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[en] A method is described for the analysis of the natural 14CO concentration in the atmosphere. It is based on the separation of CO from ambient air, followed by the determination of its 14C-content by proportional gas counting employing a low volume counter. The method is described in detail. The concentration of 14CO at midlatitudes (510N) exhibits a marked seasonal variation with a winter maximum of 25 +- 2 molecules/cm3 and a minimum of 11 +- 1 molecules/cm3 during summer. Measurements at different latitudes show a decrease of the 14CO concentration from the pole to the equator. Interpretations of the data are based on calculations with a 2-d-time dependent model. From the simultaneous examination of the 14CO and 12CO balance in the troposphere it is possible to derive the two most important parameters for the global cycle of atmospheric CO: the concentration of OH radicals in the troposphere - and thus the lifetime of CO - and the contribution of biological sources to the CO budget. From this work the concentration of OH radicals in the troposphere is estimated to average around 6.5 x 105 molecules/cm3, biological sources are shown to contribute about 12.5 x 1014g 12CO per year to the CO budget. (orig.)
[en] The lifetime of the excited state of a atom or molecule is often determined from the rate of fluorescence decay originating as a function of buffer gas pressure, an accurate determination is made of the rates of collision induced transitions away from the excited state. Deconvolution can in principle be employed to resolve fluorescence times shorter than the response times of the system. However, attainable reproducibility and accuracy in actual experiments usually set a limit beyond which no meaningful results are expected. Prudence thus dictates that the results of deconvolution be viewed with extreme caution whenever fluorescence time much shorter than the response of times of the system are indicated
[en] The site of attack of OH radicals on dihydrouracil and five methylated derivatives was determined by pulse radiolysis using N,N,N',N'-tetramethylphenylenediamine (TMPD) to detect oxidizing radicals and tetranitromethane (TNM) as well as K3Fe(CN)6 to detect reducing radicals. In dihydrouracil OH radicals abstract preferentially an H atom at C(6) giving the 6-yl radical (>= 90 %) which at pH equivalent to 6.5 reduces TNM and K3Fe(CN)6 at almost diffusion-controlled rates. Only a small fraction of OH radicals abstract the H atom at C(5) (<= 10%). The resulting 5-yl radical oxidizes TMPD to TMPD+ at pH 7-8. With the methylated derivatives of dihydrouracil, OH radicals react less selectively, especially in the case of N(1)-methyl derivatives. This methyl group is activated to a similar degree as the methylene group at C(6). In 1-Medihydrouracil the yield of N(1)-CH2 radicals is about 29%. Radicals at the other methyl substituents are generated to a lesser extent (<= 10%) and are relatively unreactive towards oxidizing agents such as TNM and K3Fe(CN)6 as well as towards the reducing agent, TMPD. Although methyl substitution opens new routes for OH attack the preferred site of H abstraction remains C(6) (> 60%). (author)
[en] A novel chitosan derivative with double quaternary ammonium salt.2-pyridine-acetyl-N-trimethyl chitosan chloride (PATMCS) was synthesized and the antioxidant activity of PATMCS against hydroxyl radicals was assessed. The results indicated that PATMCS had potent hydroxyl scavenging activity. The IC50 of PATMCS was 0.13 mg/mL. PATMCS showed 100% scavenging effect at a dose of 0.8 mg/mL which markedly better than that of N-trimethyl chitosan chloride (TMCS). It was confirmed that quaternary chitosan derivatives showed potent antioxidant activity. PATMCS has double quaternary ammonium salt structure in the molecules. Therefore, the antioxidant activity of PATMCS was better than TMCS. The above results are theoretically fundamental for further development and making use of chitosan resources to prepare new antioxidants
[en] The sites of reactions in 5-hydroxy-6-methyluracil (HMU) and its dimeric and tetrameric associates are studied theoretically in terms of the electrostatic potential (ESP). The ESP approach allows us to construct the most stable combinations of HMU associates and to study HMU self-assembly without simulating all possible associates. A self-assembly scheme is proposed for HMU. N1–H···O1 bonds initially form between the molecules. If the first recognition center (RC) is occupied, N3–H···O2 bonds form with the participation of the third RC. The resulting ribbons with alternating contacts 1–1 and 3–3 are cross-linked through the electrostatic attraction of hydroxyl groups and the forces of dispersion between methyl groups.
[en] Highlights: • Rate constant has been measured for the reaction of OH with C2H5O2, C3H7O2 and C4H9O2. • A fast reaction of OH radicals with I-atoms is suspected. • Fast rate constant for all peroxy radicals has been measured. The rate constants for the reaction of three peroxy radicals with OH have been measured using different precursors. Peroxy radicals have been prepared by either the reaction of Cl-atoms with C2H6, C3H8 and n-C4H10 or through photolysis of the corresponding alkyliodide. Using Cl-atoms, the following rate constants have been measured: Experiments using 248 nm photolysis of the corresponding alkyliodides as precursor enhances the rate constants by a factor of two, bringing up the suspicion of a fast reaction between I-atoms and OH radicals.