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51.
This paper is aimed at dissecting and discussing the effect of high pressure on chirogenesis, thus unveiling the role of this universal force in astrochemical and primeval Darwinian scenarios. The first part of this contribution revisits the current status and recent experiments, most dealing with crystalline racemates, for which generation of metastable conglomeratic phases would eventually afford spontaneous resolution and hence enantioenriched mixtures. We then provide an in-depth thermodynamic analysis, based on previous studies of non-electrolyte solutions and dense mixtures accounting for the existence of positive excess volume upon mixing, to simulate the mirror symmetry breaking, the evolution of entropy production and dissipation due to enantiomer conversion. Results clearly suggest that mirror symmetry breaking under high pressure may be a genuine phenomenon and that enantioenrichment from initial scalemic mixtures may also take place. 相似文献
52.
Celia Blanco Dr. Joaquim Crusats Dr. Zoubir El‐Hachemi Albert Moyano Dr. Sabino Veintemillas‐Verdaguer Dr. David Hochberg Prof. Josep M. Ribó 《Chemphyschem》2013,14(17):3982-3993
Simulations of a chemical kinetics model, based on the free‐energy relationships of classical primary nucleation theory, show that the deracemization phenomenon in systems of achiral or fast racemizing compounds yielding enantiopure crystals as the more stable solid phase is a true spontaneous mirror symmetry breaking process (SMSB). That is, the achievement of a stationary chiral state is more stable than the racemic one. The model translates the free‐energy relationships determined by the existence of a critical size cluster to a chemical kinetics model, in which the consideration of forward and backward reaction rate constants avoids the misuse of network parameters that violate thermodynamic constraints (microreversibility principle), which would lead to apparent in silico SMSB. The model provides qualitative agreement for deracemizations by mechanical attrition of visible crystals, as well as for those obtained under temperature gradients. The analysis of the effect of the system parameters to obtain a SMSB scenario shows that the network possesses the principal characteristics of SMSB networks: 1) an enantioselective autocatalytic stage, corresponding to the non‐linear kinetics of enantioselective (homochiral) cluster‐to‐cluster growth, and 2) the mutual inhibition step originating in the backward flow of chiral clusters towards smaller achiral clusters, or even to a racemizing monomer. The application of such a SMSB kinetic model to enantioselective polymerizations and to chiral biopolymers is discussed. 相似文献
53.
Zakhariya Manevitch Dmitry Lev Malka Hochberg Mila Palhan Aaron Lewis Claes D. Enk 《Photochemistry and photobiology》2010,86(2):476-479
Onychomycosis is caused by dermatophyte infection of the nail. Though laser energy has been shown to eliminate dermatophytes in vitro, direct laser elimination of onychomycosis is not successful due to difficulties in selectively delivering laser energy to the deeper levels of the nail plate without collateral damage. Femtosecond (fsec) infrared titanium sapphire lasers circumvent this problem by the nonlinear interactions of these lasers with biological media. This quality, combined with the deeply penetrating nature of the near-infrared radiation, allows elimination of deeply seeded nail dermatopytes without associated collateral damage. Nail cuttings obtained from patients with onychomycosis caused by Trichophyton rubrum underwent fsec laser irradiation using increasing laser intensities with the focus scanned throughout the whole thickness of the nail specimen. The efficacy of the laser treatment was evaluated by subculture. Scanning electron microscopy was used to determine fsec laser-induced collateral damage. We found that a fsec laser fluence of 7 × 1031 photons m−2 s−1 or above successfully inhibited the growth of the fungus in all samples examined, whereas laser intensities above 1.7 × 1032 photons m−2 s−1 affected the structure of the nail plate. Our findings suggest that T. rubrum-mediated onychomycosis may be treated by fsec laser technology. 相似文献
54.
55.
Munshi G Mustafa Sudhir Raniwala T Awes B Rai RS Bhalerao JG Contreras RV Gavai SK Ghosh P Jaikumar GC Mishra AP Mishra H Mishra B Mohanty J Nayak J-Y Ollitrault SC Phatak L Ramello R Ray PK Sahu AM Srivastava DK Srivastava VK Tiwari 《Pramana》2006,67(5):961-981
This is the report of Heavy Ion Physics and Quark-Gluon Plasma at WHEPP-09 which was part of Working Group-4. Discussion and
work on some aspects of quark-gluon plasma believed to have created in heavy-ion collisions and in early Universe are reported. 相似文献
56.
57.
Results regarding the pebbling number of various graphs are presented. We say a graph is of Class 0 if its pebbling number equals the number of its vertices. For diameter d we conjecture that every graph of sufficient connectivity is of Class 0. We verify the conjecture for d = 2 by characterizing those diameter two graphs of Class 0, extending results of Pachter, Snevily and Voxman. In fact we use this characterization to show that almost all graphs have Class 0. We also present a technical correction to Chung's alternate proof of a number theoretic result of Lemke and Kleitman via pebbling. © 1997 John Wiley & Sons, Inc. J Graph Theory 25: 119–128, 1997 相似文献
58.
RE Tribble A Azhari HL Clark CA Gagliardi Y-W Lui AM Mukhamedzhanov A Sattaroy X Tang L Trache V Burjan J Cejpek V Kroha S Piskor J Vincour F Carstoiu 《Pramana》1999,53(3):585-594
S-factors for direct capture reactions can be found at astrophysical energies from asymptotic normalization coefficients which
provide the normalization of the tail of the overlap function. For example the overlap for 8B → 7Be+p defines the S-factor for 7Be (p, γ)8B. Peripheral transfer reactions offer a technique to determine these asymptotic normalization coefficients. As a test of
the technique, the 16O(3He, d)17F reaction has been used to determine asymptotic normalization coefficients for transitions to the ground and first excited
states of 17F. The S-factors for 16O(p, γ)17F calculated from these 17F → 16O+p asymptotic normalization coefficients are found to be in very good agreement with recent measurements. Following the same
technique, the 10B(7Be, 8B)9Be and 14N(7Be, 8B)13C reactions have been used to measure the asymptotic normalization coefficient for 7Be(p, γ)8B. This result provides an indirect determination of S
17(0). 相似文献
59.
AM Cardoso SM Alexandre CM Barros AJ Correia NM Nibbering 《Rapid communications in mass spectrometry : RCM》1999,13(19):1885-1888
The collision-induced dissociation (CID) of deprotonated arylalkylamines of general formula R(1)C(6)H(4)CHR(2)CH(2)NR(3)(2) (where R(1) = H, OH, F or NO(2); R(2) = H or OH; R(3) = H or CH(3)) generated by negative chemical ionization with H(2)O and D(2)O as ionizing reagents, is discussed. The negative chemical ionization mass spectra show that, in the absence of a hydroxy group in the aromatic ring, deprotonation takes place at the benzylic position whereas the proton is lost from the OH group when present. The nitro compound forms only M(-.) ions. The CID spectra of the deprotonated molecules show that fragmentations are strongly dependent on the structural features of the molecules, namely the presence or absence of substituents in the aromatic ring or aliphatic chain. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
60.
AM Johnston CM Scrimgeour MO Henry LL Handley 《Rapid communications in mass spectrometry : RCM》1999,13(14):1531-1534
The conversion of nitrate (NO(3)(-)) to 1-phenylazo-2-naphthol (Sudan-1) has been examined as a method for natural abundance measurement of delta(15)N of NO(3)(-). The reaction results in dilution of NO(3)(-)-N with only one reagent-derived N and the product is readily concentrated from dilute samples by reverse phase chromatography. There is systematic isotopic fractionation during the reaction, but this can be allowed for by analysing known NO(3)(-) standards along with each sample set. Sudan-1 prepared from surface water samples containing approximately 50 &mgr;g NO(3)(-)-N can be analysed by automated continuous flow isotope ratio mass spectrometry with a precision of 0.2 per thousand (one standard deviation) and the accuracy is not affected by interference from other nitrogenous species in the sample or reagents. Copyright 1999 John Wiley & Sons, Ltd. 相似文献