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.     

Solvent effects on oxygen atom transfer reaction between manganese(V)-oxo corrole and alkene

Pseudo-first order reaction rate constants of 5,10,15-tris（pentafluorophenyl）corrole Mn（V）-oxo (F₁₅CMn（V）-oxo),5,15-bis（pentafluorophenyl）-10-（phenyl）corrole Mn（V）-oxo(F₁₀CMn（V）-oxo),5,15- bis（phenyl）-10-（pentafluorophenyl）corrole Mn（V）-oxo（F₅CMn（V）-oxo） and 5,10,15-tris（phenyl）corrole Mn（V）-oxo（F₀CMn（V）-oxo） with a series of alkene substrates in different solvents were determined by UV-vis spectroscopy.The results indicated that the oxygen atom transfer pathway between Mn（V）-oxo corrole and alkene is solvent-dependent.     

Band-broadening effects in preparative free-flow zone electrophoresis

Recently, we proposed a novel preparative free flow zone electrophoresis cell with extremely short residence time, which does not require external cooling (Poggel, M., Melin, T., Electrophoresis 2001, 22, 1008-1015). Within the new cell the smallest chamber dimension is not orientated perpendicular but in direction of the electric field. This alteration provides straight forward scale up opportunities. In this paper, new experimental results are reported, from which the limits of stable flow can be determined. The data suggest that not density differences but electrohydrodynamic effects are responsible for the disturbance of the laminar flow pattern, which is observed above a critical field strength. To demonstrate the efficiency of the new system, a three-component mixture consisting of bovine serum albumin (BSA), myoglobin and cytochrome c is processed, resulting in relatively high recovery and purity values of the different proteins, although a complete separation is not achieved.     

Bright Bose-Einstein gap solitons of atoms with repulsive interaction

We report on the first experimental observation of bright matter wave solitons for 87Rb atoms with repulsive atom-atom interaction. This counterintuitive situation arises inside a weak periodic potential, where anomalous dispersion can be realized at the Brillouin zone boundary. If the coherent atomic wave packet is prepared at the corresponding band edge, a bright soliton is formed inside the gap. The strength of our system is the precise control of preparation and real time manipulation, allowing the systematic investigation of gap solitons.     

Dispersion management for atomic matter waves

We demonstrate the control of the dispersion of matter wave packets utilizing periodic potentials. This is analogous to the technique of dispersion management known in photon optics. Matter wave packets are realized by Bose-Einstein condensates of 87Rb in an optical dipole potential acting as a one-dimensional waveguide. A weak optical lattice is used to control the dispersion relation of the matter waves during the propagation of the wave packets. The dynamics are observed in position space and interpreted using the concept of effective mass. By switching from positive to negative effective mass, the dynamics can be reversed. The breakdown of the approximation of constant, as well as experimental signatures of an infinite effective mass are studied.     

Bose-Einstein condensate coupled to a nanomechanical resonator on an atom chip

We theoretically study the coupling of Bose-Einstein condensed atoms to the mechanical oscillations of a nanoscale cantilever with a magnetic tip. This is an experimentally viable hybrid quantum system which allows one to explore the interface of quantum optics and condensed matter physics. We propose an experiment where easily detectable atomic spin flips are induced by the cantilever motion. This can be used to probe thermal oscillations of the cantilever with the atoms. At low cantilever temperatures, as realized in recent experiments, the backaction of the atoms onto the cantilever is significant and the system represents a mechanical analog of cavity quantum electrodynamics. With high but realistic cantilever quality factors, the strong coupling regime can be reached, either with single atoms or collectively with Bose-Einstein condensates. We discuss an implementation on an atom chip.     

β-Octafluorinated tetrakis(ethoxycarbonyl)porphyrin

Tetrakis(alloxycarbonyl)porphyrin and its β-octafluoro-substituted derivatives were synthesized via Lindsey method and transformed to their zinc complexes.Single crystal X-ray structures of corresponding Zn(Ⅱ) porphyrins revealed that β-octafluorination will give more compactness of porphyrin moieties in the crystal structure owing to the hydrogen bonding interactions involving β-fluorine atoms.An unusual six-coordinated Zn(Ⅱ) was found via intramolecular coordination of oxygen atom of meso-substituents with central Zn(Ⅱ).     

双色光量子比特

单个光子可以被置于两种不同颜色的叠加态,这种双色光子的实现将在量子信息处理中发挥重要作用。
光子是光的量子形态,它的发现是量子物理的关键一环。如今在诸如量子计算、安全通信以及精密测量等量子技术中,光子是最重要的基石之一。这些应用通常依赖对光子偏振或空间模式的量子控制。但令人诧异的是,光的颜色(频率)这一最显见的特性却很难在量子水平上加以控制。