Andreja Šarlah

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 parameters, a 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.

The Winch Model Can Explain both Coordinated and Uncoordinated Stepping of Cytoplasmic Dynein

Cytoplasmic dynein moves processively along microtubules, but the mechanism of how its heads use the energy from ATP hydrolysis, coupled to a linker swing, to achieve directed motion, is still unclear. In this article, we present a theoretical model based on the winch mechanism in which the principal direction of the linker stroke is toward the microtubule-binding domain. When mechanically coupling two identical heads (each with postulated elastic properties and a minimal ATPase cycle), the model reproduces stepping with 8-nm steps (even though the motor itself is much larger), interhead coordination, and processivity, as reported for mammalian dyneins. Furthermore, when we loosen the elastic connection between the heads, the model still shows processive directional stepping, but it becomes uncoordinated and the stepping pattern shows a greater variability, which reproduces the properties of yeast dyneins. Their slower chemical kinetics allows processive motility and a high stall force without the need for coordination.

Introduction to Micro- and Macroscopic Descriptions of Nematic Liquid Crystalline Films: Structural and Fluctuation Forces

In this Chapter we introduce some of the theoretical approaches for studying thin nematic liquid crystalline systems, both on the microscopic and macroscopic level. In the former, one models the microscopic interactions between the constituing molecules, leaves the system to evolve, and then determines its macroscopic properties. If the obtained macroscopic behaviour is in agreement with the experimental evidence the modeled interaction is considered appropriate. On the other hand, the macroscopic description takes into account the universal properties of systems in the vicinity of phase and structural transitions. This means that they are based on the fact that in the vicinity of phase changes the macroscopic properties of the system do not depend on the details of the microscopic interactions but on the symmetry properties and dimensionality of the system in question. Most of our attention is focused on the effects of confinement on to liquid crystalline order. Finally, we will be interested in the resulting disjoining pressure. The evidences in experiments will be briefly mentioned.

Soft Modes in Confined Nematic Liquid Crystals

Abstract Collective fluctuations of the liquid-crystalline order in two model geometries with a domain-like equilibrium configuration are analyzed within the Landau-de Gennes theory. The wetting-induced heterophase structure is characterized by a localized soft mode corresponding to fluctuations of the position of the interface between the two phases. In a very thin hybrid cell, the soft mode is associated with a structural transition from the configuration with a step-like profile of the tilt angle to the usual bent structure. This slowdown is exhibited by the lowest director mode. In both geometries the critical behavior is expected to be observable by the evanescent light wave scattering.

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.

Fluctuations in Confined Liquid Crystals and Pretransitional Evanescent Light Scattering

Abstract The effect of surface-induced (dis)order on collective orientational fluctuations in nematic liquid crystals is studied within Landau-de Gennes theory. Close to the nematic-isotropic phase transition the corresponding spectra are characterized by soft modes, which are associated with fluctuations of the thickness of the (dis)ordered wetting layer. In this range the dynamical part of the evanescent light scattering is dominated by the soft modes.