Anisotropic Macromolecules Research Articles

Active depletion torque between two passive rods.

The active depletion torque experienced by two anisotropic objects in an active bath is a conceptional generalization of the equilibrium entropic torque. Using Brownian dynamics simulations, we compute the active depletion torque suffered by two passive rods immersed in an ensemble of active Brownian particles. Our results demonstrate that the active depletion torque is qualitatively different from its passive counterpart. Interestingly, we find that the active depletion torque can be greatly affected by the external constraint applied on the rotational degree of freedom of the rods, and even the direction may be changed with the orientational constraint, which is in contrast to the equilibrium depletion torque. The main reason for the remarkable features of the active depletion torque is that the active particles can significantly accumulate in the vicinity of the rods due to persistent self-propulsion, which is sensitively dependent on the constraint strength and the rod configurations. Our findings could be relevant for understanding the self-assembly and dynamics of anisotropic macromolecules in living environments.

Influence of high hydrostatic pressure on the optical properties of minced fish proteins

In the functioning of various biological systems, an extremely important role is played by the dynamic properties of optically anisotropic protein macromolecules, such as translational movements, temporal correlations, and transfer processes, which are determined by the surface charge, molecular weight, molecular shape, and the nature of intermolecular interactions. An experimental study of the influence of high hydrostatic pressure on the optical properties of pike perch minced fish proteins at room temperature was carried out in this work. The intensities of the radiation transmitted through a thin plane-parallel layer of minced fish were measured, which was subjected to hydrostatic compression at a rate of ≈3 MPa/s and ≈17 MPa/s. It was found that with increasing pressure, the intensity of the transmitted light decreases nonmonotonically; in this case, the experimental plots of the I(P) dependence show the presence of broad attenuation bands of the intensity of the radiation transmitted through the sample, located at pressures of about 150, 300, and 700 MPa. The observed attenuation bands are associated with the appearance of scattering centers formed as a result of denaturation of macromolecules of proteins of various types with different functional purposes contained in the muscle tissue of pike perch.

Influence of high hydrostatic pressure on the optical properties of minced fish proteins

In the functioning of various biological systems, an extremely important role is played by the dynamic properties of optically anisotropic protein macromolecules, such as translational movements, temporal correlations, and transfer processes, which are determined by the surface charge, molecular weight, molecular shape, and the nature of intermolecular interactions.

Anisotropic dynamics of a self-assembled colloidal chain in an active bath.

Anisotropic macromolecules exposed to non-equilibrium (active) noise are very common in biological systems, and an accurate understanding of their anisotropic dynamics is therefore crucial. Here, we experimentally investigate the dynamics of isolated chains assembled from magnetic microparticles at a liquid-air interface and moving in an active bath consisting of motile E. coli bacteria. We investigate both the internal chain dynamics and the anisotropic center-of-mass dynamics through particle tracking. We find that both the internal and center-of-mass dynamics are greatly enhanced compared to the passive case, i.e., a system without bacteria, and that the center-of-mass diffusion coefficient D features a non-monotonic dependence as a function of the chain length. Furthermore, our results show that the relationship between the components of D parallel and perpendicular with respect to the direction of the applied magnetic field is preserved in the active bath compared to the passive case, with a higher diffusion in the parallel direction, in contrast to previous findings in the literature. We argue that this qualitative difference is due to subtle differences in the experimental geometry and conditions and the relative roles played by long-range hydrodynamic interactions and short-range collisions.

Spheroidal Microparticle Monolayers Characterized by Streaming Potential Measurements

An efficient method was developed enabling the synthesis of spheroidal polymer microparticles. Thorough physicochemical characteristics of the particles were acquired comprising the size, shape, electrophoretic mobility, and the diffusion coefficient. The particles were monodisperse, and their shape was well-fitted by prolate spheroids having the axis ratio equal to 4.17. Knowing the diffusion coefficient, their hydrodynamic diameter of 449 nm was calculated, which matched the value derived from Brenner's analytical expression. Particle deposition kinetics on mica and silicon/silica substrates, modified by poly(allylamine hydrochloride) (PAH) adsorption, was studied by optical microscopy and AFM imaging. The validity of the random sequential adsorption model was confirmed. Additionally, monolayers of the particles on these substrates were thoroughly characterized in situ by the streaming potential measurements for different ionic strengths. These measurements confirmed that the ζ potential change with the spheroidal particle coverage is less abrupt than for spheres and agrees with theoretical predictions. Exploiting these results, a useful analytical expression was derived that allows one to calculate the spheroidal particle coverage in situ via the streaming potential measurements. This expression, especially accurate for low coverage range, can be used for a quantitative interpretation of adsorption and desorption kinetics of anisotropic macromolecules, e.g., proteins on solid substrates.

Strong orientational coordinates and orientational order parameters for symmetric objects

Recent advancements in the synthesis of anisotropic macromolecules and nanoparticles have spurred an immense interest in theoretical and computational studies of self-assembly. The cornerstone of such studies is the role of shape in self-assembly and in inducing complex order. The problem of identifying different types of order that can emerge in such systems can, however, be challenging. Here, we revisit the problem of quantifying orientational order in systems of building blocks with non-trivial rotational symmetries. We first propose a systematic way of constructing orientational coordinates for such symmetric building blocks. We call the arising tensorial coordinates strong orientational coordinates (SOCs) as they fully and exclusively specify the orientation of a symmetric object. We then use SOCs to describe and quantify local and global orientational order, and spatiotemporal orientational correlations in systems of symmetric building blocks. The SOCs and the orientational order parameters developed in this work are not only useful in performing and analyzing computer simulations of symmetric molecules or particles, but can also be utilized for the efficient storage of rotational information in long trajectories of evolving many-body systems.

Birefringence in solutions and films of poly(N-methyl-N-vinylacetamide) macromolecules

The flow birefringence of aqueous solutions of poly(N-methyl-N-vinylacetamide) macromolecules for samples of various molecular masses is studied. It is found that the sign of birefringence changes from positive to negative with a decrease in molecular mass in accordance with the manifestation of the macroform effect on condition that the value of the inherent optical anisotropy of macromolecules is negative. With the use of the macroform-effect theory in the Maxwell phenomenon of polymer solutions, the inherent optical anisotropy of the repeating polymer unit is estimated as (a ‖−a ⊥) = −(5 ± 1) × 10−25 cm3. In turn, the value of (a ‖−a ⊥) makes it possible to separate structural and orientational contributions to the effect of spontaneous birefringence for films of various thicknesses that are cast from aqueous solutions of poly(N-methyl-N-vinylacetamide). The parameter of orientational order of chain fragments for this polymer in the near-surface layers of the films is −(0.05 ± 0.01).

Near-Interface Brownian Motion of Anisotropic Particles

Anisotropic microscopic objects are ubiquitous such as polymers, biological cells and filamentous macromolecules. Near interfaces, the thermal motion of these objects is strongly constrained due to the hydrodynamic interactions, often impacting the overall behavior of the biophysical systems. Thus, understanding this wall-effect is a key to describe many surface-related problems. Unlike the well-studied case of spheres, however, both the experimental and theoretical studies of the near-wall Brownian motion of anisotropic bodies have been elusive due to the lack of ideal imaging techniques and its intrinsically complex system. Here we present the experimental and computational study of the rotational Brownian motion of silicon nanowires tethered on a substrate. A uniquely developed interference method enables the direct visualization of the microscopic rotations of nanowires in three dimensions with high angular ( 200Hz) with normal CCD camera. The quantitative measurement at short time scales revealed the anisotropic reduction in their rotational diffusivities as a function of the inclined angles. At long time scales, the rotational diffusivity of their free ends into the inclined direction decreased more than 40-80 % in total. We then developed an implicit hydrodynamic model from a string-of-beads idealization, which is equivalent to the infinite-order Stokes superposition of solvent flow around the beads and its implicit solution for their hydrodynamic velocities. The calculation showed excellent agreement with the experimental observations. The demonstrated interferometric method, together with the versatile numerical simulation, provides a systematic approach for studying a variety of colloidal rheology as model biophysical systems. Our observation provides insights into the fundamental diffusive processes, useful for understanding the near-interface behavior of anisotropic microorganisms and macromolecules as well as property of the surfaces themselves.

Hydrodynamic, conformational, and optical properties of cellulose tridecanoate molecules in solutions

Cellulose tridecanoates of various molecular weights were synthesized, and their characteristics in chloroform and tetrachloroethane solutions were studied by methods of translational diffusion, velocity sedimentation, flow birefringence (Maxwell’s method), and viscometry. The effects of solvent and temperature on the conformational characteristics of the macromolecules under consideration were examined. The anisotropy of the monomeric unit of cellulose tridecanoates was studied, and the contribution of pendant chains to the optical anisotropy of macromolecules of cellulose esters with aliphatic substituents was analyzed.

Multiple Topologies from Glycopolypeptide–Dendron Conjugate Self-Assembly: Nanorods, Micelles, and Organogels

Glycopolypeptides (GPs) were synthesized by ring-opening polymerization of glycosylated N-carboxyanhydride monomer and attached to hydrophobic dendrons at one chain end by "click" reaction to obtain amphiphilic anisotropic macromolecules. We show that by varying polypeptide chain length and dendron generation, an organogel was obtained in dimethylsulfoxide, while nanorods and micellar aggregates were observed in aqueous solutions. Assemblies in water were characterized by electron microscopy and dye encapsulation. Secondary structure of the GP chain was shown to affect the morphology, whereas the chain length of the poly(ethylene glycol) linker between the GP and dendron did not alter rod-like assemblies. Bioactive surface chemistry of these assemblies displaying carbohydrate groups was demonstrated by interaction of mannose-functionalized nanorods with ConA.

Brownian Motion of Graphene

Brownian motion is a manifestation of the fluctuation-dissipation theorem of statistical mechanics. It regulates systems in physics, biology, chemistry, and finance. We use graphene as prototype material to unravel the consequences of the fluctuation-dissipation theorem in two dimensions, by studying the Brownian motion of optically trapped graphene flakes. These orient orthogonal to the light polarization, due to the optical constants anisotropy. We explain the flake dynamics in the optical trap and measure force and torque constants from the correlation functions of the tracking signals, as well as comparing experiments with a full electromagnetic theory of optical trapping. The understanding of optical trapping of two-dimensional nanostructures gained through our Brownian motion analysis paves the way to light-controlled manipulation and all-optical sorting of biological membranes and anisotropic macromolecules.

Translational and rotational coupling in Brownian rods near a solid surface

An anisotropic macromolecule confined between two surfaces displays Brownian motion predominantly in the plane parallel to these surfaces. It can be expected that both the rotational and translational diffusion coefficients are strongly affected by hydrodynamic interactions with the walls. This work studies the more extreme case in which a rodlike particle comes into contact with a wall or in very close proximity (order of 100 nm). Experimental data have been gathered and analyzed demonstrating the rod tethering on a surface. This is compared with numerical simulations to allow estimates of proximity to the surface. The experimental data show that particle tethered motion is subject to varied degrees of constraining which imply subtle deviations in the Brownian dynamical behavior. The key finding is that a rotational-translational coupling occurs which is markedly different from the translational and rotational movements normally predicted for anisotropic macromolecules.

Directional Brownian diffusion dynamics with variable magnitudes

The coupling effects of rotational and translational Brownian motion in anisotropic macromolecules result in differentiated diffusion constants parallel and perpendicular to the particle’s long axis in the short term. Investigations of these dynamics allow for a greater understanding of how directional alignment can be harnessed for applications. We investigate the dynamics using carbon nanofibers here and the results infer that significant reduction in the diffusion constants can result from probable hydrodynamic interactions. The diffusion constant perpendicular to the particle’s long axis is particularly reduced.

Brownian Motion of an Ellipsoid

We studied the Brownian motion of isolated ellipsoidal particles in water confined to two dimensions and elucidated the effects of coupling between rotational and translational motion. By using digital video microscopy, we quantified the crossover from short-time anisotropic to long-time isotropic diffusion and directly measured probability distributions functions for displacements. We confirmed and interpreted our measurements by using Langevin theory and numerical simulations. Our theory and observations provide insights into fundamental diffusive processes, which are potentially useful for understanding transport in membranes and for understanding the motions of anisotropic macromolecules.

Exact Scaling Laws for Electrical Conductivity Properties of Nematic Polymer Nanocomposite Monodomains

The purpose of this paper is to connect two critical aspects of nanocomposite materials engineering: the knowledge of the orientational distribution of quiescent or flowing anisotropic macromolecules, and homogenization theory of composites with spheroidal inclusions at low volume fractions. The nano-elements considered herein are derived from the class of high-aspect-ratio nematic polymers, either rod-like or platelet spheroids. By combining the two features, we derive the effective electrical conductivity tensor in closed form. Scaling properties of enhanced conductivity versus volume fraction and weak shear rate become explicit. The most dramatic effect is that the effective conductivity tensor inherits hysteresis, bi-stability, and discontinuous jumps from the isotropic–nematic first-order phase transition. These formulas reveal finer estimates that depend on a competition between two inherently extreme parameters in nematic polymer nanocomposites: the molecular aspect ratio and the conductivity ratio of the inclusions and matrix. Herein, we confine our attention to steady monodomain orientational distributions at rest and in weak shear flows, which serve as benchmarks and guides for future extensions and numerical approaches.

Interaction between charged anisotropic macromolecules: application to rod-like polyelectrolytes.

In this paper we propose a framework allowing one to compute the effective interactions between two anisotropic macromolecules, thereby generalizing the Derjaguin, Landau, Verwey, and Overbeek theory [E. J. W. Verwey and J. T. G. Overbeek, Theory of the Stability of Lyophobic Colloids (Elsevier, Amsterdam, 1948)] to nonspherical finite size colloids. We show in particular that the effective interaction potential remains anisotropic at all distances and provide an expression for the anisotropy factor. We then apply this framework to the case of finite rod-like polyelectrolytes. The calculation of the interaction energy requires the numerical computation of the surface charge profiles, which result here from a constant surface potential on the rod-like colloids. However, a simplified analytical description is proposed, leading to an excellent agreement with the full numerical solution. Conclusions on the phase properties of rod-like colloids are proposed in this context.

Spectral Analysis of Light Rayleigh Scattered on Macromolecules Reoriented by a Laser Beam

The spectral analysis of light Rayleigh scattered by monodisperse solutions of rigid, anisotropic macromolecules in the reorienting electric field of a laser beam is presented. Formulas for the spectral lineshapes of the polarized and depolarized intensities of scattered light are derived, taking into account the rotational and translational diffusion of the macromolecules in the laser field. From our numerical computations, the influence of the shape and size of the macromolecules and the influence of the reorientation parameter of their polarizability ellipsoid on the lineshapes of the respective scattered light components at a low degree of reorientation in the laser field are determined.

Refractive Index Measurements to Study the Structural Behavior of Anisotropic Macromolecules Confined between Two Mica Surfaces

A study was performed with a surface force apparatus (SFA) in which the automation of a mica surface displacement at very low speed and the use of multiple beam interferometry yield force profiles and variations of the refractive index of the macromolecular layers adsorbed between the two mica plates of the device. The performances of such an SFA are illustrated by the identification of the two molecular types of the plant lectin concanavalin A (Con A) when the anisotropic dimer Con A is converted into an isotropic tetramer form. Thus, the SFA can be a good model device for studying macromolecular interactions submitted to a variable compression stress in a confined space.

Experimental study of electro-optical rotation of helical polypeptides

The optical rotation of anisotropic macromolecules depends on the direction of propagation of linearly polarized light through the molecule. The molecular optical activity is thus a tensor property, the components of which characterize the molecular structure. A study is reported in which these components have been measured for three polypeptides in helix-promoting solvents. These were poly(γ-benzyl- l-glutamate), poly(γ-benzyl- d-glutamate) and poly(β-benzyl- l-aspartate). The measurement technique involved imposing orientational alignment upon the molecules. This alignment was achieved using pulsed electric fields and the optical rotation recorded over a range of visible light wavelengths. From the rates of response to the field changes, helical lengths were obtained. The implications for molecular conformations are discussed.

Allowance for the anisotropy of the internal field in determining the optical anisotropy of macromolecules by dynamic birefringence

A consistent calculation of the effect of the anisotropy of the internal field in the Lorentz-Lorenz expressions and in the expressions relating to dynamic birefringence of liquids or polymer solutions to the optical anisotropy of the components of their molecules or the statistical segments of the macromolecules has been made. As distinct from the approaches used previously, it is assumed, in calculating the internal field in the anisotropic medium, that the void has an ellipsoidal shape. The absolute value of the anisotropy is greater by a factor of 1.3–1.8 than the values adopted in the literature.