Folding at the speed limit

Many small proteins seem to fold by a simple process explicable by conventional chemical kinetics and transition-state theory. This assumes an instant equilibrium between reactants and a high-energy activated state. In reality, equilibration occurs on timescales dependent on the molecules involved, below which such analyses break down. The molecular timescale, normally too short to be seen in experiments, can be of a significant length for proteins. To probe it directly, we studied very rapidly folding mutants of the five-helix bundle protein lambda(6-85), whose activated state is significantly populated during folding. A time-dependent rate coefficient below 2 micro s signals the onset of the molecular timescale, and hence the ultimate speed limit for folding. A simple model shows that the molecular timescale represents the natural pre-factor for transition state models of folding.     

Sex releases the speed limit on evolution

Explaining the evolutionary maintenance of sex remains a key problem in evolutionary biology. One potential benefit of sex is that it may allow a more rapid adaptive response when environmental conditions change, by increasing the efficiency with which selection can fix beneficial mutations. Here I show that sex can increase the rate of adaptation in the facultatively sexual single-celled chlorophyte Chlamydomonas reinhardtii, but that the benefits of sex depend crucially on the size of the population that is adapting: sex has a marked effect in large populations but little effect in small populations. Several mechanisms have been proposed to explain the benefits of sex in a novel environment, including stochastic effects in small populations, clonal interference and epistasis between beneficial alleles. These results indicate that clonal interference is important in this system.     

An upper limit to the masses of stars

There is no accepted upper mass limit for stars. Such a basic quantity eludes both theory and observation, because of an imperfect understanding of the star-formation process and because of incompleteness in surveying the Galaxy. The Arches cluster is ideal for investigating such limits, being large enough to expect stars at least as massive as approximately 500 solar masses (approximately 500 Mo; based on a typical mass function), and young enough for its most massive members to still be visible. It is also old enough to be free of its natal molecular cloud, it is at a well-established distance, and it is close enough for us to discern its individual stars. Here I report an absence of stars with initial masses greater than 130 Mo in the Arches cluster, whereas the typical mass function predicts 18. I conclude that this indicates a firm limit of 150 Mo for stars; the probability that the observations are consistent with there being no upper limit is 10(-8).     

具功能性反应的食饵与捕食者系统的定性分析

应用数学生态学和微分方程定性理论,建立并讨论了一个具功能性反应的食饵与捕食者系统,在给定参数满足一定的条件下,证明了该系统极限环的存在性和唯一性,以及该系统的正平衡点全局渐近稳定。     

干燥速率曲线测定实验设备改进的一点设想

流化床干燥操作实验同时涉及传热和传质两大化工过程,是化工原理实验教学中的重要内容。该文针对目前流化床设备中存在的不足,提出了改进的设想。     

提高跳远助跑速度的方法

跳远是一个速度性项目,助跑速度起着决定性的作用,跳远运动员的训练要以速度训练为指导思想。本文采用文献资料法和实验对照法,对提高跳远速度的方法进行探索,结果证明这些方法能有效地提高跳远助跑速度,对提高跳远的运动成绩有显著的作用。     

使用内存映射实现目录快速浏览

在客户/服务器模式(Client/Server)中,经常出现服务器繁忙,延长对用户请求的响应时间,针对此问题,本文提出一种新方法来解决,使用内存映射实现目录快速浏览. 通过设计适当的数据结构来表示表示目录结点信息,设计索引文件类,通过类[4]中的方法来实现这个功能,并利用VC++6.0来运行程序,结果表明这种思想是完全可行的,并且提高了访问速度,达到了预期的目的.     

Asymptotic behavior of the solutions to the one-dimensional nonisentropic hydrodynamic model for semiconductors

In this paper, the asymptotic behavior of the global smooth solutions to the Cauchy problem for the one-dimensional nonisentropic Euler-Poisson (or full hydrodynamic) model for semiconductors, where the energy equation with non-zero thermal conductivity coefficient are contained, is discussed. The global existence of smooth solutions for the Cauchy problem with small perturbed initial data is proved. In particular, that the solutions converge to the corresponding stationary solutions exponentially fast as t → ∞ is showed. Biography: LI Yeping(1972–), male, Associate professor, Ph.D., research direction: nonlinear partial differential equations.     

一类二阶线性方程的积分解与解的渐近展开式

用推广的Laplace变换求一类二阶线性方程的积分解和解的渐近展开式.     

对祖冲之的密率推算方法的猜想

圆周率π自古以来一直是人们日常生活中的一个非常重要的常数,祖冲之是我国古代杰出的数学家,在圆周率的研究上为人类作出了不朽的贡献,求出了约率22/7和密率355/113,已非常精确。这一结果是如何推算的一直为人们所考究,笔者就此作些尝试。     

一类非线性自治大系统的全局稳定性

本文研究非线性自治大系统 dx_i/dt=sum from i=1 to r(f_(ij)(x_j)) (i=1,…,r) 这里,x_i∈R~(ni),f_(ij)∈C(R~(nj),R~(ni),f_(ij)(o)=0,得到保证其零解为全局一致渐近稳定的充分条件(见定理1)。     

怎样用单片机来检测转速及角位移

介绍一种利用单片机来检测旋转体的电路,该电路结构简单,安装调试方便,工作稳定可靠,在工厂生产中可广泛的应用.     

如何以数学文化助力高职高专极限教学

以"可误主义"数学哲学为指导思想,挖掘极限概念的数学文化内涵,提高高职高专极限教学质量,顺应高等数学教育主体大众化、高等数学教育目的多元化趋势.以"无形"的数学文化推动极限概念教学.     

提高DBASE-Ⅲ库文件输入速度的措施

本文对DBASE-Ⅱ数据库管理系统的库文件操作中提高数据输入速度的问题进行论述,以供选用DBASE-Ⅱ建库的用户参考。     

加快斯考德法收敛速度的探讨

本文通过理论分析，结合作者编制的通风网络分风解算“ＮＳＳＰ”程序的实际验证，探讨了加快斯考德法收敛速度的技巧，从而进一步使该算法稳定，求解迅速，以满足通风系统优化对计算方法的不断要求。     

具有无限长区域的非线性奇摄动问题的渐近解

利用匹配渐近展开法,研究了形如y″+kxy′+εkk-2yny′=0的具有无限长区域的非线性奇摄动问题,分别用收缩变换和伸长变换两种方法得到了该问题的一致有效的渐近解,推广了已有的结果.     

The observation of the Earth tide is irrelevant to the speed of gravity

Recently, the observations on the earth tide at Shiquanhe and Wushi were used to show that gravity propagates at the speed of light. Based on the brief review of general relativity and the Newtonian gravitational theory, we re-analyze the experiments and cannot reach the same conclusion from the observations.