Spin models for orientational ordering of colloidal molecular crystals

Two-dimensional colloidal suspensions exposed to periodic external fields exhibit a variety of molecular crystalline phases. There two or more colloids assemble at lattice sites of potential minima to build new structural entities, referred to as molecules. Using the strength of the potential and the filling fraction as control parameter, phase transition to unconventional orientationally ordered states can be induced. We introduce an approach that focuses at the discrete set of orientational states relevant for the phase ordering. The orientationally ordered states are mapped to classical spin systems. We construct effective hamiltonians for dimeric and trimeric molecules on triangular lattices suitable for a statistical mechanics discussion. A mean-field analysis produces a rich phase behavior which is substantiated by Monte Carlo simulations.Comment: 19 pages, 21 figures; misplacement of Fig.3 fixe

Minimum requirements for motility of a processive motor protein.

Motor proteins generally have a two-way coupling between the ATP hydrolysis site, the lever movement and the binding affinity for their track, which allows them to perform efficient stepping. Here we explore the minimal requirements for directed motility based on simpler schemes in which the binding/unbinding from the track is decoupled from the ATPase cycle. We show that a directed power stroke alone is not sufficient for motility, but combined with an asymmetry in force-induced unbinding rates it can generate stepping. The energetic efficiency of such stepping is limited to approximately 20%. We conclude that the allosteric coupling between the ATP hydrolysis and the track binding is not strictly necessary for motility, but it greatly improves its efficiency

Geometry of the model.

<p>(A) A model monomeric motor with the linker in two possible positions, post-powerstroke (<i>M</i>) and pre-powerstroke (<i>M</i>*). The end of the linker moves for <i>d</i>_PS between the two positions. (B) In a dimeric motor the ends of the linkers are coupled through an elastic connector. (C) Asymmetric unbinding rates. The distance parameter (<i>δ</i>₊, <i>δ</i>₋) depends on the direction of the force acting on the bond.</p

States in the dimeric motor.

<p>(A) The bound states. The arrows show the extension of the elastic connector joining the two linkers. (B) Transitions between the states. Lower plane: Transitions between the states with one head unbound (states <i>m</i>, <i>m</i>*). Upper plane: Transitions between states with both heads bound. Arrows between the two planes show binding/unbinding of the lead (red) and the trail (blue) head.</p

Motor properties in the high MT affinity regime with .

<p>(A) The average motor displacement per hydrolysis of one ATP molecule of an unloaded motor as a function of the kinetic rate for different values of the asymmetry parameter. (B) The force–velocity diagram for two values of the kinetic rate , (blue) or 0.01 (red). (C) The stall force as a function of . (D) The force dependence of the work produced by the motor for parameters as in (B). In all plots (dotted line), 0.33 (dashed line), and 1.0 (solid line).</p

Reversible model.

<p>(A) The model includes the reverse recovery stroke (<i>k</i>_-RS) and the reverse power stroke (<i>k</i>_-PS). (B) The maximum work per ATP as a function of the distance parameter () and the connector stiffness <i>K</i> while other parameters have the optimal values.</p

The maximum work per ATP produced by the motor.

<p>(A,B) The maximum work per ATP as a function of two dimensionless kinetic parameters for two levels of asymmetry in the force-induced unbinding rates: (A) and and (B) . In each plot, the maximum is marked by a cross. (C) The work per ATP in the optimal kinetic regime as a function of the mean distance parameter and its asymmetry distance <i>δ</i>_<i>a</i>. The dashed line separates different bond types with respect to their behavior under backward load: The area below the dashed line describes a slip bond, the area above the line a catch bond, and the line itself an ideal bond.</p

Combination of Antimicrobial and Endotoxin-Neutralizing Activities of Novel Oleoylamines

A combination of antimicrobial and endotoxin-neutralizing activities is desired in order to prevent progression from infection to sepsis due to the release of lipopolysaccharide from dying gram-negative bacteria. Lipopolyamines have emerged as a new type of endotoxin-neutralizing compound, but their antimicrobial activity has not been investigated. We synthesized a series of 10 oleoylamines differing in the polyamino head group, particularly in the number and separation between nitrogen atoms and the position of the oleoyl moiety. Compounds showed activity against both gram-negative and gram-positive bacteria in the micromolar range. Compounds were able to provide penetration of ethidium bromide into bacteria, indicating effects on the bacterial membrane. Oleoylamines neutralized endotoxin in Limulus amoebocyte lysate assays and by neutralization of tumor necrosis factor alpha release in human blood. Comparison of biological activities of compounds identified structural properties responsible for antimicrobial activity, and quantitative structure-property relationship analysis provided a quantitative model for prediction of activity of oleoylamines