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111.
Cojocaru B Laferrière M Carbonell E Parvulescu V García H Scaiano JC 《Langmuir : the ACS journal of surfaces and colloids》2008,24(9):4478-4481
Singlet oxygen has been characterized spectroscopically as a product of the exposure of suspensions of zeolites containing oxidation catalysts. Spectroscopic and lifetime studies show that a part of the singlet oxygen formed reacts within the zeolite porous structure, while a significant fraction escapes and becomes available for reaction in the bulk media. The liquid phase plays a key role in determining intra- and extracavity dynamics. 相似文献
112.
Petroselli G Dántola ML Cabrerizo FM Capparelli AL Lorente C Oliveros E Thomas AH 《Journal of the American Chemical Society》2008,130(10):3001-3011
UV-A radiation (320-400 nm) induces damage to the DNA molecule and its components through different photosensitized reactions. Among these processes, photosensitized oxidations may occur through electron transfer or hydrogen abstraction (type I) and/or the production of singlet molecular oxygen ((1)O2) (type II). Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have investigated the photosensitized oxidation of 2'-deoxyguanosine 5'-monophosphate (dGMP) by pterin (PT) in aqueous solution under UV-A irrradiation. Kinetic analysis was employed to evaluate the participation of both types of mechanism under different pH conditions. The rate constant of (1)O2 total quenching (k(t)) by dGMP was determined by steady-state analysis of the (1)O2 NIR luminescence, whereas the rate constant of the chemical reaction between (1)O2 and dGMP (k(r)) was evaluated from kinetic analysis of concentration profiles obtained by HPLC. The results show that the oxidation of dGMP photosensitized by PT occurs through two competing mechanisms that contribute in different proportions depending on the pH. The dominant mechanism in alkaline media involves the reaction of dGMP with (1)O2 produced by energy transfer from the PT triplet state to molecular oxygen (type II). In contrast, under acidic pH conditions, where PT and the guanine moiety of dGMP are not ionized, the main pathway for dGMP oxidation involves an initial electron transfer between dGMP and the PT triplet state (type I mechanism). The biological implications of the results obtained are also discussed. 相似文献
113.
New Copper,Palladium and Nickel Catalytic Systems: An Evolution towards More Efficient Procedures
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Raul SanMartin María Teresa Herrero Esther Domínguez 《Chemical record (New York, N.Y.)》2016,16(3):1082-1095
Metal‐catalysed reactions are a fundamental tool in synthetic chemistry. Increasingly challenging transformations can be accomplished only by means of certain metal catalysts. However, there still remains the need for a substantial decrease of the amount of catalyst, for better reuse or recycling of such active species, and for the avoidance of relatively toxic solvents in favour of environmentally friendly media. These facts apply to copper‐, palladium‐, and nickel‐catalysed cross‐coupling reactions, direct arylations, and oxidative processes. This account summarises our research on the last reactions, featuring an evolution towards more sustainable procedures in this field. 相似文献
114.
Petroselli G Erra-Balsells R Cabrerizo FM Lorente C Capparelli AL Braun AM Oliveros E Thomas AH 《Organic & biomolecular chemistry》2007,5(17):2792-2799
UV-A radiation (320-400 nm) induces damages to the DNA molecule and its components through photosensitized reactions. Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have investigated the photosensitization of 2'-deoxyadenosine-5'-monophosphate (dAMP) by pterin (PT) in aqueous solution under UV-A radiation. The effect of pH was evaluated, the participation of oxygen was investigated and the products analyzed. Kinetic studies revealed that the reactivity of dAMP towards singlet oxygen (1O2) is very low and that this reactive oxygen species does not participate in the mechanism of photosensitization, although it is produced by PT upon UV-A excitation. In contrast, analysis of irradiated solutions by means of electrospray ionization mass spectrometry strongly suggested that 8-oxo-7,8-dihydro-2'-deoxyadenosine-5'-monophosphate (8-oxo-dAMP) was produced, indicating that the photosensitized oxidation takes place via a type I mechanism (electron transfer). 相似文献
115.
Cabrerizo FM Dántola ML Petroselli G Capparelli AL Thomas AH Braun AM Lorente C Oliveros E 《Photochemistry and photobiology》2007,83(3):526-534
Pterins (PTs) belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. We have investigated the reactivity of conjugated PTs (folic acid [FA], 10-methylfolic acid [MFA], pteroic acid [PA]) and unconjugated PTs (PT, 6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT]) with singlet oxygen (1O2) in aqueous solutions, and compared the efficiencies of chemical reaction and physical quenching. The chemical reactions between 1O2, produced by photosensitization, and PT derivatives were followed by UV-visible spectrophotometry and high-performance liquid chromatography, and corresponding rate constants (k(r)) were evaluated. Whenever possible, products were identified and quantified. Rate constants of 1O2 total quenching by the PT derivatives investigated were obtained from steady-state 1O2 luminescence measurements. Results show that the behavior of conjugated PTs differs considerably from that of unconjugated derivatives, and the mechanisms of 1O2 physical quenching by these compounds and of their chemical reaction with 1O2 are discussed in relation to their structural features. 相似文献
116.
Pérez Pavón JL del Nogal Sánchez M Fernández Laespada ME Moreno Cordero B 《Journal of chromatography. A》2007,1175(1):106-111
A sensitive method is presented for the fast analysis of seven fuel oxygenates (methanol, ethanol, tert-butyl alcohol (TBA), methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME) and diisopropyl ether (DIPE)) and benzene, toluene, ethylbenzene and p-xylene (BTEX) in water samples. The applicability of a headspace (HS) autosampler in combination with a GC device equipped with a programmable temperature vaporizer (PTV) and a MS detector is explored. The proposed method achieves a clear improvement in sensitivity with respect to conventional headspace methods due to the use of the PTV. Two different packed liners with materials of different trapping strengths (glass wool and Tenax-TA) were compared. The benefits of using Tenax-TA instead of glass wool as packed material for the measurement of the 11 compounds emerged as better signal-to-noise ratios and hence better detection limits. The proposed method is extremely sensitive. The limits of detection are of the order of ng/L for six of the compounds studied and of the order of microg/L for the rest, with the exception of the most polar and volatile compound: methanol. Precision (measured as the relative standard deviation for a level with an S/N ratio close to 3) was equal to or lower than 15% in all cases. The method was applied to the determination of the analytes in natural matrixes (tap, river and sea water) and the results obtained can be considered highly satisfactory. The methodology has much lower detection limits than the concentration limits proposed in drinking water by the US Environmental Protection Agency (EPA) and the European Union for compounds under regulation. 相似文献
117.
Studart AR Amstad E Gauckler LJ 《Langmuir : the ACS journal of surfaces and colloids》2007,23(3):1081-1090
The stabilization of nanoparticles in concentrated aqueous suspensions is required in many manufacturing technologies and industrial products. Nanoparticles are commonly stabilized through the adsorption of a dispersant layer around the particle surface. The formation of a dispersant layer (adlayer) of appropriate thickness is crucial for the stabilization of suspensions containing high nanoparticle concentrations. Thick adlayers result in an excessive excluded volume around the particles, whereas thin adlayers lead to particle agglomeration. Both effects reduce the maximum concentration of nanoparticles in the suspension. However, conventional dispersants do not allow for a systematic control of the adlayer thickness on the particle surface. In this study, we synthesized dispersants with a molecular architecture that enables better control over the particle adlayer thickness. By tailoring the chemistry and length of these novel dispersants, we were able to prepare fluid suspensions (viscosity < 1 Pa.s at 100 s-1) with more than 40 vol % of 65-nm alumina particles in water, as opposed to the 30 vol % achieved with a state-of-the-art dispersing agent. This remarkably high concentration facilitates the fabrication of a wide range of products and intermediates in materials technology, cosmetics, pharmacy, and in all other areas where concentrated nanoparticle suspensions are required. On the basis of the proposed molecular architecture, one can also envisage other similar molecules that could be successfully applied for the functionalization of surfaces for biosensing, chromatography, medical imaging, drug delivery, and aqueous lubrication, among others. 相似文献
118.
Herbert Schumann Esther C. E. Rosenthal Jrg Demtschuk Stefan Mühle 《无机化学与普通化学杂志》2000,626(10):2161-2166
Organometallic Compounds of the Lanthanides. 139 Mixed Sandwich Complexes of the 4 f Elements: Enantiomerically Pure Cyclooctatetraenyl Cyclopentadienyl Complexes of Samarium and Lutetium with Donor‐Functionalized Cyclopentadienyl Ligands The reactions of [K{(S)‐C5H4CH2CH(Me)OMe}], [K{(S)‐C5H4CH2CH(Me)NMe2}] and [K{(S)‐C5H4CH(Ph)CH2NMe2}] with the cyclooctatetraenyl lanthanide chlorides [(η8‐C8H8)Ln(μ‐Cl)(THF)]2 (Ln = Sm, Lu) yield the mixed cyclooctatetraenyl cyclopentadienyl lanthanide complexes [(η8‐C8H8)Sm{(S)‐η5 : η1‐C5H4CH2CH(Me)OMe}] ( 1 a ), [(η8‐C8H8)Ln{(S)‐η5 : η1‐C5H4CH2CH(Me)NMe2}] (Ln = Sm ( 2 a ), Lu ( 2 b )) and [(η8‐C8H8)Ln{(S)‐η5 : η1‐C5H4CH(Ph)CH2NMe2}] (Ln = Sm ( 3 a ), Lu ( 3 b )). For comparison, the achiral compounds [(η8‐C8H8)Ln{η5 : η1‐C5H4CH2CH2NMe2}] (Ln = Sm ( 4 a ), Lu ( 4 b )) are synthesized in an analogous manner. 1H‐, 13C‐NMR‐, and mass spectra of all new compounds as well as the X‐ray crystal structures of 3 b and 4 b are discussed. 相似文献
119.
Emilio Calvo Esther Gutiérrez Juan Carlos Santos 《International Journal of Game Theory》2000,29(2):177-188
We consider multichoice NTU games, i.e., cooperative NTU games in which players can participate in the game with several
levels of activity. For these games, we define and characterize axiomatically the multichoice consistent value, which is a generalization of the consistent NTU value for NTU games and of the multichoice value for multichoice TU games.
Moreover, we show that this value coincides with the consistent NTU value of a replicated NTU game and we provide a probabilistic
interpretation.
Received: May 1998/Final version: January 2000 相似文献
120.
Songlan Sun Gaia De Angelis Dr. Stefania Bertella Marie J. Jones Dr. Graham R. Dick Prof. Esther Amstad Prof. Jeremy S. Luterbacher 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2024,136(5):e202312823
Concerns over the sustainability and end-of-life properties of fossil-derived surfactants have driven interest in bio-based alternatives. Lignocellulosic biomass with its polar functional groups is an obvious feedstock for surfactant production but its use is limited by process complexity and low yield. Here, we present a simple two-step approach to prepare bio-based amphiphiles directly from hemicellulose and lignin at high yields (29 % w/w based on the total raw biomass and >80 % w/w of these two fractions). Acetal functionalization of xylan and lignin with fatty aldehydes during fractionation introduced hydrophobic segments and subsequent defunctionalization by hydrogenolysis of the xylose derivatives or acidic hydrolysis of the lignin derivatives produced amphiphiles. The resulting biodegradable xylose acetals and/or ethers, and lignin-based amphiphilic polymers both largely retained their original natural structures, but exhibited competitive or superior surface activity in water/oil systems compared to common bio-based surfactants. 相似文献