Monomer Diffusion Coefficient Research Articles

The effect of fatty acids, ionic strength, and electric fields on the microscopic dynamics of BSA aggregates

This paper presents the microscopic dynamics of the concentrated suspensions of bovine serum albumin (BSA) proteins and their aggregates by dynamic light scattering (DLS) experiments. The effects of fatty acids binding to BSA, as well as the ionic strength and weak electric field, are discussed for affecting the stability of BSA suspensions against calcium-induced aggregation. By variation of the ionic strength, in the absence of an external electric field, DLS experiments show that monomer–BSA interactions (in the essentially fatty acid-free case) are overall repulsive but that, nevertheless, aggregation occurs to some extent. Also, the diffusive properties of different types of BSA are explored under an applied low-AC electric field by means of in situ electric small-angle depolarized DLS experiments, which reveal a significant decrease of the translational BSA–monomer diffusion coefficient with increasing frequency, while the aggregates indicate orientational motion via rotation on applying an electric field. These observations are interpreted in terms of (localized) orientation interactions obtained as oscillations in the intermediate scattering correlation function, as well as the anomalous slower relaxations as resulting in effective (collective) dynamics between monomeric BSA and their protein aggregates.

Vapor-Phase Synthesis of Molecularly Imprinted Polymers on Nanostructured Materials at Room-Temperature.

Molecularly imprinted polymers (MIPs) have recently emerged as robust and versatile artificial receptors. MIP synthesis is carried out in liquid phase and optimized on planar surfaces. Application of MIPs to nanostructured materials is challenging due to diffusion-limited transport of monomers within the nanomaterial recesses, especially when the aspect ratio is >10. Here, the room temperature vapor-phase synthesis of MIPs in nanostructured materials is reported. The vapor phase synthesis leverages a >1000-fold increase in the diffusion coefficient of monomers in vapor phase, compared to liquid phase, to relax diffusion-limited transport and enablethe controlled synthesis of MIPs also in nanostructures with high aspect ratio. As proof-of-concept application, pyrrole is used as the functional monomer thanks to its large exploitation in MIP preparation; nanostructured porous silicon oxide (PSiO2 ) is chosen to assess the vapor-phase deposition of PPy-based MIP in nanostructures with aspect ratio >100; human hemoglobin (HHb) is selected as the target molecule for the preparation of a MIP-based PSiO2 optical sensor. High sensitivityand selectivity, low detection limit, high stability and reusability are achieved in label-free optical detection of HHb, also in human plasma and artificial serum. The proposed vapor-phase synthesis of MIPs is immediately transferable to other nanomaterials, transducers, and proteins.

Polyamide composite membranes for enhanced organic solvent nanofiltration performance by metal ions assisted interfacial polymerization method

AbstractHerein, thin‐film composite membranes consisting of poly(m‐phenyleneisophthalamide) substrate and polyamide active layer were constructed by transition metal ion‐assisted interfacial polymerization method. As compared to the traditional polyamide membranes, a much thinner polyamide layer (33 vs. 200 nm) can be synthesized with higher permeance (3.2 vs. 0.62 L m−2 h−1 bar−1) in the organic solvent nanofiltration. Similarly, the prepared membranes maintained a high rejection (>99%) for various dyes. Optimal membranes prepared by using Co2+ exhibited strong tolerance to various organic solvents with good long‐term stability. Positron annihilation spectroscopy and other characterization methods were used to investigate the relationships between the membrane microstructures and the enhanced separation performance. Based on molecular dynamics simulation, it was found that the diffusion coefficient of polyethyleneimine monomer decreased by about 18 times after adding Co2+ to the aqueous solution (forming coordination interaction). This procedure has great potential and sustainability for practical organic solvent nanofiltration applications.

Optimizing the surface properties of nanofiltration membrane by tailoring the diffusion coefficient of amine monomer

High-performance hollow fiber NF membranes with network structure were successfully prepared by introducing macromolecular polyvinyl alcohol (PVA) into aqueous phase to change the diffusion coefficient of amine monomer (piperazine, PIP) in organic phase. Based on Wilke-Change equation, a theoretical model (DAO) dealing with viscosity of aqueous phase, molecular weight of PVA, and diffusion coefficient of amine monomer in organic phase was proposed. The applicability of the DAO model was verified by ultraviolet spectrophotometry (UV) monitoring and one-dimensional diffusion model. Subsequently, the structure of NF membranes was analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results suggested that the diffusion coefficient of amine monomer was negatively correlated with aqueous phase's viscosity and polymer's molecular weight. With PVA205 (Molecular weight is 205000 Da) used to change the viscosity of aqueous phase, the diffusion coefficient of PIP (in n-hexane) was close to 2.70 × 10−6 cm2/s, and the resultant NF membrane (NF-PVA205-1) showed excellent performance; specifically, NF-PVA205-1 achieved a flux as 107 L/m2·h (4 bar), which is 3.5-times higher than that of pure polyamide NF membrane, and the rejection rate for sodium sulfate can reach 97.9%. Such a facile fabrication method proposed in this study can balance the "trade-off" effect of traditional NF membrane.

Preparation of acid-resistant nanofiltration membrane with dually charged separation layer for enhanced salts removal

Acid-resistant nanofiltration (NF) membrane with precise ionic separation performance and high permeability is imperative for acidic wastewater reuse. However, the trade-off between ions rejection and water permeability is especially serious for acid resistant NF membrane because of the uneven separation layer structure. Herein, we dissect the two-layer reverse interfacial polymerization (r-IP) process for the preparation of high-performance acid-resistant membrane by investigating monomer reactions and phase interface properties. Mixed organic monomers of 1,4-phenylene diisocyanate and trimesoyl chloride are used to react with the negatively charged aqueous monomer 3-aminobenzenesulfonamide to fabricate interlayer with high negative charge density, and then performs a secondary r-IP reaction with another positively charged aqueous monomer polyethyleneimine to construct a positively charged top layer. By changing the monomer compositions and concentrations, the relationship between monomer reaction and membrane performance is elucidated. Importantly, when aqueous additives of ethanol and glycerol are used to regulate the monomer diffusion coefficient, the membrane surface properties and permselectivity can be improved. Compared to the blank, the prepared membrane shows a 200% water permeability enhancement (2.6 Lm-2h-1bar−1) and desirable rejections of both Na2SO4 (97.7%) and MgCl2 (93.0%). Moreover, the smooth and hydrophilic surface endows the membrane with excellent anti-fouling performance at acidic pH. Benefiting from the robust structure of poly(amide-sulfonamide) and polyurea in the backbone, the membrane shows impressive acid resistance in 10 wt% H2SO4 and long-term stability in heavy metal removal. This work opens new pathways to prepare acid stable NF membrane with desirable permselectivity.

Diffusing Diffusivityin Dynamics of Unentangled Polymer Melts

AbstractThe exact results for Rouse chain dynamics in the crowded environment modeled bydiffusing diffusivity(N. Ahamad and P. Debnath,Phys. A2020,549, 124335) in explaining polymer melt dynamics is explored. Theory for mode accounts for anomalous and non‐Gaussian diffusion in the center of mass (c.m.) and monomer dynamics of united atom molecular dynamics simulation trajectories of unentangled polyethylene melts, with excellent agreement. Accurate estimates of scaling relations for c.m. and monomer mean square displacement with time interval are obtained. The corresponding c.m. and monomer diffusion coefficients as a function of confirmdiffusing diffusivityin polymer melts. It is shown shown that the dependent parameters of the modified Rouse model expressed in terms of all modes are reducible to an identical set of per mode parameter values on application to real data. Analysis of velocity autocorrelation functions indicates that the theory is tested on simulation trajectories of polymer melts with friction. The theory estimates for additional power‐law exponents in the exponential of displacement distributions agree with the data. The results show fairly convincingly that the Rouse model withdiffusing diffusivitymay find scope as an alternate theoretical formalism for polymeric liquids.

Mechanism of Post-Radiation-Chemical Graft Polymerization of Styrene in Polyethylene.

Structural and morphological features of graft polystyrene (PS) and polyethylene (PE) copolymers produced by post-radiation chemical polymerization have been investigated by methods of X-ray microanalysis, electron microscopy, DSC and wetting angles measurement. The studied samples differed in the degree of graft, iron(II) sulphate content, sizes of PE films and distribution of graft polymer over the polyolefin cross section. It is shown that in all cases sample surfaces are enriched with PS. As the content of graft PS increases, its concentration increases both in the volume and on the surface of the samples. The distinctive feature of the post-radiation graft polymerization is the stepped curves of graft polymer distribution along the matrix cross section. A probable reason for such evolution of the distribution profiles is related to both the distribution of peroxide groups throughout the sample thickness and to the change in the monomer and iron(II) salt diffusion coefficients in the graft polyolefin layer. According to the results of electron microscope investigations and copolymer wettability during graft polymerization, a heterogeneous system is formed both in the sample volume and in the surface layer. It is shown that the melting point, glass transition temperature and degree of crystallinity of the copolymer decreases with the increasing proportion of graft PS. It is suggested that during graft polymerization a process of PE crystallite decomposition (melting) and enrichment of the amorphous phase of graft polymer by fragments of PE macromolecules occurs spontaneously. The driving force of this process is the osmotic pressure exerted by the phase network of crystallites on the growing phase of the graft PS.

Effect of Ionic Strength on the Aggregation Propensity of Aβ1-42 Peptide: An In-silico Study

Background: Aggregation of misfolded proteins under stress conditions in the cell might lead to several neurodegenerative disorders. Amyloid-beta (Aβ1-42) peptide, the causative agent of Alzheimer’s disease, has the propensity to fold into β-sheets under stress, forming aggregated amyloid plaques. This is influenced by factors such as pH, temperature, metal ions, mutation of residues, and ionic strength of the solution. There are several studies that have highlighted the importance of ionic strength in affecting the folding and aggregation propensity of Aβ1-42 peptide. Objective: To understand the effect of ionic strength of the solution on the aggregation propensity of Aβ1-42 peptide, using computational approaches. Materials and Methods: In this study, Molecular Dynamics (MD) simulations were performed on Aβ1-42 peptide monomer placed in (i) 0 M, (ii) 0.15 M, and (iii) 0.30 M concentration of NaCl solution. To prepare the input files for the MD simulations, we have used the Amberff99SB force field. The conformational dynamics of Aβ1-42 peptide monomer in different ionic strengths of the solutions were illustrated from the analysis of the corresponding MD trajectory using the CPPtraj tool. Results: From the MD trajectory analysis, we observe that with an increase in the ionic strength of the solution, Aβ1-42 peptide monomer shows a lesser tendency to undergo aggregation. From RMSD and SASA analysis, we noticed that Aβ1-42 peptide monomer undergoes a rapid change in conformation with an increase in the ionic strength of the solution. In addition, from the radius of gyration (Rg) analysis, we observed Aβ1-42 peptide monomer to be more compact at moderate ionic strength of the solution. Aβ1-42 peptide was also found to hold its helical secondary structure at moderate and higher ionic strengths of the solution. The diffusion coefficient of Aβ1-42 peptide monomer was also found to vary with the ionic strength of the solution. We observed a relatively higher diffusion coefficient value for Aβ1-42 peptide at moderate ionic strength of the solution. Conclusion: Our findings from this computational study highlight the marked effect of ionic strength of the solution on the conformational dynamics and aggregation propensity of Aβ1-42 peptide monomer.

The dynamics of surface adsorption and foam formation of carbonate modified nonionic surfactants

For a group of nonionic surfactants with polar head groups synthesized by copolymerization of ethylene oxide and carbon dioxide, the adsorption dynamics at the water-air surface were investigated by dynamic surface tension measurements using the maximum bubble pressure technique. From the apparent diffusion coefficients at different concentrations, the diffusion coefficients of surfactant monomers and micelles were deduced and compared to conventional nonionic surfactants. These transport properties were correlated with results from foamability studies in Bartsch tests as well as sparging tests. The obtained foam volumes show a good correlation with the diffusion coefficients of the surfactant monomers multiplied with the concentration of available monomers, the cmc. The same relation is found for the foam stability, expressed in the foam half lifetime. The correlations show for surfactant solutions well above the critical micelle concentration that the transport of surfactant monomers to the interface rather than the micelles is the essential step in both, the foam formation as well as the foam stabilization, following a simple Fickian diffusion law.

Molecular Dynamics of Polyrotaxane in Solution Investigated by Quasi-Elastic Neutron Scattering and Molecular Dynamics Simulation: Sliding Motion of Rings on Polymer.

In this study, we investigated the molecular dynamics of polyrotaxane (PR), composed of α-cyclodextrins (CDs) and a poly(ethylene glycol) (PEG) axial chain, in solution by means of quasi-elastic neutron scattering (QENS) measurements and full-atomistic molecular dynamics (MD) simulations. From QENS experiments, we estimated the diffusion coefficients of CD and PEG monomers in PR, which are in quantitative agreement with those obtained by MD simulations. By analyzing the simulation results, we succeeded, for the first time, in observing and quantifying the sliding motion of CD along a PEG chain. The diffusion coefficient for the sliding motion is almost 6 times lower than that of the translational diffusion of CD in PR at room temperature. The retardation of the sliding motion is caused by the energy barrier on PEG produced by molecular interactions between CD and PEG. We propose a simple equation to describe the diffusion coefficient of the sliding dynamics in PR by combining the Einstein-Stokes diffusion model and a one-dimensional jump diffusion model. This work provides a general strategy for the molecular designs to control the sliding motion in PR.

Long-time dynamics and hydrodynamic correlations in quasi-two-dimensional anisotropic colloidal mixtures

The physical properties of multicomponent anisotropic colloidal dispersions are still far from being fully understood. This is mainly due to the fact that dealing with nonspherical particles and highly directional interactions is, from both experimental and theoretical points of view, a complicated task. In fact, experiments are scarce, and we still lack of model colloidal dispersions that allow us to simultaneously simplify and capture the complexity of such systems. In this contribution, we report on an experimental study of the hydrodynamic correlations and the long-time dynamics in anisotropic colloidal mixtures. The latter are composed of monomers and dimers highly confined between two parallel walls. The diffusive behavior is studied from low to intermediate particle concentrations. As concentration increases, crowding plays a significant role on the translational and rotational diffusion coefficients of monomers and dimers at short and long times. Nevertheless, in the short-time regime, the ratio between the dimer diffusion coefficients, parallel and perpendicular to the main axis of the dimer, becomes independent on the composition and total packing fraction; it depends only on the two translational hydrodynamic friction coefficients of the dimer. At long times, the dimer mean-square displacements, both parallel and perpendicular, reach the same value; i.e., the long-time diffusion coefficients seem not to be sensitive to the particle anisotropy and the crowded environment at which the dimer diffuses. Interestingly, both dynamical scenarios are analogous to the reported case for an isolated ellipsoid of similar dimensions, even though the dimer experiences collisions and hydrodynamic interactions with monomers and other dimers. Our findings also point out that the two-dimensional hydrodynamic correlations, monomer-monomer and monomer-dimer, differ mainly at short distances due to the difference in the shape of both types of species. However, contrary to the case of particles in a three-dimensional unbounded fluid, such hydrodynamic correlations decay faster, exhibiting a dipole-like long-ranged behavior with a ${r}^{\ensuremath{-}2}$ dependence, similar to the results previously reported for a monodisperse quasi-two-dimensional colloidal dispersion. Our measurements are corroborated by means of molecular dynamics computer simulations that explicitly include the information of the host solvent.

Mechanistic study on comonomer effect in ethylene/1-hexene copolymerization with TiCl4/MgCl2 model Ziegler-Natta catalysts

Two MgCl2-supported Ziegler-Natta model catalysts were prepared by contacting activated MgCl2 with deficient or excess amount of TiCl4. Ethylene/1-hexene copolymerization with the catalysts was conducted under different 1-hexene feed, and active center concentration of the reaction system was determined by quench-labeling method using thiophene-2-carbonyl chloride as the quencher. The catalytic activity was only moderately enhanced (increment <50%) by adding 1-hexene in ethylene polymerization with the model catalyst (Cat-1) of 0.1 wt% Ti content, and the active center ratio ([C*]/[Ti]) was only slightly increased (from 64 to 72 mol%). The comonomer activation effect in Cat-1 catalyzed copolymerization was mainly attributed to increase of apparent propagation rate constant by the comonomer, which can be explained by reduction of mass transfer barrier in the polymer/catalyst particles because of larger monomer diffusion coefficients in the copolymer phase than in the more crystalline polyethylene phase. In contrast, the activity was nearly tripled by adding 1-hexene when the catalyst of 1.47% Ti content (Cat-2) was used, meanwhile its [C*]/[Ti] was also tripled (from 15 to 49 mol%). The strong comonomer activation effect in Cat-2 catalyzed copolymerization was caused by marked increase of active center concentration. In this case, fragmentation of the polymer/catalyst particles was intensified by adding the comonomer, and more active centers that were originally inaccessible to the cocatalyst and monomer were exposed through the particle fragmentation. According to the results of this work, the comonomer effect in conventional supported Ziegler-Natta catalysts can be explained by the combination of physical factor (increase in diffusion coefficient) and chemical factor (exposure of inaccessible active center precursors through particle fragmentation).

Regression Analysis for Nucleation-Elongation Model of Supramolecular Assembly: How To Determine Nucleus Size.

Nucleation-elongation is known to give satisfactory descriptions of many supramolecular polymerization systems in thermal equilibrium. Its key feature is the necessity to form a "nucleus" consisting of a certain number of monomer units before being able to grow into a longer polymer chain. The size of the nucleus has significant implications for the understanding of the supramolecular polymerization mechanism. Here we investigate how experiments can give information on the nucleus size by regression analysis of various types of measurements. The measurements of free monomer concentrations, diffusion coefficients, and calorimetric response as functions of concentration or temperature are considered. The nucleation-elongation model with a general value for the nucleus size is used to provide mathematical expressions for these experimental observables. Numerical experiments are performed where experimental errors are simulated by computer-generated random numbers, and it is investigated whether least-squares fitting analyses can give the correct values of the nucleus size in the presence of experimental errors. It is recommended that the calorimetric measurements such as differential scanning calorimetry (DSC) or isothermal titration calorimetry (ITC) be performed under various conditions to correctly determine the nucleus size experimentally.

Toward a Theory of Diffusion of a Nonionic Surfactant with Variable Aggregation Number in a Micellar System

Since the aggregation number of micelles always grows with concentration, and, in some cases this dependence is noticeable even for spherical micelles, there is a need to revise the theory of micellization, in which the aggregation number is assumed to be constant. This work reformulates the theory of diffusion of nonionic surfactants in micellar solutions with regard to the variability of the aggregation number. A new formula, which expresses the diffusion coefficient of a surfactant via the diffusion coefficients of monomers and micelles, contains an additional factor capable of increasing the diffusion coefficient with the surfactant concentration. However, this factor is not overly strong, and the “old” part of the formula acts in the opposite direction; as a result, the conventional decrease in the diffusion coefficient of a nonionic surfactant remains prevailing. The analytical consideration has been supplemented with numerical calculations, the results of which are presented in the tables.

The Formation of Amyloid Fibril on Two-Dimensional Surface

Several neurodegenerative diseases are accompanied by fibrillar aggregates or amyloids in different parts of the brain. The formation of amyloid fibrils is assisted by the existence of membranes and crowded environment in the brain. However, the detailed mechanism of the way membrane surfaces and other interfaces affect the fibril formation rate is still unclear. In recent work, we have demonstrated a label-free procedure method to study surface effect on fibril growth. In this method, we prepare samples by controlling incubation time and drying process to pause the fibril growth on a hydrophilic surface. Using atomic force microscopy (AFM), the fibril's morphologies and length distribution can be characterized over time. As such, we can study the kinetics of fibril growth on various surfaces. In addition to the kinetic information, we also obtain the dispersion of these surface-mediated fibrils. In the case where fibrils are formed at the interface through a diffusion-limited aggregation process, the dispersion reflects the diffusion coefficient of monomers on surfaces. Overall, the combination of experimental data and the theoretical approach presented here elucidate the importance of surface effects in amyloid formation of short peptides.

Modeling of kinetically limited growth rate for solution-synthesized germanium nanocrystals

Solution synthesis is a common method for preparing semiconductor nanocrystals (NCs). For such solution synthesis, many investigations have considered diffusion-limited growth, in which the diffusion of reactants through the boundary layer (BL) limits the NC growth rate. These studies often model the growth rate with a diffusion BL thickness much larger than the NC size and with unphysically low diffusion constants on the order of 10−12 cm2 s−1. In this work, we have examined the growth of Ge NCs synthesized by injecting Ge amide precursors into a solution of 1-octadecene, oleylamine, and hexadecylamine. We have previously established this low-temperature, low-pressure synthesis route. The resulting Ge growth rate compares well with our model, in which we consider both BL diffusion and surface kinetics of Ge precursors and organic ligand adsorbates. Our modeling results suggest that the NC growth is limited not by diffusion, but by the surface adsorption and desorption kinetics. The BL thickness in the stirred reaction vessel is calculated to be on the same order of magnitude as the crystal radius; therefore, the surface kinetics cannot be ignored. Furthermore, the synthesis temperature is near 300 °C, where the Ge monomer diffusion coefficient within the growth solution is substantially increased and estimated to be on the order of 10−5 cm2 s−1. These considerations agree well with our experimentally measured growth rate and strongly suggest that the NC size evolution is controlled primarily by the surface kinetics.

Global properties and propensity to dimerization of the amyloid-beta (12-28) peptide fragment through the modeling of its monomer and dimer diffusion coefficients and electrophoretic mobilities.

Neuronal activity loss may be due to toxicity caused mainly by amyloid-beta (1-40) and (1-42) peptides forming soluble oligomers. Here the amyloid-beta (12-28) peptide fragment (monomer) and its dimer are characterized at low pH through the modeling of their diffusion coefficients and effective electrophoretic mobilities. Translational diffusion coefficient experimental values of monomer and dimer analogs of this peptide fragment and monomer and dimer mixtures at thermodynamic equilibrium are used as reported in the literature for different monomer initial concentrations. The resulting electrokinetic and hydrodynamic global properties are employed to evaluate the amyloid-beta (12-28) peptide fragment propensity to dimerization through a thermodynamic theoretical framework. Therefore equilibrium constants are considered at pH 2.9 to elucidate one of the amyloidogenic mechanisms involving the central hydrophobic region LVFFA of the peptide spanning residues 17-21 associated with phenylalanine at positions 19 and 20 in the amino acid sequence of amyloid-beta peptides. An analysis demonstrating that peptide aggregation is a concentration-dependent process is provided, where both pair and intraparticle charge regulation phenomena become relevant. It is shown that the modeling of the effective electrophoretic mobility of the amyloid-beta (12-28) peptide fragment is crucial to understand the effect of hydrophobic region LVFFA in the amyloidogenic process.

Diffusion-controlled adsorption kinetics at the interface between air and aqueous micellar solution of heptaethylene glycol monododecyl ether

The diffusion-controlled adsorption kinetics of micellar surfactant C12E7 (heptaethylene glycol monododecyl ether) solutions was studied theoretically and experimentally. The corrected diffusion equation, which was used to describe the diffusion of the monomers in the micellar solutions, was solved under the initial and boundary conditions by means of Laplace transformation. The dynamic surface adsorption γ(t) as a function of surface lifetime t, monomer diffusion coefficient D and the demicellization constant was derived. The dynamic surface tensions γ(t) of aqueous submicellar and micellar solutions were measured via maximal bubble pressure method. By analyzing the experimental data, the determined demicellization constant of C12E7 at 25°C was between 100–116 s−1.

Adsorption behavior of non-conventional eco-friendly tyrosine glycerol ether surfactants

Due to the increasing demand for environmentally friendly surfactants, a novel non-conventional glycerol ether surfactant with 12 carbon atoms in the hydrophobic alkyl chain and the amino acid tyrosine in the hydrophilic head (TyrGE12) is synthesized. Measurements of dynamic surface tension by maximum bubble pressure tensiometry and drop profile tensiometry are carried out for alkali aqueous solutions over a wide concentration range and for surface ages spanning from milliseconds to hours. Critical Micelle Concentration (CMC) value, standard free energy of adsorption and other physicochemical properties are calculated based on static (equilibrium) surface tension measurements. Dynamic surface tension data are analyzed using approximate solutions of the Ward and Tordai model to estimate diffusion coefficients for short and long adsorption times. For concentrations above the CMC, the effective diffusion coefficient (D*) of monomers is also estimated. Diffusion coefficients for short adsorption times lie in a physically reasonable range whereas for long adsorption times the computed very low values indicate the presence of an adsorption barrier. Interfacial dilatational storage modulus and loss modulus measurements indicate a dominant elastic character of the adsorption layer for all the examined concentrations and oscillation frequencies. Local peaks in the values of both moduli, below and above CMC, imply that structural changes in the adsorption layer might be partially affected by the presence of micelles.

Surface-induced assembly of apolipoprotein A-I: Implications for symmetry-driven non-cooperative clustering

In condensed matter physics the geometry of a crystal is determined by the mechanism of condensation. In biology, the link between clustering mechanisms and the shape of a protein crystal is not well defined. To gain more insight into the problem, we studied clustering of apolipoprotein A-I (apo A-I) on a solid surface using AFM. The amyloidogenic protein apo A-I is the main protein component of high density lipoprotein and thus reduces the risk of atherosclerosis. We found that apo A-I clustered to form nano-scale, symmetrical clusters on mica. Statistical analysis of size distribution for several thousand clusters suggested that the clustering reaction followed a non-cooperative kinetic scheme characterized by a single equilibrium constant of 0.92·106M−1 and a change in free energy (ΔG) of −0.03kJmole−1/residue. This is close to ΔG of−0.04kJmole−1/residue for apo A-I binding to phospholipid membrane; and 30-fold smaller than ΔG for β-amyloid formation by apo A-I. The high symmetry of the clusters is consistent with an isotropic diffusion coefficient of protein monomers on the surface of the substrate. This previously unrecognized link between protein clustering mechanism and the symmetry of the growth pattern may have important implications in medicine, pharmaceutics and polymer science.