parent compound diuron. On this account, no further studies on identification of degradation products and their toxicity are needed. This study presents an example of a systematic and simple first tier method to assess the toxicity of degradation products.

Protective effects of carnosine and homocarnosine on ferritin and hydrogen peroxide-mediated DNA damage.

Previous studies have shown that one of the primary causes of increased iron content in the brain may be the release of excess iron from intracellular iron storage molecules such as ferritin. Free iron generates ROS that cause oxidative cell damage. Carnosine and related compounds such as endogenous histidine dipetides have antioxidant activities. We have investigated the protective effects of carnosine and homocarnosine against oxidative damage of DNA induced by reaction of ferritin with H(2)O(2). The results show that carnosine and homocarnosine prevented ferritin/H(2)O(2)-mediated DNA strand breakage. These compounds effectively inhibited ferritin/H(2)O(2)-mediated hydroxyl radical generation and decreased the mutagenicity of DNA induced by the ferritin÷H(2)O(2) reaction. Our results suggest that carnosine and related compounds might have antioxidant effects on DNA under pathophysiological conditions leading to degenerative damage such as neurodegenerative disorders.

Investigation of pH at different dentinal sites after placement of calcium hydroxide dressing by two methods.

OBJECTIVE: The dentin pH at different sites following placement of calcium hydroxide paste using 2 different methods was evaluated. STUDY DESIGN: One hundred twenty Extracted teeth were instrumented and randomly divided into 6 different groups. Then they were dressed with calcium hydroxide by 2 different methods using paper points or Lentulo spiral, according to their respective groups, and stored. After the experimental period of time, half of the samples were cut transversally into slices and the other half split longitudinally and rinsed with distilled water. The dentin pH of cervical, middle, and apical thirds was measured from the root canal wall surface and 1 mm away from the root canal lumen in the inner dentin. The results were compared and statistically analyzed. RESULTS: The highest pH values were obtained on the root canal walls when the calcium hydroxide was placed with Lentulo spiral filler (P > .05). There was a significant reduction in pH values in the inner dentin. When the cross sections of teeth were compared at 7 days, the Lentulo group was statistically different only in the apical third. CONCLUSIONS: Placement of the calcium hydroxide paste with a Lentulo spiral with subsequent compaction with the blunt end of a sterile paper point obtained a higher pH value on the canal walls and in the inner dentin than paper points only.

The pK(a) of the internucleotidic 2'-hydroxyl group in diribonucleoside (3'-->5') monophosphates.

Ionization of the internucleotidic 2'-hydroxyl group in RNA facilitates transesterification reactions in Group I and II introns (splicing), hammerhead and hairpin ribozymes, self-cleavage in lariat-RNA, and leadzymes and tRNA processing by RNase P RNA, as well as in some RNA cleavage reactions promoted by ribonucleases. Earlier, the pK(a) of 2'-OH in mono- and diribonucleoside (3'-->5') monophosphates had been measured under various nonuniform conditions, which make their comparison difficult. This work overcomes this limitation by measuring the pK(a) values for internucleotidic 2'-OH of eight different diribonucleoside (3'-->5') monophosphates under a set of uniform noninvasive conditions by 1H NMR. Thus the pK(a) is 12.31 (+/-0.02) for ApG and 12.41 (+/-0.04) for ApA, 12.73 (+/-0.04) for GpG and 12.71 (+/-0.08) for GpA, 12.77 (+/-0.03) for CpG and 12.88 (+/-0.02) for CpA, and 12.76 (+/-0.03) for UpG and 12.70 (+/-0.03) for UpA. By comparing the pK(a)s of the respective 2'-OH of monomeric nucleoside 3'-ethyl phosphates with that of internucleotidic 2'-OH in corresponding diribonucleoside (3'-->5') monophosphates, it has been confirmed that the aglycons have no significant effect on the pK(a) values of their 2'-OH under our measurement condition, except for the internucleotidic 2'-OH of 9-adeninyl nucleotide at the 5'-end (ApA and ApG), which is more acidic by 0.3-0.4 pK(a) units.

Indole-3-propionate: a potent hydroxyl radical scavenger in rat brain.

The hydroxyl radical scavenging activity of indole-3-propionate was evaluated by kinetic competition studies with the hydroxyl radical trapping reagent 2,2'-azino-bis-(3-ethyl-benz-thiazoline-6-sulfonic acid) (ABTS) and by measuring hydroxyl radical-initiated lipid peroxidation in the rat striatum. Using ABTS, the indole was shown to act as a potent hydroxyl radical scavenger with a rate constant of 7.8x1010 mol l-1 s-1. Hydroxyl radical-initiated lipid peroxidation, determined by measuring tissue malondialdehyde formation, was inhibited dose-dependently both in vitro and in vivo. Indole-3-propionate reacts with hydroxyl radicals at a diffusion controlled rate and can thereby provide on-site protection against the oxidative damage of biomolecules induced by these highly reactive and toxic oxygen intermediates. While it remains to be established if endogenous brain tissue levels of indole-3-propionate are sufficiently high to have a significant impact on total antioxidative capacity, the compound itself or a structurally related agent may be useful as an antioxidant adjuvant to combat hydroxyl radical-mediated oxidative stress.

pH changes in root dentin over a 4-week period following root canal dressing with calcium hydroxide.

Root canals in extracted human teeth were cleaned and shaped and subsequently dressed with a calcium hydroxide root canal dressing. pH Changes in the root dentin were measured over a 4-wk period with microelectrodes in small cavities at apical and cervical levels in inner and outer dentin. The pH increased within hours in the inner dentin, peaking at pH 10.8 cervically and 9.7 apically. However, 1 to 7 days elapsed before the pH began to rise in the outer root dentin, reaching peak levels of pH 9.3 cervically and 9.0 apically after 2 to 3 wk. The results show that hydroxyl ions derived from a calcium hydroxide dressing do diffuse through root dentin. They diffuse faster and reach higher levels cervically than apically. Surface pH measurements showed that hydroxyl ions do not diffuse in more than a minor way through the intact root surface.