The influences of fluorines in chemistry have emerged as a breakthrough in various arenas of bio-organic and medicinal chemistry. But its incorporation in β-turn design and its implications for supramolecular chemistry remains in a rudimentary stage. Inspired by the diversity displayed by the isomers of mono-fluorinated phenylalanine in biological sciences, here our effort is to modulate the solid state conformational analysis of three terminally protected synthetic tripeptides Boc-(Y)-F-Phe-Aib-Xaa-OMe, where (Y is (2)-F-Phe, Xaa; Leu in peptide I, (3)-F-Phe, Xaa; Leu in peptide II and (4)-F-Phe, Xaa; Ile in peptide III). Interestingly, all the three peptides display a conformational preference for β-turns, stabilized by 4→1 intramolecular hydrogen bonding. Our investigation further demonstrates that mere interchange of positions of fluorines in mono-fluorinated phenylalanine in peptides I–III introduces significant diversity in supramolecular chemistry. X-ray crystallography sheds some light at atomic resolution. Furthermore, this supramolecular heterogeneous behavior is evident from the morphologies obtained from the materials of all the three peptides grown from acetone to petroleum ether solution, studied by field emission scanning electron microscopy. Thus, these monofluorinated peptides I–III may serve as prominent candidates in understanding the structure and function of misfolded disease causing peptides like prion and Alzheimer's amyloid. 相似文献
Magneto-electronic properties of asymmetric bilayer nanographene ribbons are enriched by geometric structures, interlayer atomic interactions, magnetic quantization and finite-size confinement. There are drastic changes on the band symmetry, the degeneracy of the partial flat bands, the number of band-edge states, the energy dispersion, the carrier density, and the spatial symmetry of the wave function. Quasi-Landau levels might be converted into oscillating bands where extra band-edge states are created. When the upper ribbon is located at the ribbon centre, the Landau wave functions are completely destroyed. Meanwhile, a charge transfer between different layers or different sublattices in the same layer occurs. Furthermore, the density of states, reflecting the band structure, is also severely altered in terms of the number, structure, energy, and height of the prominent peaks. 相似文献
Crystallization kinetics of Mg84Ni12.5Y3.5 amorphous ribbons produced by the melt-spinning method, was studied by DSC analysis and X-ray diffraction. The effect of heating rate (from 2 to 240 K min–1) was investigated in the temperature range from 298 to 673 K. The results showed that the crystallization process took place in two stages: a) crystallization of part of the amorphous matrix to an intermediate phase and hcp-Mg, and b) transformation of the intermediate phase and the remaining amorphous material to Mg2Ni+Mg (solid solution of Y in Mg). Increasing the heating rate from 2 to 240 K min–1 results in increases of the temperature difference between the two-step crystallization of the first stage transformation processes from 33 to 56 K and in increases of the temperature difference between the two-stage transformation from 62 to 97 K.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
ABSTRACTHierarchical self-assembly underpins much of the diversity of form and function seen in soft systems, yet the pathways by which they achieve their final form are not always straightforward – intermediate steps, kinetic effects and finite sizes of aggregates all influence the self-assembly pathways of these systems. In this paper, we use molecular dynamics simulations of binary mixtures of spheres and ellipsoidal discs to investigate the self-assembly of anisotropic aggregates with internal structures. Through this, the full aggregation pathways of spontaneously chiral, multi-bilayer and multi-layer assemblies have been tracked and characterised via a semi-qualitative analysis. This includes the unambiguous identification of first-, second- and third-generation hierarchical assemblies within a single simulation. Given the significant challenge of tracking full aggregation pathways in experimental systems, our findings strongly support the notion that molecular simulation has much to contribute to improving our understanding of hierarchical self-assembling systems. 相似文献
Three dimensional (3D) reconstruction and modelling software was evaluated to find a procedure suitable for visualization of small subcellular structures in transmission electron microscope images. The method applied in this study demonstrates bizarre alterations of the structure of synaptic bodies (SBs) in pinealocytes of the guinea-pig pineal gland caused by constant illumination. It can, in general, be used for any 3D reconstruction from serial sections.
Pineal glands of five guinea-pigs (two kept under a LD cycle of 12:12 h; three kept in constant light, for 4 months) were investigated. SBs consist of an electron-dense centre with attached vesicles. Under normal lighting conditions most SBs are flat plates (about 35 nm in thickness), which eventually may be bent. The proteins comprising the molecular basis of SBs, mainly RIBEYE A and B are polymerised in a regular manner in these plates. This is not the case in other SBs, which appear as spheres or irregular lumps. SBs lie in groups in which usually some of the plates are arranged in parallel arrays
Constant illumination caused different changes in morphology: many of the SBs lie in ‘paired fields’, i.e. appear in groups attached to the cell membranes of two pinealocytes directly opposite to each other. Some of the SBs in such groups are strongly bent, showing blebs and irregular thickened areas, others seem to aggregate and show inclusions of cytoplasm. Further goblet-like, shield-like and other bizarre forms of SBs occurred and the relative number of spheroid and lump-like SBs increased. Protrusions on larger SBs suggest detachment or fusion of SB material to a greater extent than in the control animals. There is a reduction of areas in which the polymerisation of the SB proteins remains well ordered, i.e. where the typical thickness of 35 nm is maintained. It remains unclear why this polymerisation pattern is only partly affected by constant light. 相似文献
The phases, microstructure, and magnetic properties of Co80Zr18−xNbxB2 (x=1, 2, 3, and 4) melt-spun ribbons were investigated. The small substitution of Nb for Zr in the Co–Zr–B melt-spun ribbons resulted in the improvement of magnetic properties, especially the coercivity. The main effect of added Nb on the coercovity of Co–Zr–Nb–B melt-spun ribbons, originated from modification of the grain size of Co11Zr2 phase. The coercivity of the Co–Zr–Nb–B melt-spun ribbons depends on the annealing temperature. The optimal magnetic properties of Hc=5.1 kOe, and (BH)max=3.4 MGOe were obtained in the Co80Zr15Nb3B2 melt-spun ribbons annealed at 600 °C for 3 min. 相似文献