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The current rule to predict the enantiopreference of subtilisin toward secondary alcohols is based on the size of the substituents at the stereocenter and implies that the active site contains two differently sized pockets for these substituents. Several experiments are inconsistent with the current rule. First, the X-ray structures of subtilisin show there is only one pocket (the S1' pocket) approximately the size of a phenyl group to bind secondary alcohols. Second, the rule often predicts the incorrect enantiomer for reactions in water. To resolve these contradictions, we refine the current rule to show that subtilisin binds only one substituent of a secondary alcohol and leaves the other in solvent. To test this refined empirical rule, we show that the enantioselectivity of a series of secondary alcohols in water varied linearly with the difference in hydrophobicity (log P/P0) of the substituents. This hydrophobicity difference accounts for the solvation of one substituent in water. 相似文献
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Shi G Wu JT Li Y Geleziunas R Gallagher K Emm T Olah T Unger S 《Rapid communications in mass spectrometry : RCM》2002,16(11):1092-1099
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Monte Carlo study of impact ionization in n-type InAs induced by intense ultrashort terahertz pulses
Electron impact ionization induced in n-type InAs by single-cycle pulses of picosecond and subpicosecond duration has been investigated by Monte Carlo method. It is established that the rate of generation of electron–hole pairs decreases with the decrease of the pulse duration. The impact ionization threshold field is found to depend linearly on frequency for the fields oscillating at frequencies much higher than reciprocal momentum relaxation time. Good agreement between calculations and available experimental data has been obtained. 相似文献
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Florian Capito Romas Skudas Bernd Stanislawski Harald Kolmar 《Colloid and polymer science》2013,291(7):1759-1769
The effect of low ionic strength leading to reduced polyelectrolyte–protein interactions has been shown by in silico and in vitro experiments, suggesting polyelectrolyte rigidity increasing at low ionic strength, thus leading to reduced interactions with proteins. This contribution elucidates polyelectrolyte–protein precipitation in the 0–2.6-mS?cm?1 ionic strength regime with polyelectrolyte rigidity determinations, using viscosimetry at these conditions, also considering protein charge distributions, using different proteins. Precipitation yields increased from 5 to 40 % at low ionic strength to up to 90 % at intermediate ionic strength, depending on protein and polyelectrolyte type, using lysozyme and three different monoclonal antibodies. Comparing precipitation behavior of the monoclonal antibodies, a qualitative correlation between required polyelectrolyte flexibility to enhance protein precipitation and protein average charge as well as hydrophobicity of the antibodies was discovered. Antibodies with lower average charge and less hydrophobicity required more flexible polyelectrolytes to enhance precipitation behavior by allowing interaction of the polyelectrolytes with proteins, attaching to positively charged protein patches while “circumnavigating” negatively charged protein areas. In contrast, antibodies with higher protein average charge showed increasing precipitation yields up to 90 % already at lower ionic strength, associated with then more rigid polyelectrolyte structures. Therefore, designing polyelectrolytes with specific chain flexibility could help to improve precipitation behavior toward specific target proteins in polyelectrolyte-driven purification techniques. 相似文献
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Phonon generation by electrons is supplied in n-type Si crystals in electric fields E100 kV/cm at the lattice temperature of 80 K employing the ensemble Monte Carlo technique. Electron transfer between equivalent energy valleys is accounted for the g-type- and f-type phonon absorption and emission. Acoustic phonons are accounted for the quasi-elastic scattering of electrons within the energy valleys. Excess phonon number is determined using numerical data on phonon generation rate and experimental values of phonon lifetimes. The feasibility of stimulated emission of infrared-range photons due to direct optical transitions between the phonon bands is discussed. 相似文献
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Yu. Ya. Volkolupov A. N. Dovbnya V. V. Zakutin M. A. Krasnogolovets N. G. Reshetnyak V. P. Romas’ko 《Technical Physics》2001,46(7):873-876
High-power electrons beams generated in a single injection magnetron gun with secondary-emission cathodes and in a set of such guns are studied. Hollow electron beams of current 50–100 A, electron energy 30–100 kV, and peak power 1–5 MW are obtained. The beams can be used as electron sources in accelerators and ordinary and multibeam high-power microwave devices. 相似文献
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In this paper the response of an amperometric biosensor at mixed enzyme kinetics and diffusion limitations is modelled in
the case of the substrate and the product inhibition. The model is based on non-stationary reaction–diffusion equations containing
a non-linear term related to non-Michaelis–Menten kinetics of an enzymatic reaction. A numerical simulation was carried out
using a finite difference technique. The complex enzyme kinetics produced different calibration curves for the response at
the transition and the steady-state. The biosensor operation is analysed with a special emphasis to the conditions at which
the biosensor response change shows a maximal value. The dependence of the biosensor sensitivity on the biosensor configuration
is also investigated. Results of the simulation are compared with known analytical results and with previously conducted researches
on the biosensors. 相似文献