时滞速度反馈作用下弹性梁的主共振分析

采用时滞速度反馈控制策略对轴力作用下的弹性梁进行振动控制.根据Newton第二定律建立压电耦合弹性梁的非线性振动控制模型,运用直接法得到时滞反馈作用下弹性梁主共振的一阶近似解,得出系统响应与控制参数的关系.结果表明,主共振的响应存在多解和跳跃现象,调节控制增益和时滞值可以有效抑制大幅振动.     

Aryl Radical Geometry Determines Nanographene Formation on Au(111)

The Ullmann coupling has been used extensively as a synthetic tool for the formation of C?C bonds on surfaces. Thus far, most syntheses made use of aryl bromides or aryl iodides. We investigated the applicability of an aryl chloride in the bottom‐up assembly of graphene nanoribbons. Specifically, the reactions of 10,10′‐dichloro‐9,9′‐bianthryl (DCBA) on Au(111) were studied. Using atomic resolution non‐contact AFM, the structure of various coupling products and intermediates were resolved, allowing us to reveal the important role of the geometry of the intermediate aryl radicals in the formation mechanism. For the aryl chloride, cyclodehydrogenation occurs before dehalogenation and polymerization. Due to their geometry, the planar bisanthene radicals display a different coupling behavior compared to the staggered bianthryl radicals formed when aryl bromides are used. This results in oligo‐ and polybisanthenes with predominantly fluoranthene‐type connections.     

环形线负载驱动变曲率反射镜技术

从薄板弹性理论出发,对可实现曲率变化的环形线负载驱动模型进行分析,给出了基于该模型的大镜厚比变曲率反射镜的形变方程.以较小的驱动力实现较大的中心形变为目标,利用MATLAB软件对不同反射镜厚度、驱动环半径下的反射镜形变情况进行模拟计算,结果表明,反射镜厚度范围在2~4 mm之间、驱动环半径数值在反射镜有效半径1/2处最佳.以此为依据,设计并研制了口径为100 mm、厚度为3 mm的铍青铜环形线负载驱动变曲率反射镜结构及原型样片,给出了变曲率反射镜整体结构前10阶的振动模态分析结果.完成装配后,反射镜原型样片的面形精度接近λ/30(λ为波长).对该结构进行极限曲率变化和面形精度保持的验证实验,通过对变曲率反射镜结构进行改进,环形线负载驱动能够实现超过30个波长(632.8 nm)的中心形变,且面形精度的变化与反射镜中心矢高的变化呈弱相关.     

用从头算方法研究GeCl2分子的非谐振力场和光谱常数

用从头算方法的MP2和CCSD（T）方法结合cc-pVTZ基组计算了二氯化锗同位素（⁷⁰GeCl₂、⁷²GeCl₂和⁷⁶GeCl₂）分子的平衡结构、光谱常数和非谐振力场.二氯化锗的几何结构、转动常数、振转相互作用常数、谐频、非谐振常数、四次和六次离心畸变常数、三次和四次力常数的计算结果与实验结果符合较好,二氯化锗分子的同位素效应较小,可能的原因是Ge同位素的质量变化相对较小.两种方法计算的结果均与实验结果符合,但CCSD（T）方法比MP2计算结果的偏差稍大一些,可能的原因是CCSD（T）方法在描述过共价Cl原子的电子相关时不够充分.     

The secondary Bjerknes force between two gas bubbles under dual-frequency acoustic excitation

The secondary Bjerknes force is one of the essential mechanisms of mutual interactions between bubbles oscillating in a sound field. The dual-frequency acoustic excitation has been applied in several fields such as sonochemistry, biomedicine and material engineering. In this paper, the secondary Bjerknes force under dual-frequency excitation is investigated both analytically and numerically within a large parameter zone. The unique characteristics (i.e., the complicated patterns of the parameter zone for sign change and the combination resonances) of the secondary Bjerknes force under dual-frequency excitation are revealed. Moreover, the influence of several parameters (e.g., the pressure amplitude, the bubble distance and the phase difference between sound waves) on the secondary Bjerknes force is also investigated numerically.     

Origin of intrinsic 3(10)-helix versus strand stability in homopolypeptides and its implications for the accuracy of the Amber force field

Current all-atom force fields often fail to recognize the native structure of a protein as the lowest free energy minimum. One possible cause could be the mathematical form of the potential based on the assumption that the conformation of a residue is independent of its neighbors. Here, using quantum mechanical (QM) methods (MP2/6-31g**//HF/6-31g** and MP2/cc-pVDZ//cc-pVDZ//HF/cc-pVDZ), the intrinsic correctness of the gas phase terms (without solvation) of the Amber ff03 and ff99 potentials are examined by testing their ability to reproduce the relative 3(10)-helix versus extended structure stabilities in the gas phase for 1-7-residue alanine, valine, leucine, and isoleucine homopolypeptides. The 3(10)-helix versus extended state stability strongly depends on chain length and less on the amino acid identity. The helical conformation becomes lower in energy than the extended conformation for all tested peptides longer than two residues, and its stability increases with the increase of chain length. The ff03 potential better describes the 3(10)-helix versus extended state energy than ff99 and also reproduces the curvature of the relative helix-extended state energies. Therefore, the mathematical form of the Amber potential is sufficient to describe the local effect of 3(10)-helix versus extended structure stabilization in the gas phase. However, the energy curves are shifted and the backbone geometries differ compared with the QM results. This may cause significant geometric discrepancies between native and predicted structures. Therefore, extant molecular mechanics force fields, such as Amber, need refinement of their parameters to correctly describe helix-extended state energetics and geometry of major conformations.     

Comparison of solvation-effect methods for the simulation of peptide interactions with a hydrophobic surface

In this study we investigated the interaction behavior between thirteen different small peptides and a hydrophobic surface using three progressively more complex methods of representing solvation effects: a united-atom implicit solvation method [CHARMM 19 force field (C19) with Analytical Continuum Electrostatics (ACE)], an all-atom implicit solvation method (C22 with GBMV), and an all-atom explicit solvation method (C22 with TIP3P). The adsorption behavior of each peptide was characterized by the calculation of the potential of mean force as a function of peptide-surface separation distance. The results from the C22/TIP3P model suggest that hydrophobic peptides exhibit relatively strong adsorption behavior, polar and positively-charged peptides exhibit negligible to relatively weak favorable interactions with the surface, and negatively-charged peptides strongly resist adsorption. Compared to the TIP3P model, the ACE and GBMV implicit solvent models predict much stronger attractions for the hydrophobic peptides as well as stronger repulsions for the negatively-charged peptides on the CH(3)-SAM surface. These comparisons provide a basis from which each of these implicit solvation methods may be reparameterized to provide closer agreement with explicitly represented solvation in simulations of peptide and protein adsorption to functionalized surfaces.     

Implementation and application of helix-helix distance and crossing angle restraint potentials

Based on the definition of helix-helix distance and crossing angle introduced by Chothia et al. (J Mol Biol 1981, 145, 215), we have developed the restraint potentials by which the distance and crossing angle of two selected helices can be maintained around target values during molecular dynamics simulations. A series of assessments show that calculated restraint forces are numerically accurate. Since the restraint forces are only exerted on atoms which define the helical principal axes, each helix can rotate along its helical axis, depending on the helix-helix intermolecular interactions. Such a restraint potential enables us to characterize the helix-helix interactions at atomic details by sampling their conformational space around specific distance and crossing angle with (restraint) force-dependent fluctuations. Its efficacy is illustrated by calculating the potential of mean force as a function of helix-helix distance between two transmembrane helical peptides in an implicit membrane model.     

Glycerol facilitates the disaggregation of recombinant adeno-associated virus serotype 2 on mica surface

Preparation of distributed virus on a solid substrate is a prerequisite for investigation of the properties and individualism of virus, while many previous studies showed that virus has a tendency to aggregate on solid substrates. In this communication, we report a novel approach by which well-separated recombinant adeno-associated virus serotype 2 (rAAV2) could be prepared on bare mica surface. The key technique in this approach is the addition of less than 3% (v/v) glycerol into the virus solution and subsequently deposition onto mica surface for the sample preparation. The possible mechanisms are also briefly discussed.