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91.
Life is made of the intimate interaction of metabolism and genetics, both built around the chemistry of the most common elements of the Universe (hydrogen, oxygen, nitrogen, and carbon). The transmissible interaction of metabolic and genetic cycles results in the hypercycles of organization and de-organization of chemical information, of living and non-living. The origin-of-life quest has long been split into several attitudes exemplified by the aphorisms "genetics-first" or "metabolism-first". Recently, the opposition between these approaches has been solved by more unitary theoretical and experimental frames taking into account energetic, evolutionary, proto-metabolic and environmental aspects. Nevertheless, a unitary and simple chemical frame is still needed that could afford both the precursors of the synthetic pathways eventually leading to RNA and to the key components of the central metabolic cycles, possibly connected with the synthesis of fatty acids. In order to approach the problem of the origin of life it is therefore reasonable to start from the assumption that both metabolism and genetics had a common origin, shared a common chemical frame, and were embedded under physical-chemical conditions favourable for the onset of both. The singleness of such a prebiotically productive chemical process would partake of Darwinian advantages over more complex fragmentary chemical systems. The prebiotic chemistry of formamide affords in a single and simple physical-chemical frame nucleic bases, acyclonucleosides, nucleotides, biogenic carboxylic acids, sugars, amino sugars, amino acids and condensing agents. Thus, we suggest the possibility that formamide could have jointly provided the main components for the onset of both (pre)genetic and (pre)metabolic processes. As a note of caution, we discuss the fact that these observations only indicate possible solutions at the level of organic substrates, not at the systemic chemical level. 相似文献
92.
Faoro R Kadankov M Parisi D Veronesi S Tonelli M Petrov V Griebner U Segura M Mateos X 《Optics letters》2012,37(9):1517-1519
Stable passive Q-switching of a Tm: LiYF4 laser is obtained using polycrystalline Cr2+: ZnS as a saturable absorber. The achieved maximum pulse energy of 0.9 mJ and peak power of 65 kW for a pulse duration of ~14 ns represent substantial improvement and highest values for a passively Q-switched diode-pumped Tm laser operating at ~1.9 μm. 相似文献
93.
94.
Ethyl 2-diazo-4,4,4-trifluoro-3-oxobutanoate is a highly versatile intermediate for the synthesis of a wide range of trifluoromethyl heterocycles. With the use of rhodium(II) or copper(II) catalyzed carbene X-H insertion reactions as key steps, a diverse set of trifluoromethyl-oxazoles, -thiazoles, -imidazoles, -1,2,4-triazines, and -pyridines are available from the diazoketoester, either directly in a single step or with just one additional step. 相似文献
95.
Solid damping in micro electro mechanical systems 总被引:1,自引:1,他引:0
This paper focuses on the problem of the numerical evaluation of dissipation induced by thermoelastic coupling in microelectromechanical
systems. An ad hoc conceived, FE based, numerical procedure for the evaluation of the thermoelastic dissipation is proposed
and the numerical results are compared with analytical solutions. In order to introduce in the numerical response a dependence
on the size of the resonating devices, which is experimentally observed at very small dimensions, a new enhanced non-local
coupled thermoelastic model is proposed and the first results are discussed.
An erratum to this article can be found at 相似文献
96.
Hans-Jörg G. Diersch Volker Clausnitzer Volodymyr Myrnyy Rodrigo Rosati Mattias Schmidt Holger Beruda Bruno J. Ehrnsperger Raffaele Virgilio 《Transport in Porous Media》2010,83(3):437-464
The flow and deformation processes in swelling porous media are modeled for absorbent hygiene products (e.g., diapers, wipes,
papers etc.). The first part of the article derives the fundamental equations for the hysteretic unsaturated flow, liquid
absorption, and large deformation. The final set of model equations consists of balance equations of mobile and absorbed (immobile)
liquid combined with a series of constitutive relationships. The resulting equation system is strongly nonlinear and requires
advanced numerical strategies for solving. The second part of the article focuses on numerical solution and presents simulation
results for 2D and 3D applications. 相似文献
97.
Giovanni Romano Marina Diaco Raffaele Barretta 《Continuum Mechanics and Thermodynamics》2010,22(3):177-187
The First Principle of Continuum Thermodynamics is formulated as a variational condition whose test fields are piecewise constant
virtual temperatures. Lagrange multipliers theorem is applied to relax the constraint of piecewise constancy of test fields.
This provides the existence of square summable vector fields of heat flow through the body fulfilling a virtual thermal work
principle, analogous to the virtual work principle in Mechanics. The issue of compatibility of thermal gradients is dealt
with and expressed by the complementary variational condition. Primal, complementary and mixed variational inequalities leading
to computational methods in heat-conduction boundary-value problems are briefly discussed. 相似文献
98.
A quartz crystal microbalance can be used as a biosensor if biological receptor molecules (ligands) are attached to the crystal.
To improve the sensitivity, the surface area of the gold electrodes on the crystal was increased by rendering the electrodes
porous. This increased the response up to a factor of 3. A protein model system with human anti-myoglobin as ligand and sheep
skeletal myoglobin at different concentrations as analyte was used. The quartz crystal was mounted in a flow-cell, where immobilisation,
binding and regeneration of the surface were carried out. The kinetics of the model system was monitored under various experimental
conditions.
Received February 2, 2000. Revision June 30, 2000. 相似文献
99.
The dynamics of Lagrangian systems is formulated with a differential geometric approach and according to a new paradigm of the calculus of variations. Discontinuities in the trajectory, non-potential force systems and linear constraints are taken into account with a coordinate-free treatment. The law of dynamics, characterizing the trajectory in a general non-linear configuration manifold, is expressed in terms of a variational principle and of differential and jump conditions. By endowing the configuration manifold with a connection, the general law is shown to be tensorial in the velocity of virtual flows and to depend on the torsion of the connection. This result provides a general expression of the EULER-LAGRANGE operator. POINCARÉ and LAGRANGE forms of the law are recovered as special cases corresponding, respectively, to the connection induced by natural and mobile reference frames. For free motions, the geodesic property of the trajectory is directly inferred by adopting the LEVI-CIVITA connection induced by the kinetic energy. 相似文献
100.
Dr. Bruno Mattia Bizzarri Dr. Paola Manini Dr. Valeria Lino Prof. Marco d'Ischia Dr. Michail Kapralov Prof. Eugene Krasavin Klaudia Mráziková Prof. Jiří Šponer Dr. Judit E. Šponer Prof. Ernesto Di Mauro Prof. Raffaele Saladino 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(65):14919-14928
Formation and structural modification of oxygenated polycyclic aromatic hydrocarbons (oxyPAHs) by UV irradiation on minerals have recently been proposed as a possible channel of PAH transformation in astrochemical and prebiotic scenarios of possible relevance for the origin of life. Herein, it is demonstrated that high-energy proton-beam irradiation in the presence of various meteorites, including stony iron, achondrite, and chondrite types, promotes the conversion of two representative oxyPAH compounds, 1-naphthol and 1,8-dihydroxynaphthalene, to complex mixtures of oxygenated and oligomeric derivatives. The main identified products include polyhydroxy derivatives, isomeric dimers encompassing benzofuran and benzopyran scaffolds, and, notably, a range of quinones and perylene derivatives. Addition of urea, a prebiotically relevant chemical precursor, expanded the range of identified species to include, among others, quinone diimines. Proton-beam irradiation of oxyPAH modulated by nitrogen-containing compounds such as urea is proposed as a possible contributory mechanism for the formation and processing of insoluble organic matter in meteorites and in prebiotic processes. 相似文献