"Up" versus "down" alignment and hydration structures of solutes at the air/water interface revealed by heterodyne-detected electronic sum frequency generation with classical molecular dynamics simulation

We unambiguously demonstrate the "up" versus "down" alignment of a pair of prototypical solute molecules adsorbed at the air/water interface for the first time using heterodyne-detected electronic sum frequency generation spectroscopy. This molecular alignment is also reproduced by classical molecular dynamics (MD) simulation theoretically. Furthermore, the MD simulation indicates distinctly different interface-specific hydration structures around the two solute molecules, which dictate the molecular alignment at the interface. It is concluded that the hydrophilicity difference between the terminal functional groups of the solute governs the molecular orientation and surrounding hydration structures at the interface.     

OBSERVATIONS ON DEFORMATION BEHAVIOR OF HIGH PERFORMANCE FIBERS BY POLARIZING OPTICAL MICROSCOPY

By means of polarizing optical microscopy (POM), deformation behavior of four kinds of fibers, i.e. ultra highmolecular weight polyethylene (UHMW-PE) fiber, polyvinyl alcohol (PVA) fiber, polyethylene terephthalate (PET) fiber,and wholly aromatic (p-hydroxybenzoic acid/2-hydroxy-6-naphthoic acid) copolyester [P(HBA/HNA)]/PET (ACPET blend)fiber, in axial compression, axial impacting, and bending was observed. In compression, kink bands formed at an angle of55～60° to the fiber axis in 10-times-drawn UHMW-PE fiber, 75～80° in 40-times-drawn sample, 80° in PVA fiber, and 90°in the ACPET blend fiber. In impacting and bending, band angles of UHMW-PE, PVA and PET fibers are nearly the same asthose formed in compression, indicating that slip systems do not change. For any of the four kinds of fiber, band spacingexhibits great differences in compression, in impacting, and in bending, which may be attributed to the differences in thedegrees of strain or stress concentration.     

High molecular recognition: design of "Keys"

Molecular recognition between molecules is one of the most fundamental processes in biology and chemistry. The recognition process is largely driven by non-covalent forces such as hydrogen bonding, electrostatics, van der Waals forces, pi-pi interactions, and conformational energy. The complementarity between the receptor and substrate is very similar to the "lock and key" function, first described by Emil Fischer over 100 years ago, - the lock being the molecular receptor such as a protein or enzyme and the key being the substrate such as a drug, that is recognized to give a defined receptor-substrate complex. This review focuses on the design of specific ligand systems as "Keys" to enable the induced fit of these keys into the target macromolecules, protein/enzyme (Locks) with particular emphasis on protein recognition.     

PVA/PAA水凝胶纤维的电刺激响应性能

以过硫酸胺为引发剂,在PVA水溶液中原位聚合丙烯酸单体,得到的PVA／PAA混合水溶液在凝固浴硫酸胺饱和水溶液中纺丝制备了物理缠结和氢固定网络形式的PVA／PAA水凝胶纤维。该纤维于NaCl溶液中在直流电场作用下具有电流－刺激敏感性,表现为溶胀、收缩、弯曲行为。纤维的弯曲速度和最大弯曲度随电场强度和凝胶网络中PAA含量的增加而增大,随电解质溶液离子强度的变化出现临界最大值。纤维向负极弯曲的过程中,在电场下自由离子和反庆子迁移引起的渗透压主导作用,弯曲过程主要是溶胀弯曲;向正极弯曲过程中,由于电化学反应和电场作用下产生的PH梯度导致凝胶网络构像变化主导作用,弯曲主要是收缩弯曲;弯曲由负极向正极转化过程中,两种机理对弯曲的影响相对平衡。     

Peptide‐Induced Hierarchical Long‐Range Order and Photocatalytic Activity of Porphyrin Assemblies

Long‐range structural order and alignment over different scales are of key importance for the regulation of structure and functionality in biology. However, it remains a great challenge to engineer and assemble such complex functional synthetic systems with order over different length scales from simple biologically relevant molecules, such as peptides and porphyrins. Herein we describe the successful introduction of hierarchical long‐range order in dipeptide‐adjusted porphyrin self‐assembly by a thermodynamically driven self‐orienting assembly pathway associated with multiple weak interactions. The long‐range order and alignment of fiber bundles induced new properties, including anisotropic birefringence, a large Stokes shift, amplified chirality, and excellent photostability as well as sustainable photocatalytic activity. We also demonstrate that the aligned fiber bundles are able to induce the epitaxially oriented growth of Pt nanowires in a photocatalytic reaction.     

Induction of a heterochiral helix through the covalent chiral domino effect originating in the "Schellman motif"

We report unique phenomena where the transition from a homochiral helix to a heterochiral helix occurs by increasing the chain length of the l-sequence. Peptides composed of the l-Leu sequences with different lengths and the achiral nona-sequence at the C-terminal side were used here. Conformation of their peptides in solution was investigated mainly by using CD analysis in various solvents, or additionally by IR and NMR. When the l-sequence has a sufficient length, a left-handed helicity was induced in the achiral sequence. Notably, the polymeric l-sequence produced a heterochiral helix that switches the helix sense around the boundary of the chiral/achiral sequence. Energy calculation demonstrated that a stable heterochiral helix favors a bending form, while a homochiral helix takes a relatively straight form. Such a bending form was suggested to be advantageous to solvent effects. The "Schellman motif" has been recognized as a local heterochiral structure in protein helices. We propose a nucleation model of a heterochiral helix through the covalent chiral domino effect derived from the Schellman motif. The present findings not only offer us novel design of a heterochiral helix but also support an elementary model for the origins of homochiral-heterochiral structures from primitive chiral/achiral sequences.     

The dynamical response of a hydrogel fiber to electrochemical stimulation

The preparation of a smart hydrogel fiber based on chitosan/poly(ethylene glycol) is presented. The dynamics of this hydrogel fiber in response to electric stimulation is reported. The effects of a number of factors have been systematically studied, including the fiber diameter, concentration of the crosslinking agent, electric potential imposed across the fiber, pH, and ionic strength of the bath solution. Fiber deformation is expressed in terms of the curvature at the midlength of the fiber for various times. The number of bending to a given extent within a given time period is used to describe the rate of cyclic deformation. Our experimental results show a stable reversibility of bending behavior under the applied electric field. The bending curvature is proportional to the intensity of the applied electric potential. Although adequate mechanical properties are maintained, the rate of deformation can be improved via the adjustment of a number of the aforementioned extrinsic factors. These observations are interpreted in terms of fiber stiffness, fixed charge density, and swelling pressure, which depend on the hydrogel equilibrium states in different pH and ionic environments along with the electrochemical reactions under the electric field. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 236–246, 2001     

石墨多壳层形成物的离壳生长现象

用高分辨透射电子显微镜(HRTEM)表征了在特定电弧放电条件下(碳阴极,钯管阳极)产生的石墨多壳层结构。看到了各种各样的,由多层石墨壳组成,大小从约2nm到20nm以上的多面体。对它们当中一些多面体所表现出来的自组装(self-assembly)"离壳生长"(out-of-shellgrowth)现象作了鉴定,并对其生长机制进行了讨论。     

Flexural properties of sandwich composite panels with glass laminate aluminum reinforced epoxy facesheets strengthened by SMA wires

Sandwich composite panels are widely used and significant in structural applications such as aerospace, shipbuilding and transportation, etc. This is due to their specific properties such as specific stiffness, strength and energy absorption. Still, many innovations are required to develop and upgrade their mechanical properties in various loadings and conditions, specifically in bending loads. One of the methods to enhance the properties of sandwich structures is to employ various advanced materials in these structures to change their acquired properties. In the present research work, sandwich composite panels made by fiber metal laminate like glass laminate aluminum reinforced epoxy (GLARE) as the facesheets and PVC polymer foam as the core material are investigated in flexural (bending) loading condition. To change or enhance the behaviour of sandwich panel in bending loads, shape memory alloy wires are also embedded in between glass fiber reinforced epoxy composite layers in fiber metal laminate facesheets. The shape memory wires are also pre-strained in fiber reinforced epoxy composite in sandwich panels. To study the flexural properties of sandwich panels with fiber metal laminate facesheets, the effect of shape memory alloy wires and also the effect of pre-straining of the wires, three types of sandwich panels are considered and made including panels without shape memory alloy wire, two wires with 0% tensile pre-strain, and two wires with 5% tensile pre-strain for the same cross section. Due to the importance of bending properties in structural applications, the sandwich composite specimens are subjected to flexural test according to ASTM standards. The maximum of 13% increase in maximum bending load and 84% increase in breaking load for the specimens with 0% pre-strained wires are achieved. Also, the maximum displacement and the energy absorption for the specimen with 5% pre-strain was enhanced by 26.5% and 37%, respectively. The energy absorption during the flexural test is greater in case of the specimen with pre-strained wires. Moreover, the specimens with pre-strained wires show better integrity of the structure after the failure in bending. The results represent the advantage effect of shape memory alloy wires on sandwich composite panel's behaviour in bending.     

Photoinduced Bending Behavior of Cross‐linked Azobenzene Liquid‐Crystalline Polymer Films with a Poly(oxyethylene) Backbone

Cross‐linked azobenzene liquid‐crystalline polymer films with a poly(oxyethylene) backbone are synthesized by photoinitiated cationic copolymerization. Azobenzene moieties in the film surface toward the light source are simultaneously photoaligned during photopolymerization with unpolarized 436 nm light and thus form a splayed alignment in the whole film. The prepared films show reversible photoinduced bending behavior with opposite bending directions when different surfaces of one film face to ultraviolet light irradiation.

     

Energy level alignment at metal/organic semiconductor interfaces: "pillow" effect, induced density of interface states, and charge neutrality level

A unified model, embodying the "pillow" effect and the induced density of interface states (IDIS) model, is presented for describing the level alignment at a metal/organic interface. The pillow effect, which originates from the orthogonalization of the metal and organic wave functions, is calculated using a many-body linear combination of atomic orbitals Hamiltonian, whereby electron long-range interactions are obtained using an expansion in the metal/organic wave function overlap, while the electronic charge of both materials remains unchanged. This approach yields the pillow dipole and represents the first effect induced by the metal/organic interaction, resulting in a reduction of the metal work function. In a second step, we consider how charge is transferred between the metal and the organic material by means of the IDIS model: Charge transfer is determined by the relative position of the metal work function (corrected by the pillow effect) and the organic charge neutrality level, as well as by an interface parameter S, which measures how this potential difference is screened. In our approach, we show that the combined IDIS-pillow effects can be described in terms of the original IDIS alignment corrected by a screened pillow dipole. For the organic materials considered in this paper, we see that the IDIS dipole already represents most of the realignment induced at the metal/organic interface. We therefore conclude that the pillow effect yields minor corrections to the IDIS model.     

Effects of bending and torsion rigidity on deformation and breakage of flexible fibers: a direct simulation study

To understand the various mechanisms of fiber deformation of flexible fiber suspensions, we carry out a direct simulation study to analyze the effect of fiber rigidity on fiber motion in simple shear flow. Such a study may be used to investigate the critical parameters controlling the breakage of flexible fibers during processing. We model the fiber as a series of rigid spheres connected by stiff springs. The stretching, bending, and torsional rigidities are determined by Young's modulus and shear modulus to realistically model the fiber rigidity. The model correctly predicts the orbit period of fiber rotation, T ?γ, as well as the trend of critical flow strength, η ?γ/E, versus fiber aspect ratio, r(p), at which breakage occurs in simple shear flow.     

Rheological behavior of biocomposites of silk fibroin fiber and poly(ε‐caprolactone): Effect of fiber network

Rheological behavior was examined for biocomposites of rod‐like silk fibroin (SF) fiber and poly(ε‐caprolactone) (PCL) to investigate an effect(s) of the SF fiber network therein on the mechanical properties. At 160 °C where PCL was a homogeneous melt, linear viscoelastic tests revealed that the SF/PCL composites hardly relax to behave essentially as elastic solids (more precisely, plastic solids before yielding) at low frequencies. The corresponding equilibrium modulus G₀ increased strongly with the SF volume fraction ?_SF (G₀ ～ ?) and was attributable to the elastic bending of the SF fibers incorporated in the network. The Doi‐Kuzuu model for non‐Brownian rods was modified for the SF/PCL composites by incorporating the rod–rod contact at equilibrium. The G₀ calculated from this model was satisfactorily close to the data, in both ?_SF dependence and magnitude, lending support to the assignment of the composite elasticity to the fiber bending. The storage modulus G′ measured under large‐amplitude oscillatory shear (LAOS) was smaller than the linear viscoelastic G′, and this difference between the linear and nonlinear moduli was enhanced for the composites with a larger SF content and at lower frequencies. This nonlinear effect was attributable to a decrease of the effective fiber–fiber contacts sustaining the elasticity under LAOS. Under steady shear, the SF/PCL composites exhibited nonlinear (plastic) flow behavior associated with the stress overshoot, and their apparent viscosity was comparable to/lower than the viscosity of neat PCL matrix. The overshoot became much less significant on application of a second shear immediately after the first shear, while the overshoot was partly recovered after a quiescent rest between the first and second shears. These nonlinear features were attributable to slippage between shear‐oriented fibers and PCL matrix. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1957–1970, 2009     

Epitaxial clusters in single crystal hosts

It is a general phenomenon for single crystal hosts that insoluble materials precipitate in the form of epitaxially aligned clusters. Such precipitates (or inclusions) are therefore coherently aligned and offer a unique model system for X-ray diffraction studies. In this paper, the general phenomenon of epitaxial clusters is discussed, and specific X-ray experiments on krypton as well as lead clusters in aluminum are presented. A hypothesis for the origin of epitaxial alignment is described and interfacial roughening is discussed. Also discussed is superheating and supercooling at the melting transition.     

Revealing the Hierarchical Mechanical Strength of Single Cellulose Acetate Electrospun Filaments through Ultrasonic Breakage

The tensile strength of single cellulose acetate electrospun fibers is determined through sonication‐induced fragmentation in water using a model previously developed by Terentjev and co‐workers. The fragmentation of the electrospun fibers results in a gradual shortening of their length until a constant modal length is achieved. A single electrospun CA fiber tensile strength of ≈150 MPa (55–280 MPa) is determined based on fracture statistics. It is also observed that the fragmented fibers show bunches of nanofilaments at their ends with similar diameters to those of round structures observed in the cross‐section of the initial electrospun fibers (≈38 nm). The sonication of these nanofilaments gives rise to spherical particles with similar diameter dimensions, which allows the estimation of a value of the tensile strength of the order of 2 MPa for these nanostructures. The aggregation and the alignment of the nano filaments inside the electrospun fiber should be the source of its higher strength value.

     

Large-scale fabrication of aligned single-walled carbon nanotube array and hierarchical single-walled carbon nanotube assembly

Aligned single-walled carbon nanotubes (SWNTs) and hierarchical SWNT assembly were fabricated by electrospinning. The high fiber elongation and high DC electric field applied during the electrospinning process result in the orientation of the SWNTs along the axial direction of the fiber. The alignment of the electropsun composite fiber transfers this local SWNT orientation to macroscopically aligned SWNTs. After removing the polymer component from the aligned composite fiber, we produced large area aligned SWNTs. The results show that the directional control of SWNT alignment and debundling of SWNTs into individual tubes can be simultaneously realized.     

Method to assess packing quality of transmembrane alpha-helices in proteins. 2. Validation by "correct vs misleading" test

We describe a set of tests designed to check the ability of the new "membrane score" method (see the first paper of this series) to assess the packing quality of transmembrane (TM) alpha-helical domains in proteins. The following issues were addressed: (1) Whether there is a relation between the score (S(mem)) of a model and its closeness to the "nativelike" conformation? (2) Is it possible to recognize a correct model among misfolded and erroneous ones? (3) To what extent the score of a homology-built model is sensitive to errors in sequence alignment? To answer the first question, two test cases were considered: (i) Several models of bovine aquaporin-1 (target protein) were built on the structural templates provided by its homologs with known X-ray structure. (ii) Side chains in the spatial models of visual rhodopsin and cytochrome c oxidase were rebuilt based on the backbone scaffolds taken from their crystal structures, and the resulting models were iteratively fitted into the full-atom X-ray conformations. It was shown that the higher the S(mem) value of a model is, the lower its root-mean-square deviation is from the "correct" (crystal) structure of a target. Furthermore, the "membrane score" method successfully identifies the rhodopsin crystal structure in an ensemble of "rotamer-type" decoys, thus providing the way to optimize mutual orientations of alpha-helices in models of TM domains. Finally, being applied to a set of homology models of rhodopsin built on its crystal structure with systematically shifted alignment, the approach demonstrates a prominent ability to detect alignment errors. We therefore assume that the "membrane score" method will be helpful in optimization of in silico models of TM domains in proteins, especially those in GPCRs.     

The nature of "unusual" electro-optical transients observed for DNA

Unusual electro-optical transients have been observed for many different polymers and colloidal systems. These effects provoked serious confusion, because a simple-minded interpretation can be completely misleading. The case of double helical DNA is of particular interest, because DNA has been studied in more detail than other systems and because of its biological function. DNA is subject to bending, which implies a loss of symmetry. Due to its high charge density, non-symmetric conformations must have a non-symmetric distribution of charges leading to a torque of considerable magnitude in the presence of external electric fields. The dipole moment describing this torque must be calculated in a coordinate system with its origin at the center of diffusion. The resulting dipole values are in the range of thousands of Debye units. Because the new dipole type is analogous to but not identical with permanent dipoles, the notation “quasi-permanent” dipole is suggested. Application of this concept, using commonly accepted parameters for DNA and established procedures for calculation of electro-optical transients, leads to “unusual” transients. Thus, these transients must be expected from well-known parameters of DNA double helices. The influence of the quasi-permanent dipole moment may be amplified considerably by hydrodynamic coupling. This effect has been demonstrated for the case of smoothly bent rods.

Both model calculations and experiments illustrate the danger of getting data that may be completely misleading. For example, depending on pulse amplitudes and/or pulse lengths, electro-optical decays may be accelerated artificially due to superposition of decay components with opposite amplitudes. Experiments show that unusual transients and apparent acceleration effects disappear, when high frequency sine pulses are used for the electro-optical analysis of DNA.

Electro-optical effects depend upon the internal dynamics of the object under investigation. In general, the dynamics of DNA bending was assumed to be fast compared to rotational diffusion. Because stacking rearrangements in single stranded nucleic acids are relatively slow and recently the dynamics of the B–A transition was observed in the time range >1 μs, it is likely that there are also relatively slow rearrangements between bending conformers. Bending transitions are expected to be relatively fast, when there are no activation barriers in the bending pathway, and may be slow, when activation barriers must be passed between bending conformers.     

Synthesis of "solid solution" and "core-shell" type cobalt--platinum magnetic nanoparticles via transmetalation reactions

In this article, we report the synthesis of "solid solution" and "core-shell" types of well-defined Co--Pt nanoalloys smaller than 10 nm. The formation of these alloys is driven by redox transmetalation reactions between the reagents without the need for any additional reductants. Also the reaction proceeds selectively as long as the redox potential between the two metals is favorable. The reaction between Co(2)(CO)(8) and Pt(hfac)(2) (hfac = hexafluoroacetylacetonate) results in the formation of "solid solution" type alloys such as CoPt(3) nanoparticles. On the other hand, the reaction of Co nanoparticles with Pt(hfac)(2) in solution results in "Co(core)Pt(shell)" type nanoalloys. Nanoparticles synthesized by both reactions are moderately monodispersed (sigma < 10%) without any further size selection processes. The composition of the alloys can also be tuned by adjusting the ratio of reactants. The magnetic and structural properties of the obtained nanoparticles and reaction byproducts are characterized by TEM, SQUID, UV/vis, IR, EDAX, and XRD.     

轻质柔性聚酰亚胺纸基电磁屏蔽材料的制备与性能

基于聚酰亚胺纤维纸基导电骨架(PI-CP)的三维网络结构, 通过聚吡咯(PPy)的气相沉积及无钯化学镀工艺在PI-CP上进行镍基金属的层层组装, 制备了夹芯结构的镍/聚吡咯@聚酰亚胺纤维纸基电磁屏蔽材料 (Ni/PPy@PI-CP); 对其形貌、 结构、 导电性能、 力学性能及电磁屏蔽性能进行了表征; 并考察了温度、 弯曲牢度和折叠次数对Ni/PPy@PI-CP导电性能和电磁屏蔽性能的影响. 研究结果表明, Ni/PPy@PI-CP-3的电磁屏蔽性能可达到70 dB以上; Ni/PPy@PI-CP-3在弯曲变形下表现出优良的导电稳定性, 其电导率损失可以忽略不计, 经过200次的反复弯曲测试, 其电导率仍保持在92.4%以上. 此外, Ni/PPy@PI-CP还具有轻质及易于加工的特性, 并具有稳定的热性能, 于300 ℃下处理后电磁屏蔽性能仍保持在80％以上.