Screening inhibitors of anthrax lethal factor

The disease anthrax is caused by lethal factor, an enzyme component of the toxin produced by the spore-forming bacterium Bacillus anthracis. Here we describe substrate molecules for this factor that offer a means for high-throughput screening of potential inhibitors for use in anthrax treatment. Our assay should help to answer the urgent call for new and specific therapies to combat this pathogen after its recent emergence as a terrorist bioweapon.     

Determinants of woody cover in African savannas

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.     

Control of ion selectivity in potassium channels by electrostatic and dynamic properties of carbonyl ligands

Potassium channels are essential for maintaining a normal ionic balance across cell membranes. Central to this function is the ability of such channels to support transmembrane ion conduction at nearly diffusion-limited rates while discriminating for K+ over Na+ by more than a thousand-fold. This selectivity arises because the transfer of the K+ ion into the channel pore is energetically favoured, a feature commonly attributed to a structurally precise fit between the K+ ion and carbonyl groups lining the rigid and narrow pore. But proteins are relatively flexible structures that undergo rapid thermal atomic fluctuations larger than the small difference in ionic radius between K+ and Na+. Here we present molecular dynamics simulations for the potassium channel KcsA, which show that the carbonyl groups coordinating the ion in the narrow pore are indeed very dynamic ('liquid-like') and that their intrinsic electrostatic properties control ion selectivity. This finding highlights the importance of the classical concept of field strength. Selectivity for K+ is seen to emerge as a robust feature of a flexible fluctuating pore lined by carbonyl groups.     

关于谐振腔中的几个问题

论述了具有理想导体壁的谐振腔中各不同谐振频率的电磁场分量所满足的方程和边值条件，以及总能量与各不同频率的分量场的场能间的关系。结果表明：不同频率的谐分场间的互作用能对总场能没有贡献，总场能等于不同频率的场单独存在时的场能之和；且电场与磁场间有π/2的位相差，其场能的时间平均值相等。     

Zn_(1-x)Cd_xSe/ZnSe异质结中的束缚极化子

对半导体单异质结系统 ,引入三角势近似异质结势 ,考虑电子、杂质与声子的相互作用 ,利用改进的 LLP变分法讨论在界面附近束缚于正施主杂质的极化子基态能量 .对 Zn1-xCdx Se/Zn Se系统的杂质态结合能进行了数值计算 ,给出了结合能、声子贡献随杂质位置、电子面密度和组分的变化关系 .结果表明 ,杂质 -声子相互作用显著且声子对结合能的作用为负 .     

"长河二号"系统在我国南海的定位误差分析

运用C编程语言和MATLAB工具仿真分析了"长河二号"系统在南海的几何定位因子分布,仿真结果表明"长河二号"系统在南海大部分海区定位误差较小,但是在部分海区定位误差较大,尤其是在(φ17°N,λ119°E～φ21°N,λ121°E)附近海区甚至不能定位.并对两种降低"长河二号"系统定位误差的方法(建立罗兰-C小台链,多台链交叉定位)进行了几何定位因子评估,评估结果表明这几种方法是可行的.     

蓝牙技术与无线局域网

对目前以蓝牙技术和无线局域网(802．11标准系列)为代表的短距离无线接入技术作了研究和介绍，并对蓝牙技术和无线局域网的体系结构、网络拓扑结构、底层特性、电源管理、安全性多方面进行了比较和研究。     

硫酸钾型蔬菜专用肥开发研制

化肥生产企业根据土壤养分及作物营养需求开发研制价格适宜、配方合理的补钾专用肥，满足平衡施肥的需要。     

低碳视角下的计程车计费系统的设计

目前,低碳生活,低碳经济已经成为人们关注的热点问题。从低碳视角出发,依据软件设计思想原理,用java编程模拟计程车数据处理模块。对现有的计程车计费系统进行优化,在给出车费的同时显示耗油量、二氧化碳排放量,使乘客快速的知道乘车过程中的碳消耗量,便于他们改变消费模式和生活方式,从而充分发掘消费生活领域节能减排的巨大潜力。