3,287 research outputs found
The heat kernel expansion for the electromagnetic field in a cavity
We derive the first six coefficients of the heat kernel expansion for the
electromagnetic field in a cavity by relating it to the expansion for the
Laplace operator acting on forms. As an application we verify that the
electromagnetic Casimir energy is finite.Comment: 12 page
Kinetics of the template-directed oligomerization of guanosine 5'-phosphate-2-methylimidazolide: Effect of temperature on individual steps of reactionion
Non-enzymatic, template-directed reactions have been proposed as models for prebiological polynucleotide synthesis. Chemically activated mononucleotides react in the presence of a polynucleotide, acting as the template in a Watson-Crick base-pairing fashing, and form the complementary daughter polynucleotide. Phosphoimidazolide-activated nucleotides have been used successfully as substrates in these reactions. The kinetics of the guanosine 5'-monophosphate-2-methylimidazolide (2-MelmpG) reaction in aqueous pH 8.0 solutions in the presence and in the absence of polycytidylate (poly(C)) were studied, acting as the template at 6, 23, and 37 C. In the absence of the template, the major reaction pathway of 2-MelmpG is hydrolysis of the P-N bond to form the unreactive guanosine 5'-monophosphate (5'-GMP) and 2-methylimidazole. Concentrated solution of 2-MelmpG (greater than 0.02 M) in the absence of the template form only a small amount dinucleotide, (pG)2, but in the presence of poly(C), oligoguanylates, (pG)n with 2 less than or = n less than or = 40, can be detected. We were able to determine the rate constants for individual steps of this reaction. A summary of the conclusions is presented
Homogenization and Scattering Analysis of Second-Harmonic Generation in Nonlinear Metasurfaces
We propose an extensive discussion on the homogenization and scattering
analysis of second-order nonlinear metasurfaces. Our developments are based on
the generalized sheet transition conditions (GSTCs) which are used to model the
electromagnetic responses of nonlinear metasurfaces. The GSTCs are solved both
in the frequency domain, assuming an undepleted pump regime, and in the
time-domain, assuming dispersionless material properties but a possible
depleted pump regime. Based on these two modeling approaches, we derive the
general second-harmonic reflectionless and transmissionless conditions as well
as the conditions of asymmetric reflection and transmission. We also discuss
and clarify the concept of nonreciprocal scattering pertaining to nonlinear
metasurfaces
Product and rate determinations with chemically activated nucleotides in the presence of various prebiotic materials, including other mono- and polynucleotides
We are investigating the reactions of ImpN's in the presence of a number of prebiotically plausible materials, such as metal ions, phosphate, amines and other nucleotides and hope to learn more about the stability/reactivity of ImpN's in a prebiotic aqueous environment. We find that, in the presence of phosphate, ImpN's form substantial amounts of diphosphate nucleotides. These diphosphate nucleotides are not very good substrates for template directed reactions, but are chemically activated and are known to revert to the phosphoimidazolides in the presence of imidazole under solid state conditions. With respect to our studies of the oligomerization reaction, the determination of the dimerization rate constant of a specific ImpN (guanosine 5'-phospho 2 methylimidazolide) both in the absence and the presence of the template leads to the conclusion that at 37 C the dimerization is not template directed, although the subsequent polymerization steps are. In other words, this specific polynucleotide synthesizing system favors the elongation of oligonucleotides as compared with the formation of dimers and trimers. This favoring of the synthesis of long as opposed to short oligonucleotides may be regarded as a rudimentary example of natural selection at the molecular level
NGF-response of EGF-dependent progenitor cells obtained from human sympathetic ganglia
SIGNALLING molecules are thought to play a significant role in determining the fate of neural crest progenitor cells. The human sympathetic chain was identified at 6.5, 7.5, 8.2, 10.2 and 11.4 postconception (PC) weeks demonstrating low affinity nerve growth factor (NGF) receptors, and was processed for tissue culture. In the presence of epidermal growth factor (EGF), floating spheres of proliferating progenitor cells were developed in vitro. In the absence of EGF progenitor cells differentiated into tyrosine hydroxylase (TH)-immunoreactive neuronal and TH-negative flat cells. NGF treatment significantly increased neurite outgrowth and survival of TH-immunoreactive cells. The multipotent cells we isolated differ from previously reported sympathoadrenal progenitors in that they give rise to TH immunoreactive neurones precociously sensitive to NGF
Towards Self-Replicating Chemical Systems Based on Cytidylic and Guanylic Acids
This project is aimed towards a better understanding of template-directed reactions and, based on this, towards the development of efficient non-enzymatic RNA replicating systems. These systems could serve as models for the prebiotic synthesis of an RNA world. The major objectives of this project were: (a) To elucidate the mechanistic aspects of template-directed (TD) chemistry, (b) to identify the conditions, environmental and other, that favor "organized chemistry" and stereo selective polymerization of nucleotides and (c) to search and, hopefully, find catalysts that will improve the efficiency of these reactions. Enhanced efficiency is expected to facilitate the road towards a self-replicating chemical system based on all four nucleic acid bases. During the first nine months of the granting period from January 1997 to October 1997, we have made substantial progress towards the first two objectives. During this period our activities were directed towards (1) synthesizing activated nucleotides to be used as substrates, (2) using these substrates in order to determine the effect of the leaving group (imidazole (Im), 2-methylimidazole (2-MeIm), and 2,4-dimethylimidazole (2,4-diMeIm)) in the product distribution, (3) developing techniques for analysis of mixtures by LC/MS, (4) creating a protocol in order to obtain kinetic parameters of the dimerization reaction and (5) analyzing kinetic data obtained with the poly(C)/2-MeImpG system. With the exception of item (5), the experimental work for the projects (1) - (4) is still in progress. A list of publications and manuscripts resulted from this research is enclosed
Analisi dell’anisotropia microstrutturale in materiali compositi rinforzati con fibre corte
Synchrotron light microtomography has proved to be particularly efficient in order to analyze
the microstructural characteristics in terms of reinforce fibre distribution and orientation in glass fibre reinforced
composites. The spatial distribution of fibre within the polymeric matrix could be detected even in
case of fibre characterized by a small diameter (10 micrometers average diameter). Differences in orientation
distribution within a sample could be measured using the Mean Intercept Length (MIL) and the fabric
tensor. The results presented herein refer to a sample of a 30% by weight glass fibre reinforced polyamide 6,
extracted form a thin plate
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