Effects of thioamide substitutions on the conformation and stability of alpha- and 3(10)-helices

Thiopeptides, formed by replacing the amide oxygen atom with a sp(2) sulfur atom, are useful in protein engineering and drug design because they confer resistance to enzymatic degradation and are predicted to be more rigid. This report describes our free molecular dynamics simulations with explicit water and free energy calculations on the effects of thio substitutions on the conformation of alpha-helices, 3(10)-helices, and their relative stability. The most prominent structural effect of thio substitution is the increase in the hydrogen bond distance from 2.1 A for normal peptides to 2.7 A for thiopeptides. To accommodate for the longer C[double bond]S...H-N hydrogen bond, the (phi, psi) dihedral angles of the alpha-helix changed from (-66 degrees, -42 degrees) to (-68 degrees, -38 degrees), and the rise per turn increased from 5.5 to 6.3 A. For 3(10)-helices, the (phi, psi) dihedral angles (-60 degrees, -20 degrees) and rise per turn (6.0 A) changed to (-66 degrees, -12 degrees) and 6.8 A, respectively. In terms of relative stability, the most prominent change upon thio substitution is the decrease in the free energy difference, Delta A(alpha --> 3(10)), from 14 to 3.5 kcal/mol. Therefore, normal peptides are less likely to form 3(10)-helix than are thiopeptides. Component analysis of the Delta A(alpha --> 3(10)) reviews that the entropy advantage of the 3(10)-helix for both Ac-Ala(10)-NHMe and Act-Alat(10)-NHMe is attributed to the 3(10)-helix being more flexible than the alpha-helix. Interestingly, upon thio substitution, this differential flexibility is even more apparent because the alpha-helix conformation of Act-Alat(10)-NHMe becomes more rigid due to the bulkier sulfur atom.     

掺杂镨的纳米二氧化钛光催化降解2,4-二硝基苯酚的研究

以钛酸丁酯为原料,用溶胶-凝胶(sol-gel)法制备了掺杂镨的纳米TiO2粉末.采用X光衍射仪对粉体的物相进行了表征.样品经500 ℃焙烧2 h后,0.5%(摩尔分数)Pr3+-TiO2纳米粉末为单一的锐钛型结构.通过粉体对2,4-二硝基苯酚的降解情况对其光催化活性进行了测试,结果表明与纯TiO2相比,Pr3+-TiO2的光催化活性有较大提高,当Pr3+的掺入量为0.5%(摩尔分数)时催化活性最高.以高压汞灯为光源,2,4-二硝基苯酚的初始浓度为50 mg·L-1,催化剂0.5%(摩尔分数)Pr3+-TiO2投加量为1.0 g·L-1时,2,4-二硝基苯酚的光催化降解效果最好.     

Wild ginseng grows in Myanmar

Ginseng, the underground parts of plants of Panax species, has been used in oriental traditional medicine for centuries. Unfortunately, because of extensive exploitation over thousands of years, the natural source of these species has been almost exhausted. Recently, we have found a wild ginseng growing in Myanmar. Here, by a combination of chemical composition study and gene sequence analysis, we unambiguously demonstrate that the wild ginseng is actually P. zingiberensis, commonly known as ginger ginseng. This ginseng was an indigenous to the southwestern China. However, now it is seriously threatened to brink of extinction and is put on the highest level of protection in China. Therefore, an appropriate protection measure is highly recommended to preserve this valuable resource, since this Myanmar ginseng might turn out to be the last P. zingiberensis, which could ever be seen in the planet.     

Link between the kinetic‐ and exchange‐energy functionals in the generalized gradient approximation

An approximate kinetic‐energy functional of the generalized gradient approximation form was derived following the “conjointness conjecture” of Lee, Lee, and Parr. The functional shares the analytical form of its gradient dependency with the exchange‐energy functionals of Becke and Perdew, Burke, and Ernzerhof. The two free parameters of this functional were determined using the exact values of the kinetic energy of He and Xe atoms. A set of 12 closed‐shell atoms was used to test the accuracy of the proposed functional and more than 30 others taken from the literature. It is shown that the conjointness conjecture leads to a very good class of kinetic‐energy functionals. Moreover, the functional developed in this work is shown to be one of the most accurate despite its simple analytical form. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Zeta-potentials of self-assembled surface micelles of ionic surfactants adsorbed at hydrophobic graphite surfaces

The zeta-potentials of the self-assembled surface ionic surfactants (sodium dodecyl sulfate—SDS and hexadecyltrimethyl ammonium bromide—CTAB) on graphite surfaces were determined both from streaming potential and electrophoretic mobility measurements. The adsorption of the surfactants at graphite–liquid interfaces has been studied using atomic force microscopy (AFM) soft-contact imaging which shows the formation of linear, parallel hemicylinders with headgroups oriented towards the solution. The magnitude of the zeta-potential increased with an increase in surfactant concentration, reaching a constant value at a concentration corresponding to the point of surface micelle formation as confirmed from AFM imaging. The streaming potential and electrophoretic mobility measurements showed that the zeta-potentials of SDS and CTAB surface micelles adsorbed at the graphite surface were about −75 and +70 mV, respectively, well in agreement with the values reported for bulk phase micelles in the literature.     

Development of enzyme biosensor based on pH-sensitive field-effect transistors for detection of phenolic compounds.

This article describes a biosensor based on pH-sensitive field-effect transistors (pH-FETs) as transducer, and immobilised enzyme tyrosinase as biorecognition element, which was used for the determination of phenolic compounds in water solutions. The biologically active membrane was formed by cross-linking of tyrosinase with bovine serum albumin (BSA) in saturated glutaraldehyde (GA) vapours on the sensitive transducer surface. The main analytical characteristics were studied under different conditions as well as the possibility to optimise these working parameters. Different factors such as the pH of immobilisation, the enzyme loading, the time of exposition to glutaraldehyde vapours were investigated in regards to the influence on sensitivity, limit of detection, dynamic range, and operational and storage stability.     

An application of the phosphine-catalyzed [4 + 2] annulation in indole alkaloid synthesis: formal syntheses of (+/-)-alstonerine and (+/-)-macroline

[reaction: see text] An application of the phosphine-catalyzed [4 + 2] annulation in the formal synthesis of alstonerine and macroline is reported. A phosphine-catalyzed [4 + 2] reaction between imine 7a and allene 8 formed the D ring of the target indole alkaloids. A subsequent intramolecular Friedel-Crafts acylation provided the C ring of the bridged tetracycle. Deprotection, followed by methylation of the bridged nitrogen, deoxygenation of the C6 ketone, and reduction of the C16 carbethoxy group provided the previously known intermediate 3.