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31.
HCOOH-ring closures of OH-lactams possessing an allyl substituted alkene function solely afford pyrrolizidines and in high yield via 2-aza-Cope rearrangement and ensuing α-acyliminium cyclisation. 相似文献
32.
The NMR spin-lattice relaxation rate (R1) and the rotating-frame spin-lattice relaxation rate (R1rho) of amide 15N and carbonyl 13C (13C') of the uniformly 13C- and 15N-labeled ubiquitin were measured at different temperatures and field strengths to investigate the temperature dependence of overall rotational diffusion and local backbone motion. Correlation between the order parameter of the N-H vector, SNH2, and that of the carbonyl carbon, S2C', was investigated. The effective S2C' was estimated from the direct fit of the experimental relaxation rates and from the slope of 2R2-R1 vs. B2 using Lipari-Szabo formalism. The average SNH2 decreased by 5.9%, while the average S2C' decreased by 4.6% from 15 to 47 degrees C. At the extreme low and high temperatures the difference in the temperature dependence of the order parameters vanishes. At the intermediate temperatures they do not change by the same amount but they follow the same trend. On the same peptide plane along the protein sequence, S2C' and SNH2 are highly correlated. The results suggest that fast local motion experienced at the site of the N-H vector and carbonyl nucleus is more complicated than previously thought and it cannot be easily described by one single type of motion in a broad range of temperature. 相似文献
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34.
Samorì P Donners JJ Severin N Otten MB Rabe JP Nolte RJ Sommerdijk NA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(21):8955-8957
The isolation of single polyelectrolyte chains of water-soluble poly(isocyanodipeptide)s (PICs) bearing carboxylic acid terminated side chains occurring both at surfaces and in solution was accomplished by reducing the intermolecular interactions through complexation with cations or positively charged surfactants.Scanning force microscopy and viscosity analyses revealed that this method allows to tune the conformation of the macromolecule, which is of importance for tailoring the physicochemical properties of the material.This is particularly significant for the use of these polymer chains as seed for biomineralization processes. 相似文献
35.
Declercq NF Degrieck J Briers R Leroy O 《The Journal of the Acoustical Society of America》2004,116(6):3328-3335
It is known that a handclap in front of the stairs of the great pyramid of Chichen Itza produces a chirp echo which sounds more or less like the sound of a Quetzal bird. The present work describes precise diffraction simulations and attempts to answer the critical question what physical effects cause the formation of the chirp echo. Comparison is made with experimental results obtained from David Lubman. Numerical simulations show that the echo shows a strong dependence on the kind of incident sound. Simulations are performed for a (delta function like) pulse and also for a real handclap. The effect of reflections on the ground in front of the pyramid is also discussed. The present work also explains why an observer seated on the lowest step of the pyramid hears the sound of raindrops falling in a water filled bucket instead of footstep sounds when people, situated higher up the pyramid, climb the stairs. 相似文献
36.
There are different methods to mathematically represent a bounded beam. Perhaps the most famous method is the classical Fourier method that consists of the superposition of pure homogeneous plane waves all traveling in different directions and having an amplitude that can be found by the Fourier transform of the required profile. This method works perfectly for 2D as well as for 3D bounded beams. However, some researchers prefer the inhomogeneous wave theory to represent a bounded beam because some phenomena, e.g. the Schoch effect, are explained by this method by means of concepts that agree better with intuition. There are several papers dealing with this method for 2D gaussian beams. Until now, it has never been considered possible to represent 3D gaussian beams as well. The present paper shows a method to overcome this shortcoming and presents different sorts of 3D gaussian beams that are built up by means of inhomogeneous plane waves. 相似文献
37.
Integral representations are considered of solutions of the inhomogeneous Airy differential equation . The solutions of these equations are also known as Scorer functions. Certain functional relations for these functions are used to confine the discussion to one function and to a certain sector in the complex plane. By using steepest descent methods from asymptotics, the standard integral representations of the Scorer functions are modified in order to obtain nonoscillating integrals for complex values of . In this way stable representations for numerical evaluations of the functions are obtained. The methods are illustrated with numerical results.
38.
39.
Tineke A. Molenaar-Langeveld Nico M. M. Nibbering 《Journal of mass spectrometry : JMS》1986,21(10):637-644
It is shown by field ionization kinetics in combination with both site-specific and stereospecific D-labelling that the loss of a molecule of methanol from the molecular ion of methoxycyclohexane can occur via 1,4- and 1,3-eliminations. The 1,4-elimination predominates at molecular ion lifetimes of ≥10?10.1 s. It is found that ~19% of this reaction channel corresponds to a stereospecific cis-elimination, whereas the remaining 81% is only site-specific. At molecular ion lifetimes of between 10?10 and 10?9 s, a very sudden increase of the 1,3-elimination is observed at the expense of the 1,4-elimination. A stereospecific loss of methanol, however, is not observed at all for the 1,3-elimination within the limits of error. Possible intermediates and reaction pathways, which can account for the observations made, are discussed. 相似文献
40.
Nico M. Van Dijk 《商业与工业应用随机模型》1990,6(2):85-100
This paper studies a system of parallel service facilities or processors with mixed exponential and non-exponential queues, a non-exponential finite source input and interdependent arrival as well as departure blocking such as due to a common pool or shared resource. A concrete invariance condition upon the blocking protocol is provided. Under this condition the stationary busy source distribution is shown to be of an insensitive product form. The result unifies and extends known product form results as will be illustrated by several examples. 相似文献