Steric Pressure between Membrane-Bound Proteins Opposes Lipid Phase Separation

Christine S. Scheve, Paul A. Gonzales, Noor Momin, and Jeanne C. Stachowiak

Cellular membranes are densely crowded with a diverse population of integral and membrane-associated proteins. In this complex environment, lipid rafts, which are phase-separated membrane domains enriched in cholesterol and saturated lipids, are thought to organize the membrane surface. Specifically, rafts may help to concentrate proteins and lipids locally, enabling cellular processes such as assembly of caveolae, budding of enveloped viruses, and sorting of lipids and proteins in the Golgi. However, the ability of rafts to concentrate protein species has not been quantified experimentally. Here we show that when membrane-bound proteins become densely crowded within liquid-ordered membrane regions, steric pressure arising from collisions between proteins can destabilize lipid phase separations, resulting in a homogeneous distribution of proteins and lipids over the membrane surface. Using a reconstituted system of lipid vesicles and recombinant proteins, we demonstrate that protein–protein steric pressure creates an energetic barrier to the stability of phase-separated membrane domains that increases in significance as the molecular weight of the proteins increases. Comparison with a simple analytical model reveals that domains are destabilized when the steric pressure exceeds the approximate enthalpy of membrane mixing. These results suggest that a subtle balance of free energies governs the stability of phase-separated cellular membranes, providing a new perspective on the role of lipid rafts as concentrators of membrane proteins.

Enzyme Molecules as Nanomotors

Samudra Sengupta †, Krishna K. Dey †, Hari S. Muddana ‡, Tristan Tabouillot †‡, Michael E. Ibele†, Peter J. Butler *‡, and Ayusman Sen *†

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/ja3091615

Publication Date (Web): January 10, 2013

Copyright © 2013 American Chemical Society

Using fluorescence correlation spectroscopy, we show that the diffusive movements of catalase enzyme molecules increase in the presence of the substrate, hydrogen peroxide, in a concentration-dependent manner. Employing a microfluidic device to generate a substrate concentration gradient, we show that both catalase and urease enzyme molecules spread toward areas of higher substrate concentration, a form of chemotaxis at the molecular scale. Using glucose oxidase and glucose to generate a hydrogen peroxide gradient, we induce the migration of catalase toward glucose oxidase, thereby showing that chemically interconnected enzymes can be drawn together.

Electronic Hybridization of Large-Area Stacked Graphene Films

Jeremy T. Robinson †*, Scott W. Schmucker †, C. Bogdan Diaconescu ‡, James P. Long †, James C. Culbertson †, Taisuke Ohta ‡, Adam L. Friedman †, and Thomas E. Beechem ‡

ACS Nano, 2013, 7 (1), pp 637–644

DOI: 10.1021/nn304834p

Publication Date (Web): December 14, 2012

Copyright © 2012 American Chemical Society

Direct, tunable coupling between individually assembled graphene layers is a next step toward designer two-dimensional (2D) crystal systems, with relevance for fundamental studies and technological applications. Here we describe the fabrication and characterization of large-area (>cm²), coupled bilayer graphene on SiO₂/Si substrates. Stacking two graphene films leads to direct electronic interactions between layers, where the resulting film properties are determined by the local twist angle. Polycrystalline bilayer films have a “stained-glass window” appearance explained by the emergence of a narrow absorption band in the visible spectrum that depends on twist angle. Direct measurement of layer orientation via electron diffraction, together with Raman and optical spectroscopy, confirms the persistence of clean interfaces over large areas. Finally, we demonstrate that interlayer coupling can be reversibly turned off through chemical modification, enabling optical-based chemical detection schemes. Together, these results suggest that 2D crystals can be individually assembled to form electronically coupled systems suitable for large-scale applications.

DNA cartoon

DNA, also known as "molecule of life" is one of the most popular and intriguing bio molecule.  I drew this cartoon for my students to better understand the specific interaction between nucleobases. Here I am showing specific interaction (love) between Adenine and Thymine, two out of four (ATCG) nucleobases of DNA. 

Superomniphobic Surfaces for Effective Chemical Shielding

Shuaijun Pan †, Arun K. Kota †, Joseph M. Mabry , and Anish Tuteja *†§

J. Am. Chem. Soc., 2013, 135 (2), pp 578–581

DOI: 10.1021/ja310517s

Publication Date (Web): December 23, 2012

Copyright © 2012 American Chemical Society

Superomniphobic surfaces display contact angles >150° and low contact angle hysteresis with essentially all contacting liquids. In this work, we report surfaces that display superomniphobicity with a range of different non-Newtonian liquids, in addition to superomniphobicity with a wide range of Newtonian liquids. Our surfaces possess hierarchical scales of re-entrant texture that significantly reduce the solid–liquid contact area. Virtually all liquids including concentrated organic and inorganic acids, bases, and solvents, as well as viscoelastic polymer solutions, can easily roll off and bounce on our surfaces. Consequently, they serve as effective chemical shields against virtually all liquids—organic or inorganic, polar or nonpolar, Newtonian or non-Newtonian

Dividing cell

This may look like yet another video of a dividing cell, but there's a catch. You are looking at chromosomes (red) being pulled apart by the mitotic spindle (green), but it's not a cell, because there's no cell membrane. Like a child sucking an egg out of its shell, Ivo Telley from the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany, removed these cellular 'innards' from a fruit fly embryo, at a stage when it is essentially a sac full of membrane-less 'cells' that divide and divide without building physical barriers to separate themselves from each other.

Read more at: http://phys.org/news/2013-01-cell-isnt-technique-captures-division.html#jCp

Nano-lens microscopes can detect viruses, other objects at nanoscale

By using tiny liquid lenses that self-assemble around microscopic objects, a team from UCLA's Henry Samueli School of Engineering and Applied Science has created an optical microscopy method that allows users to directly see objects more than 1,000 times smaller than the width of a human hair.

Read more at: http://phys.org/news/2013-01-nano-lens-microscopes-viruses-nanoscale.html#jCp

Cell-Specific Multifunctional Processing of Heterogeneous Cell Systems in a Single Laser Pulse Treatment

Ekaterina Y. Lukianova-Hleb †, Martin B. G. Mutonga †, and Dmitri O. Lapotko †‡*

ACS Nano, 2012, 6 (12), pp 10973–10981

DOI: 10.1021/nn3045243

Current methods of cell processing for gene and cell therapies use several separate procedures for gene transfer and cell separation or elimination, because no current technology can offer simultaneous multifunctional processing of specific cell subsets in highly heterogeneous cell systems. Using the cell-specific generation of plasmonic nanobubbles of different sizes around cell-targeted gold nanoshells and nanospheres, we achieved simultaneous multifunctional cell-specific processing in a rapid single 70 ps laser pulse bulk treatment of heterogeneous cell suspension. This method supported the detection of cells, delivery of external molecular cargo to one type of cells and the concomitant destruction of another type of cells without damaging other cells in suspension, and real-time guidance of the above two cellular effects.

Giant-Amplitude, High-Work Density Microactuators with Phase Transition Activated Nanolayer Bimorphs

Kai Liu †‡, Chun Cheng †, Zhenting Cheng §, Kevin Wang †‡, Ramamoorthy Ramesh †‡, and Junqiao Wu

Nano Lett., 2012, 12 (12), pp 6302–6308

DOI: 10.1021/nl303405g

Various mechanisms are currently exploited to transduce a wide range of stimulating sources into mechanical motion. At the microscale, simultaneously high amplitude, high work output, and high speed in actuation are hindered by limitations of these actuation mechanisms. Here we demonstrate a set of microactuators fabricated by a simple microfabrication process, showing simultaneously high performance by these metrics, operated on the structural phase transition in vanadium dioxide responding to diverse stimuli of heat, electric current, and light. In both ambient and aqueous conditions, the actuators bend with exceedingly high displacement-to-length ratios up to 1 in the sub-100 μm length scale, work densities over 0.63 J/cm³, and at frequencies up to 6 kHz. The functionalities of actuation can be further enriched with integrated designs of planar as well as three-dimensional geometries. Combining the superior performance, high durability, diversity in responsive stimuli, versatile working environments, and microscale manufacturability, these actuators offer potential applications in microelectromechanical systems, microfluidics, robotics, drug delivery, and artificial muscles.

Six-Color Time-Resolved Förster Resonance Energy Transfer for Ultrasensitive Multiplexed Biosensing

Daniel Geißler, Stefan Stufler, Hans-Gerd Löhmannsröben, and Niko Hildebrandt

Publication Date (Web): December 11, 2012 (Article)

DOI: 10.1021/ja310317n