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131.
Konrad Skotnicki Ireneusz Janik Klaudia Sadowska Grazyna Leszczynska Krzysztof Bobrowski 《Molecules (Basel, Switzerland)》2022,27(1)
One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl (●OH) and azide (●N3) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of ●OH with 2-SeU are dominated by an absorption band with an λmax = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2●). The dimeric radical (2●) is formed in the reaction of a selenyl-type radical (6●) with 2-SeU, and both radicals are in equilibrium with Keq = 1.3 × 104 M−1 at pH 4 (below the pKa of 2-SeU). Similar equilibrium with Keq = 4.4 × 103 M−1 was determined for pH 10 (above the pKa of 2-SeU), which admittedly involves the same radical (6●) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2●−). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λmax = 390 nm, which was assigned to the ●OH adduct to the double bond at C5 carbon atom (3●) based on DFT calculations. Similar spectral and kinetic features were also observed during the ●N3-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU. 相似文献
132.
Living cationic polymerization of alkenes is discussed in terms of rapid dynamic equilibration between carbenium ions and either covalent species or onium ions. Rates of exchange between these species have strong effect on polydispersities of the synthesized polymers. In the discussed living systems, rates of exchange are faster than rates of propagation. Propagation proceeds via classic carbocationic mechanism. 相似文献
133.
Krzysztof Matyjaszewski 《Macromolecular Symposia》1992,60(1):107-117
The importance of exchange reactions between various types of active species present in the cationic polymerization of alkenes is addressed. Reactivities of carbocations, corresponding ion pairs, onium ions, and covalent species are discussed for model systems and in real polymerization. The influence of various parameters on the equilibria between carbenium ions, onium ions, and covalent species is described. Rate constants of the isomerization of active sites are estimated and correlated with polydispersities. General ways to improve control of cationic polymerization of alkenes are discussed. 相似文献
134.
Jerzy Chrusciel Krzysztof Matyjaszewski 《Journal of polymer science. Part A, Polymer chemistry》1996,34(11):2243-2252
Ring opening of 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane in the presence of various transition metal catalysts has been studied. Pd and Pt compounds are most active. Compounds with bulky ligands [e.g., Pd(PPh3)4] lead to the formation of “dimeric” cyclooctasilanes. Compounds with less bulky groups convert monomer to dimers and mixture of oligomers and polymers. Reactions with the all-trans monomer, tttt-(MePhSi)4, result in the formation of stereoregular dimer, presumably all-trans, tttttttt-(MePhSi)8, in high yields. Reactions in the presence of oxygen lead to the formation of polymers with inserted oxygen atoms in the main chain. © 1996 John Wiley & Sons, Inc. 相似文献
135.
N. Kent Dalley Weiming Jiang Krzysztof E. Krakowiak Geng Wu D. Wade Walke Jerald S. Bradshaw Reed M. Izatt 《Journal of inclusion phenomena and macrocyclic chemistry》1990,8(3):299-308
Hydrazino-crown ethers have been synthesized in only 3 or 4 steps starting from 1,2-diacetylhydrazine. The X-ray crystal structure of protonated hydrazino-19-crown-7 (2) showed that one of the hydrazino nitrogen atoms was directed outside the ring cavity. A solvent methanol molecule is held in the cavity of the host ligand by three hydrogen bonds involving two hydrogen atoms bonded to nitrogens of the ligand and the alcohol hydrogen of the methanol. The logK values for the interaction of2 with CH3NH
+
3
, Ag+, Pb2+, and Cd2+ were much less than those for the interaction of symmetrical triaza-l8-crown-6 (5) with the same cations. Hydrazino-crown2 reduced silver ions to silver metal when a solution of2 and silver ions in DMSO was allowed to stand for several days. 相似文献
136.
Magorzata Kapelko-eberska Marta Meisel Krzysztof Buksa Artur Gryszkin Antoni Szumny Bogna Latacz Bartosz Raszewski Tomasz Ziba 《Molecules (Basel, Switzerland)》2022,27(8)
The present study aimed to determine changes in the properties of starch triggered by its long-lasting (1, 2, 4, 7, 10, or 14 days) retention with citric acid (5 g/100 g) at a temperature of 40 °C. The starch citrates obtained under laboratory conditions had a low degree of substitution, as confirmed via NMR and HPSEC analyses. The prolonging time of starch retention with citric acid at 40 °C contributed to its increased esterification degree (0.05–0.11 g/100 g), swelling power (30–38 g/g), and solubility in water (19–35%) as well as to decreased viscosity of the starch pastes. Starch heating with citric acid under the applied laboratory conditions did not affect the course of DSC thermal characteristics of starch pasting. The low-substituted starch citrates exhibited approximately 15% resistance to amylolysis. 相似文献
137.
Krzysztof A. Topolski 《Czechoslovak Mathematical Journal》2008,58(4):927-947
We consider the initial-boundary value problem for first order differential-functional equations. We present the ‘vanishing
viscosity’ method in order to obtain viscosity solutions. Our formulation includes problems with a retarded and deviated argument
and differential-integral equations.
Supported by KBN grant 2 PO3A, 01811. 相似文献
138.
Król B Król P Pikus S Chmielarz P Skrzypiec K 《Colloid and polymer science》2010,288(12-13):1255-1269
Polyurethane cationomers were synthesised in the reaction of 4,4’-methylenebis(phenyl isocyanate) with polyoxyethylene glycol (M?= 2,000) or poly(tetrafluoroethyleneoxide-co-difluoromethylene oxide) α,ω-diisocyanate and N-methyl diethanolamine. Amine segments were built-in to the urethane-isocyanate prepolymer in the reaction with 1-bromobutane or formic acid, and then they were converted to alkylammonium cations. The obtained isocyanate prepolymers were then extended in the aqueous medium that yielded stable aqueous dispersions which were applied on the surfaces of test poly(tetrafluoroethylene) plates. After evaporation of water, the dispersions formed thin polymer coatings. 1H, 13C NMR and IR spectral methods were employed to confirm chemical structures of synthesised cationomers. Based on 1H NMR and IR spectra, the factors κ and α were calculated, which represented the polarity level of the obtained cationomers. The DSC, wide angle X-ray scattering and atom force microscopy methods were employed for the microstructural assessment of the obtained materials. Changes were discussed in the surface free energy and its components, as calculated independently according to the method suggested by van Oss–Good, in relation to chemical and physical structures of cationomers as well as morphology of coating surfaces obtained from those cationomers. Fluorine incorporated into cationomers (about 30%) contributed to lower surface free energy values, down to about 15 mJ/m2. That was caused by gradual weakening of long-range interactions within which the highest share is taken by dispersion interactions. 相似文献
139.
Krzysztof Ciesielski Janusz Pawlikowski 《Proceedings of the American Mathematical Society》2002,130(7):2007-2013
We show that the property
- (P)
- for every Darboux function there exists a continuous nowhere constant function such that is Darboux
- (A)
- for every subset of of cardinality there exists a uniformly continuous function such that ,
- (B)
- for an arbitrary function whose image contains a non-trivial interval there exists an of cardinality such that the restriction of to is uniformly continuous,
140.
Koziołkiewicz M Owczarek A Wójcik M Domański K Guga P Stec WJ 《Journal of the American Chemical Society》2002,124(17):4623-4627
A new method of analysis has allowed the exonucleolytic cleavage by human 3'-exonuclease to be determined. Hydrolysis by human plasma 3'-exonuclease proceeds with retention of configuration at phosphorus. The new method determines the sense of chirality at phosphorus in isotopomeric adenosine 5'-O-[(18)O]phosphorothioates. This is based on stereospecific two-step conversion of the mono-thionucleotide into the corresponding deoxyadenosine 5'-O-alpha-[(18)O]thiotriphosphate, followed by the use of terminal deoxyribonucleotidyl transferase and MALDI TOF mass spectrometry of the resulting elongated primer. Retention of configuration in the reaction of plasma 3'-exonuclease implies a two-step mechanism with two displacements on phosphorus. Inversion at each step leads to overall retention. 相似文献