Enhanced Brownian Ratchet Molecular Separation Using a Self-Spreading Lipid Bilayer

A new approach is proposed for two-dimensional molecular separation based on the Brownian ratchet mechanism by use of a self-spreading lipid bilayer as both a molecular transport and separation medium. In addition to conventional diffusivity-dependence on the ratchet separation efficiency, the difference in the intermolecular interactions between the target molecules and the lipid bilayer is also incorporated as a new separation factor in the present self-spreading ratchet system. Spreading at the gap between two ratchet obstacles causes a local change in the lipid density at the gap. This effect produces an additional opportunity for a molecule to be deflected at the ratchet obstacle and thus causes an additional angle shift. This enables the separation of molecules with the same diffusivity but with different intermolecular interaction between the target molecule and surrounding lipid molecules. Here we demonstrate this aspect by using cholera toxin subunit B (CTB)–ganglioside GM1 (GM1) complexes with different configurations. The present results will unlock a new strategy for two-dimensional molecular manipulation with ultrasmall devices

Enhanced Brownian Ratchet Molecular Separation Using a Self-Spreading Lipid Bilayer

A new approach is proposed for two-dimensional molecular separation based on the Brownian ratchet mechanism by use of a self-spreading lipid bilayer as both a molecular transport and separation medium. In addition to conventional diffusivity-dependence on the ratchet separation efficiency, the difference in the intermolecular interactions between the target molecules and the lipid bilayer is also incorporated as a new separation factor in the present self-spreading ratchet system. Spreading at the gap between two ratchet obstacles causes a local change in the lipid density at the gap. This effect produces an additional opportunity for a molecule to be deflected at the ratchet obstacle and thus causes an additional angle shift. This enables the separation of molecules with the same diffusivity but with different intermolecular interaction between the target molecule and surrounding lipid molecules. Here we demonstrate this aspect by using cholera toxin subunit B (CTB)–ganglioside GM1 (GM1) complexes with different configurations. The present results will unlock a new strategy for two-dimensional molecular manipulation with ultrasmall devices

Substrate-Induced Structure and Molecular Dynamics in a Lipid Bilayer Membrane

The solid-substrate-dependent structure and dynamics of molecules in a supported lipid bilayer (SLB) were directly investigated via atomic force microscopy (AFM) and single particle tracking (SPT) measurements. The appearance of either vertical or horizontal heterogeneities in the SLB was found to be strongly dependent on the underlying substrates. SLB has been widely used as a biointerface with incorporated proteins and other biological materials. Both silica and mica are popular substrates for SLB. Using single-molecule dynamics, the fluidity of the upper and lower membrane leaflets was found to depend on the substrate, undergoing coupling and decoupling on the SiO₂/Si and mica substrates, respectively. The anisotropic diffusion caused by the locally destabilized structure of the SLB at atomic steps appeared on the Al₂O₃(0001) substrate because of the strong van der Waals interaction between the SLB and the substrate. Our finding that the well-defined surfaces of mica and sapphire result in asymmetry and anisotropy in the plasma membrane is useful for the design of new plasma-membrane-mimetic systems. The application of well-defined supporting substrates for SLBs should have similar effects as cell membrane scaffolds, which regulate the dynamic structure of the membrane

Observation of Defocus Images of a Single Metal Nanorod

We examined an emission of light from a single metallic nanoparticle based on surface plasmon (SP) resonance for determination of three-dimensional orientation of nanoparticles as well as their optical properties. The defocused image of individual Au nanorods (Au NRs) is recorded by changing the focus distance under total internal reflection microscopy (TIRM) observations. Numerical and statistical analysis revealed that the observed light distribution patterns of Au NRs defocused images were classified into two groups. One is explained by considering that a single dipole dominates its light emission property. The other is explained by assuming the presence of multidipoles. This result leads us to a consideration that the emission of light coupled with the transverse and the longitudinal SP modes was observed reflecting the optical characteristics of NRs. Additionally, unique multiple ring patterns were also observed by placing Au NRs at the vicinity of nanoscopic structure, reflecting the distance between NRs and the wall of the structure in the scale less than a few tens of nanometers. The inclusive SP measurement for both the transverse and longitudinal axes of these anisotropic metal NRs using a defocused imaging system brings us reliable optical and conformational information

Imaging Characterization of Cluster-Induced Morphological Changes of a Model Cell Membrane

Understanding the activity of nanomaterials at the lipid bilayer surface can provide key information for the feasible design of functional bioactive agents. Herein, we used micro- and nanoscopic imaging techniques to evaluate the activity of nanometer-sized inorganic clusters and report that destruction of the lipid membrane is induced by a cluster-induced morphological change on the membrane surface. As model experiments, we used the Keggin-type polyoxometalate (POM) SiW₁₂O₄₀^4– for the inorganic cluster and a 1,2-dimyristoyl-<i>sn</i>-glycerol-3-phosphatidylcholine (DMPC) and egg phosphatidylcholine (EPC) bilayer for the cell membrane. Imaging experiments revealed vigorous desorption of the lipid bilayer from solid substrate by the formation of POM–lipid assembly through a supramolecular-type assembly process in which electrostatic and hydrophobic interactions between the POM and lipid determine the efficiency and dynamics of assembly formation and thereby determine lipid desorption. Furthermore, maximum efficiency of lipid desorption was found at the phase-transition temperature. This phase dependency was explained by the formation of a “leaky interface” between the gel and fluid domains, in which freedom in the conformational change of lipids during the formation of the POM–lipid assemblies becomes maximal

Lateral Diffusion and Molecular Interaction in a Bilayer Membrane Consisting of Partially Fluorinated Phospholipids

Fluorinated lipids and surfactants are attractive biomimetic materials for the extraction and reorganization of membrane proteins because of the biological inertness of fluorocarbons. We investigated the fundamental physical properties of a partially fluorinated phospholipid (F4-DMPC), such as phase transition, area thermal expansion, and lateral lipid diffusion, to evaluate the intermolecular interaction of F4-DMPC in the hydrophobic region quantitatively on the basis of free-volume theory. Fluorescence microscope observation of the supported lipid bilayer (SLB) of F4-DMPC showed that the phase transition between the liquid crystalline and gel phases occurred at 5 °C and that the area thermal expansion coefficient was independent of the temperature near the phase transition temperature. We performed a single particle tracking of the F4-DMPC-SLB on a SiO₂/Si substrate, to measure the diffusion coefficient and its temperature dependence. The apparent activation energy (<i>E</i>′_a) of lateral lipid diffusion, which is an indicator of intermolecular interaction, was 39.1 kJ/mol for F4-DMPC, and 48.2 kJ/mol for a nonfluorinated 1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine as a control. The difference of 9 kJ/mol in <i>E</i>′_a was significant compared with the difference due to the acyl chain species among nonfluorinated phosphatidylcholine and also that caused by the addition of cholesterol and alcohol in the bilayer membranes. We quantitatively evaluated the attenuation of intermolecular interaction, which results from the competition between the dipole-induced packing effect and steric effect at the fluorocarbon segment in F4-DMPC