Possible existence of ion pairs at the mouths of ion channels

An electrostatic calculation suggests that when an ion is bound near the mouth of a channel penetrating a low-dielectric membrane, a counter ion may form an ion pair with this ion. The tendency towards ion-pair formation is remarkably enhanced at channel mouths by forces (image forces) arising from the charges induced on the boundaries between different dielectrics. The binding constant for the formation of ion-pairs of monovalent ions is estimated under the assumption that local interactions between the counter ion and the channel wall are negligibly small. It is of the order of 1–10 molal^?1 or more for the binding of a Cl^? (F^?) counter ion to an $\text{Na}{}^{\mathrm{+}}$ ($\text{Li}{}^{\mathrm{+}}$) ion if appropriate conditions are fulfilled. The binding constant depends on the position of the binding site, the dimensions and geometries of the channel and channel mouth, and the state of ion loading of the channel, as well as the ionic species. The present results also indicate that when cation (anion) channels have anionic (cationic) groups as integrant parts of their channel walls, interactions between these charged groups and permeant ions are markedly enhanced by the image forces.     

Myosin V随机步长分布的动力学分析

Myosin V利用ATP水解所释放的自由能,朝肌动蛋白微丝正端作连续的定向运动,平均步长约为36nm。最近几年,诸多实验数据表明,myosin V步长并不固定为36nm,马达各步长值和相应步长出现概率的柱状图符合高斯分布;且在负载力大于2pN的情况下会出现“中间步长”和“后退步子”的现象。可根据已有实验数据,同时考虑马达在跃迁过程中所受的溶液摩擦阻力、常负载力和高斯随机力对其跃迁距离的影响,提出一种跃迁模型,并以此为基础对上述现象进行理论解释。     

红光和远红光处理引起的杜氏盐藻跨类囊体膜质子动力势变化

照射远红光后杜氏盐藻毫秒延迟发光快相强度明显增加,而红光处理结果则相反.低温条件明显抑制远红光引起的毫秒延迟发光快相强度的上升,而红光则仍能够有效地引起毫秒延迟发光快相强度的降低.加入消除跨类囊体膜质子梯度的尼日利亚菌素后,远红光不能引起延迟荧光强度的上升.与以前在高等植物中得到的结果相比,在杜氏盐藻中远红光处理后毫秒延迟发光快相强度增加的幅度更大,红光处理后没有出现毫秒延迟发光快相强度先增加后降低的现象.     

一种基于单分子纳米操纵的有序化测序策略

尽管包括人类在内的许多生物物种的基因组测序工作已经完成,但由于现有测序技术的限制,大部分复杂基因组还存在很多大大小小的缺口.缺口的填补以及对其他重复序列区域的测序迫切需要全新的思路和技术.基于在DNA单分子定位切割和拾取方面的实验进展,提出了一种基于原子力显微镜纳米操纵技术的单分子有序化测序策略.计算机模拟的结果表明,这一方法和策略是可行的,有助于解决目前测序工作中所遇到的一些棘手问题.     

Sward height and bite size affect the functional response of barnacle geese Branta leucopsis

Intake rate, the rate in which herbivores can process their food, is presumed to be an important factor in habitat selection down to the scale of the foraging patch. Much attention has been given to the selection of swards of high nutritional quality, but much less has been given to the influences of sward structure on patch selection in small herbivores. In this study we tested the effects of sward density and height on the functional foraging response of barnacle geese, Branta leucopsis. The functional response curve for herbivores describes how intake rate is affected by food availability. We conducted feeding trials to determine intake rate and bite size of barnacle geese on experimentally manipulated swards. Results indicate that intake rate is mainly dependent on sward height and that there is a strong correlation between bite size and intake rate. Sward density does not influence the rate of food consumption; it is, however, a crucial parameter affecting potential total yield. We conclude that bite size is the crucial parameter influencing intake rate. Bite size is explained both by sward height and individual differences in bill morphology. Furthermore, intake rate seems to be dependent on the physical structure of the grass species consumed.     

海马神经元新连接结构的初步原子力显微观察

目的:利用原子力显微技术(AFM)观察原代培养的海马神经元超微结构及其相互间的连接结构。方法:选择生长良好的海马神经元,用戊二醛固定30min后,固定于AFM基底上进行扫描和观测。结果:正常海马神经元表面光滑,起伏均匀、规律,突起及细胞之间的超微结构清晰,神经元胞体间存在膜性连接及长程非突触性突起连接。结论:神经元间存在直接膜性连接和长程非突触性突起连接结构。     

原子力显微观察巯基反应剂对乙酰胆碱酯酶立体结构的影响

目的:研究巯基及二硫键对乙酰胆碱酯酶(AChE)高级结构的作用。方法:将与巯基反应剂GSH和DTT作用前后的AChE重组到磷脂膜上,置于原子力显微镜下观察,扫描方式为空气中接触模式。结果:天然的AChE直径不均一,既有大粒子也有小粒子,平均最大直径(95±44)nm,平均半径(43±17)nm,平均高度(9.25±2.62)nm;与GSH反应后的AChE整体粒径明显变小,最大直径(31±13)nm,平均半径(15±4)nm,平均高度(4.02±0.67)nm;与DTT反应后的AChE边界模糊。结论:GSH可将AChE多聚体还原为单体;相同浓度的DTT作用不明显。     

Synthesis and structure-function study of artificial nucleases on the basis of hemin conjugates with peptide fragments of cell differentiation factor HLDF

A series of octa-hexapeptide fragments of HLDF and their conjugates with hemin were obtained by solid phase peptide synthesis. A relationship between the structure and the nuclease activity of the compounds was established. The effect of various factors (medium pH, the presence of metal ions, complexons, reducers, and buffer composition) on the DNA destruction with hemin peptides was studied. Preliminary information confirming an oxidative mechanism of this process was obtained. The cleavage of plasmid DNA under the action of hemin peptides was studied by the methods of electron microscopy, gel electrophoresis, and atomic force microscopy.     

Compaction of single supercoiled DNA molecules adsorbed onto amino mica

A model of possible conformational transitions of supercoiled DNA in vitro in the absence of proteins under the conditions of increasing degree of compaction was developed. A 3993-bp pGEMEX supercoiled DNA immobilized on various substrates (freshly cleaved mica, standard amino mica, and modified amino mica with a hydrophobicity higher than that of standard amino mica) was visualized by atomic force microscopy in air. On the modified amino mica, which has an increased density of surface positive charges, single molecules with an extremely high degree of compaction were visualized in addition to plectonemic DNA molecules. As the degree of DNA supercoiling increased, the length of the first-order superhelical axis of molecules decreased from 570 to 370 nm, followed by the formation of second-and third-order superhelical axes about 280 and 140 nm long, respectively. The compaction of molecules ends with the formation of minitoroids about 50 nm in diameter and molecules of spherical shape. It was shown that the compaction of single supercoiled DNA molecules immobilized on amino mica to the level of minitoroids and spheroids is due to the shielding of mutually repulsing negatively charged phosphate groups of DNA by positively charged amino groups of the amino mica, which has a high charge density of its surface.     

The surface structure of a complex substrate revealed by enzyme kinetics and Freundlich constants for α-amylase interaction with the surface of starch

Background

Starch is a main source of carbohydrate in human diets, but differences are observed in postprandial glycaemia following ingestion of different foods containing identical starch contents. Such differences reflect variations in rates at which different starches are digested in the intestine. In seeking explanations for these differences, we have studied the interaction of α-amylase with starch granules. Understanding this key step in digestion should help with a molecular understanding for observed differences in starch digestion rates.

Methods

For enzymes acting upon solid substrates, a Freundlich equation relates reaction rate to enzyme adsorption at the surface. The Freundlich exponent (n) equals 2/3 for a liquid-smooth surface interface, 1/3 for adsorption to exposed edges of ordered structures and 1.0 for solution–solution interfaces. The topography of a number of different starch granules, revealed by Freundlich exponents, was compared with structural data obtained by differential scanning calorimetry and Fourier transform infrared spectroscopy with attenuated total internal reflectance (FTIR-ATR).

Results

Enzyme binding rate and FTIR-ATR peak ratio were directly proportional to n and Δ_gelH was inversely related to n. Amylase binds fastest to solubilised starch and to granules possessing smooth surfaces at the solid–liquid interface and slowest to granules possessing ordered crystalline surfaces.

Conclusions

Freundlich exponents provide information about surface blocklet structures of starch that supplements knowledge obtained from physical methods.

General Significance

Nanoscale structures at the surface of starch granules influence hydrolysis by α-amylase. This can be important in understanding how dietary starch is digested with relevance to diabetes, cardiovascular health and cancer.