Thermal undulations of quasi-spherical vesicles stabilized by gravity

The classical treatment of quasi-spherical vesicle undulations has, in the present work, been reviewed and extended to systems, which are affected by a gravitational field caused by a density difference across the membrane. The effects have been studied by the use of perturbation theory leading to corrections to the mean shape and the fluctuation correlation matrix. These corrections have been included in an analytical expression for the flicker spectrum to probe how the experimentally accessible spectrum changes with gravity. The results are represented in terms of the gravitational parameter, g ₀ = ΔρgR ⁴/κ. The contributions from gravity are in most experimental situations small and thus negligible, but for values of g₀ above a certain limit, the perturbational corrections must be included. Expressions for the relative error on the flicker spectrum have been worked out, so that it is possible to define the regime where gravity is negligible. An upper limit of g₀ has also been identified, where the error in all modes of the flicker spectrum is significant due to distortion of the mean shape. Received 9 July 2002 and Received in final form 15 November 2002 RID="a" ID="a"e-mail: jonas@kemi.dtu.dk RID="b" ID="b"e-mail: ipsen@memphys.sdu.dk     

含裂纹试件的声镜实验研究

本文利用声学显微，获得金属材料含裂纹试件细观力学特征图像。通过对比分析，从定性到定量，证明声学显微技术以其自身的独到之处成为一种有效的实验手段，在细观力学的研究中可能具有广泛的应用前景。     

Identifying surface-attached nanobubbles

Surface-attached nanobubbles are spherical cap–shaped gas bubbles that remain stable in size in saturated liquids. Their positional stability resulted in a manifold of experimental techniques to record their topography, softness, and chemical composition. Here, we summarize these techniques and how to distinguish them from nongaseous objects commonly found on surfaces in contact with liquids.     

Real-time dynamics of label-free single mast cell granules revealed by differential interference contrast microscopy

We demonstrate the capability of differential interference contrast (DIC) microscopy as a simple and useful tool for studying cellular events without fluorescence labeling. By coupling an advanced DIC microscope to a computer-controlled motorized vertical stage and a high-speed, high-resolution CCD camera, real-time three-dimensional monitoring is possible in a high-throughput manner. The performance among three modes of microscopy, bright-field, dark-field and DIC, in terms of horizontal resolving power and vertical sectioning was investigated. As a model, exocytosis of rat peritoneal mast cells was recorded on the subsecond time scale. Three-dimensional tracking of granules during degranulation was achieved and granule–granule fusion before plasma membrane fusion was recorded. Electronic Supplementary Material Supplementary material is available for this article at     

An(110)2×1、3×1再构表面STM图谱模拟——单吸附组态法

应用由激子动力学方法导出的单吸附组态扫描隧道显微镜系统隧道电流与各纯相干记忆函数之间的解析公式,具体计算模拟了An(110)2×1、3×1再构表面的STM图谱.     

Trapping of a Polyketide Synthase Module after C−C Bond Formation Reveals Transient Acyl Carrier Domain Interactions

Modular polyketide synthases (PKSs) are giant assembly lines that produce an impressive range of biologically active compounds. However, our understanding of the structural dynamics of these megasynthases, specifically the delivery of acyl carrier protein (ACP)-bound building blocks to the catalytic site of the ketosynthase (KS) domain, remains severely limited. Using a multipronged structural approach, we report details of the inter-domain interactions after C−C bond formation in a chain-branching module of the rhizoxin PKS. Mechanism-based crosslinking of an engineered module was achieved using a synthetic substrate surrogate that serves as a Michael acceptor. The crosslinked protein allowed us to identify an asymmetric state of the dimeric protein complex upon C−C bond formation by cryo-electron microscopy (cryo-EM). The possible existence of two ACP binding sites, one of them a potential “parking position” for substrate loading, was also indicated by AlphaFold2 predictions. NMR spectroscopy showed that a transient complex is formed in solution, independent of the linker domains, and photochemical crosslinking/mass spectrometry of the standalone domains allowed us to pinpoint the interdomain interaction sites. The structural insights into a branching PKS module arrested after C−C bond formation allows a better understanding of domain dynamics and provides valuable information for the rational design of modular assembly lines.     

Comparison of SEM and optical analyses of DT neutron tracks in CR-39 detectors

A solid state nuclear track detector, CR-39, was exposed to DT neutrons. After etching, the resultant tracks were analyzed using both an optical microscope and a scanning electron microscope (SEM). In this communication, both methods of analyzing DT neutron tracks are discussed.     

Multiscale analysis of water uptake and erosion in biodegradable polyarylates

The role of hydration in degradation and erosion of materials, especially biomaterials used in scaffolds and implants, was investigated by studying the distribution of water at length scales from 0.1 nm to 0.1 mm using Raman spectroscopy, small-angle neutron scattering (SANS), Raman confocal imaging, and scanning electron microscopy (SEM). The measurements were demonstrated using l-tyrosine derived polyarylates. Bound- and free- water were characterized using their respective signatures in the Raman spectra. In the presence of deuterium oxide (D₂O), H-D exchange occurred at the amide carbonyl but was not detected at the ester carbonyl. Water appeared to be present in the polymer even in regions where there was little evidence for NH to ND exchange. SANS showed that water is not uniformly dispersed in the polymer matrix. The distribution of water can be described as mass fractals in polymers with low water content (∼5 wt%), and surface fractals in polymers with larger water content (15-60 wt%). These fluctuations in the density of water distribution are presumed to be the precursors of the ∼20 μm water pockets seen by Raman confocal imaging, and also give rise to the 10-50 μm porous network seen in SEM. The surfaces of these polymers appeared to resist erosion while the core of the films continued to erode to form a porous structure. This could be due to differences in either the density of the polymer or the solvent environment in the bulk vs. the surface, or a combination of these two factors. There was no correlation between the rate of degradation and the amount of water uptake in these polymers, and this suggests that it is the bound water and not the total amount of water that contributes to hydrolytic degradation.     

Phase-Locked loops lock-in range in Frequency Modulated-Atomic Force Microscope nonlinear control system

Since the mid 1980s the Atomic Force Microscope is one the most powerful tools to perform surface investigation, and since 1995 Non-Contact AFM achieved true atomic resolution. The Frequency-Modulated Atomic Force Microscope (FM-AFM) operates in the dynamic mode, which means that the control system of the FM-AFM must force the microcantilever to oscillate with constant amplitude and frequency. However, tip-sample interaction forces cause modulations in the microcantilever motion. A Phase-Locked loop (PLL) is used to demodulate the tip-sample interaction forces from the microcantilever motion. The demodulated signal is used as the feedback signal to the control system, and to generate both topographic and dissipation images. As a consequence, a proper design of the PLL is vital to the FM-AFM performance. In this work, using bifurcation analysis, the lock-in range of the PLL is determined as a function of the frequency shift (Ω) of the microcantilever and of the other design parameters, providing a technique to properly design the PLL in the FM-AFM system.     

Imaging magnetic responses of nanomagnets by XPEEM

The Spin-resolved Photoelectron Emission Microscope (SPEEM) is a permanently installed set-up at Helmholtz-Zentrum Berlin (HZB). Due to its specific contrast it is mainly used for magnetic imaging and micro-spectroscopy with quantitative analysis. A crucial point in magnetic imaging is the application of magnetic fields. Many experiments require observation of magnetic responses or the preparation of a certain magnetic state during the measurement. We present a dedicated magnetic sample holder combining magnetic field during imaging with additional temperature control. This set-up enables SPEEM to measure magnetization curves of individual Fe nanocubes (18 nm)³ in size. If additionally alternating magnetic fields are applied we can image the local magnetic AC susceptibility (χ_AC) as a function of temperature. The latter is ideally suited to visualize local variations of the Curie temperature (T_C) in nano- and microstructures.