冬季低温胁迫对长叶竹柏幼苗细胞膜的伤害

为探讨冬季低温胁迫对长叶竹柏幼苗细胞膜的影响,选取长叶竹柏的2龄幼苗为试验材料,进行不同海拔的低温胁迫对比试验．结果表明,在冬季低温胁迫下,细胞发生膜脂过氧化作用,细胞质膜的结构和功能受刭损伤,从而导致了叶片细胞膜透性增大,电解质外渗,电导率增大．同时,SOD,CAT和POD活性及膜脂过氧化产物MDA含量的测定表明,零上低温胁迫不会对植物造成严重的膜脂过氧化伤害,部分原因可能是SOD,CAT和POD活性的协调一致性．     

Membrane fusion: the process and its energy suppliers

Membrane fusion constitutes a pivotal process in eukaryotic cell physiology. Both specialized proteins and membrane lipids play key roles in fusion. Here, our current understanding of the mechanism of membrane fusion is reviewed. The focus is on the relatively simple and well-understood proteinaceous fusion machinery of enveloped viruses and the physical properties of lipids that appear to be of great relevance for fusion progression. Recent observations suggest that viral fusion proteins use packed conformational energy and bilayer-destabilizing domains to (i) bring participating membranes into intimate contact, (ii) merge proximal lipid monolayers through highly curved stalk/hemifusion intermediates, and (iii) generate a lipid-containing fusion pore, thereby terminating the fusion process. Received 4 January 2002; received after revision 3 April 2002; accepted 5 April 2002     

Large-scale production of functional membrane proteins

The preparation of sufficient amounts of high-quality samples is still the major bottleneck for the characterization of membrane proteins by in vitro approaches. The hydrophobic nature, the requirement for complicated transport and modification pathways, and the often observed negative effects on membrane properties are intrinsic features of membrane proteins that frequently cause significant problems in overexpression studies. Establishing efficient protocols for the production of functionally folded membrane proteins is therefore a challenging task, and numerous specific characteristics have to be considered. In addition, a variety of expression systems have been developed, and choice of appropriate techniques could strongly depend on the desired target membrane proteins as well as on their intended applications. The production of membrane proteins is a highly dynamic field and new or modified approaches are frequently emerging. The review will give an overview of currently established processes for the production of functionally folded membrane proteins.     

Membrane protein folding on the example of outer membrane protein A of <Emphasis Type="Italic">Escherichia coli</Emphasis>

The biophysical principles and mechanisms by which membrane proteins insert and fold into a biomembrane have mostly been studied with bacteriorhodopsin and outer membrane protein A (OmpA). This review describes the assembly process of the monomeric outer membrane proteins of Gram-negative bacteria, for which OmpA has served as an example. OmpA is a two-domain outer membrane protein composed of a 171-residue eight-stranded -barrel transmembrane domain and a 154-residue periplasmic domain. OmpA is translocated in an unstructured form across the cytoplasmic membrane into the periplasm. In the periplasm, unfolded OmpA is kept in solution in complex with the molecular chaperone Skp. After binding of periplasmic lipopolysaccharide, OmpA insertion and folding occur spontaneously upon interaction of the complex with the phospholipid bilayer. Insertion and folding of the -barrel transmembrane domain into the lipid bilayer are highly synchronized, i.e. the formation of large amounts of -sheet secondary structure and -barrel tertiary structure take place in parallel with the same rate constants, while OmpA inserts into the hydrophobic core of the membrane. In vitro, OmpA can successfully fold into a range of model membranes of very different phospholipid compositions, i.e. into bilayers of lipids of different headgroup structures and hydrophobic chain lengths. Three membrane-bound folding intermediates of OmpA were discovered in folding studies with dioleoylphosphatidylcholine bilayers. Their formation was monitored by time-resolved distance determinations by fluorescence quenching, and they were structurally distinguished by the relative positions of the five tryptophan residues of OmpA in projection to the membrane normal. Recent studies indicate a chaperone-assisted, highly synchronized mechanism of secondary and tertiary structure formation upon membrane insertion of -barrel membrane proteins such as OmpA that involves at least three structurally distinct folding intermediates.     

膜反应器的开发进展

本文是有关膜反应器的一篇文献综述。文中详细讨论了膜反应器所用膜材料的种类、性能以及在各种化学反应中的应用。     

立式旋转圆筒型过滤器精密平衡过滤高粘性液浆的特性研究

本研究使用旋转圆筒型陶瓷过滤膜在定压条件下对高粘性液浆的精密平衡过滤特性进行测定的同时,对操作因子以及过滤速度随时间的变化、液浆浓度的影响进行了探讨.     

船舷侧板遭球鼻首撞击时的刚塑性分析及简化计算

本文从碰撞变形机理出发,采用薄膜理论,以数学表达式来描述舷侧板遭受球鼻首撞击时的变形模态,分别导出了舷侧板大变形下的撞击力P(λ),撞击能U(λ)和最大残余变形λ的理论公式;文中还从工程应用出发,导出了简化计算的解析式;经过实验验证,理论公式和简化计算公式,均与实验值吻合较好.     

VAc／BuA共聚物膜柔软性的研究

应用透射电镜，示差扫描量热计技术研究了不同配比的醋酸乙烯，丙烯酸丁酯共聚物膜的柔软性，通过差势扫描测得的玻璃化温度表明膜的柔软性随丙烯酸丁酯的用量的增加而增加。膜的柔软性与粒子大小，引发剂的用量，Ｐ到及加料方式间的关系也进行了研究。