94 research outputs found
The low-density/high-density liquid phase transition for model globular proteins
The effect of molecule size (excluded volume) and the range of interaction on
the surface tension, phase diagram and nucleation properties of a model
globular protein is investigated using a combinations of Monte Carlo
simulations and finite temperature classical Density Functional Theory
calculations. We use a parametrized potential that can vary smoothly from the
standard Lennard-Jones interaction characteristic of simple fluids, to the ten
Wolde-Frenkel model for the effective interaction of globular proteins in
solution. We find that the large excluded volume characteristic of large
macromolecules such as proteins is the dominant effect in determining the
liquid-vapor surface tension and nucleation properties. The variation of the
range of the potential only appears important in the case of small excluded
volumes such as for simple fluids. The DFT calculations are then used to study
homogeneous nucleation of the high-density phase from the low-density phase
including the nucleation barriers, nucleation pathways and the rate. It is
found that the nucleation barriers are typically only a few and that
the nucleation rates substantially higher than would be predicted by Classical
Nucleation Theory.Comment: To appear in Langmui
Quantitative plane-resolved crystal growth and dissolution kinetics by coupling in situ optical microscopy and diffusion models : the case of salicylic acid in aqueous solution
The growth and dissolution kinetics of salicylic acid crystals are investigated in situ by focusing on individual microscale crystals. From a combination of optical microscopy and finite element method (FEM) modeling, it was possible to obtain a detailed quantitative picture of dissolution and growth dynamics for individual crystal faces. The approach uses real-time in situ growth and dissolution data (crystal size and shape as a function of time) to parametrize a FEM model incorporating surface kinetics and bulk to surface diffusion, from which concentration distributions and fluxes are obtained directly. It was found that the (001) face showed strong mass transport (diffusion) controlled behavior with an average surface concentration close to the solubility value during growth and dissolution over a wide range of bulk saturation levels. The (1̅10) and (110) faces exhibited mixed mass transport/surface controlled behavior, but with a strong diffusive component. As crystals became relatively large, they tended to exhibit peculiar hollow structures in the end (001) face, observed by interferometry and optical microscopy. Such features have been reported in a number of crystals, but there has not been a satisfactory explanation for their origin. The mass transport simulations indicate that there is a large difference in flux across the crystal surface, with high values at the edge of the (001) face compared to the center, and this flux has to be redistributed across the (001) surface. As the crystal grows, the redistribution process evidently can not be maintained so that the edges grow at the expense of the center, ultimately creating high index internal structures. At later times, we postulate that these high energy faces, starved of material from solution, dissolve and the extra flux of salicylic acid causes the voids to close
Multifractal analysis of the electronic states in the Fibonacci superlattice under weak electric fields
Influence of the weak electric field on the electronic structure of the
Fibonacci superlattice is considered. The electric field produces a nonlinear
dynamics of the energy spectrum of the aperiodic superlattice. Mechanism of the
nonlinearity is explained in terms of energy levels anticrossings. The
multifractal formalism is applied to investigate the effect of weak electric
field on the statistical properties of electronic eigenfunctions. It is shown
that the applied electric field does not remove the multifractal character of
the electronic eigenfunctions, and that the singularity spectrum remains
non-parabolic, however with a modified shape. Changes of the distances between
energy levels of neighbouring eigenstates lead to the changes of the inverse
participation ratio of the corresponding eigenfunctions in the weak electric
field. It is demonstrated, that the local minima of the inverse participation
ratio in the vicinity of the anticrossings correspond to discontinuity of the
first derivative of the difference between marginal values of the singularity
strength. Analysis of the generalized dimension as a function of the electric
field shows that the electric field correlates spatial fluctuations of the
neighbouring electronic eigenfunction amplitudes in the vicinity of
anticrossings, and the nonlinear character of the scaling exponent confirms
multifractality of the corresponding electronic eigenfunctions.Comment: 10 pages, 9 figure
Conversion of self-assembled monolayers into nanocrystalline graphene: Structure and electric transport
Graphene-based materials have been suggested for applications ranging from
nanoelectronics to nanobiotechnology. However, the realization of
graphene-based technologies will require large quantities of free-standing
two-dimensional (2D) carbon materials with tuneable physical and chemical
properties. Bottom-up approaches via molecular self-assembly have great
potential to fulfil this demand. Here, we report on the fabrication and
characterization of graphene made by electron-radiation induced cross-linking
of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In
this process, the SAM is converted into a nanocrystalline graphene sheet with
well defined thickness and arbitrary dimensions. Electric transport data
demonstrate that this transformation is accompanied by an insulator to metal
transition that can be utilized to control electrical properties such as
conductivity, electron mobility and ambipolar electric field effect of the
fabricated graphene sheets. The suggested route opens broad prospects towards
the engineering of free-standing 2D carbon materials with tuneable properties
on various solid substrates and on holey substrates as suspended membranes.Comment: 30 pages, 5 figure
Role of water in Protein Aggregation and Amyloid Polymorphism
A variety of neurodegenerative diseases are associated with the formation of
amyloid plaques. Our incomplete understanding of this process underscores the
need to decipher the principles governing protein aggregation. Most
experimental and simulation studies have been interpreted largely from the
perspective of proteins: the role of solvent has been relatively overlooked.
In this Account, we provide a perspective on how interactions with water
affect folding landscapes of A monomers, A oligomer
formation, and protofilament formation in a Sup35 peptide. Simulations show
that the formation of aggregation-prone structures (N) similar to the
structure in the fibril requires overcoming high desolvation barrier. The
mechanism of protofilament formation in a polar Sup35 peptide fragment
illustrates that water dramatically slows down self-assembly. Release of water
trapped in the pores as water wires creates protofilament with a dry interface.
Similarly, one of the main driving force for addition of a solvated monomer to
a preformed fibril is the entropy gain of released water.
We conclude by postulating that two-step model for protein crystallization
must also hold for higher order amyloid structure formation starting from
N. Multiple N structures with varying water content results in a number
of distinct water-laden polymorphic structures. In predominantly hydrophobic
sequences, water accelerates fibril formation. In contrast, water-stabilized
metastable intermediates dramatically slow down fibril growth rates in
hydrophilic sequences.Comment: 27 pages, 4 figures; Accounts of Chemical Research, 201
Monitoring and Scoring Counter-Diffusion Protein Crystallization Experiments in Capillaries by in situ Dynamic Light Scattering
In this paper, we demonstrate the feasibility of using in situ Dynamic Light Scattering (DLS) to monitor counter-diffusion crystallization experiments in capillaries. Firstly, we have validated the quality of the DLS signal in thin capillaries, which is comparable to that obtained in standard quartz cuvettes. Then, we have carried out DLS measurements of a counter-diffusion crystallization experiment of glucose isomerase in capillaries of different diameters (0.1, 0.2 and 0.3 mm) in order to follow the temporal evolution of protein supersaturation. Finally, we have compared DLS data with optical recordings of the progression of the crystallization front and with a simulation model of counter-diffusion in 1D
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