Kinetics of supersaturation decay in the crystallization of lysozyme

The molecular architecture of proteins can be determined by analysing the X-ray diffraction patterns of their crystals. The technology of X-ray crystallography has reached the point, however, where the determination of the structure of a given crystal is controlled by the limited availability of the crystals themselves. Proteins can often be crystallized from pH buffered aqueous solutions of strong electrolytes. When dissolved protein in solution is more stable than crystalline protein, the appearance of crystals can be said to be under thermodynamic control. If, on the other hand, the crystals are more stable than the dissolved protein, and still crystals are slow to appear, the crystallization can be said to be under kinetic control. Using dilatometry, we have followed the rate of decay of the protein supersaturation in crystallizing solutions of chicken egg-white lysozyme under conditions of kinetic control. We have found that the rate of decay of the supersaturation is first order in the supersaturation and that the rate constant is independent of the initial protein concentration, but increases with increasing pH, decreasing temperature, and with increasing concentrations of sodium chloride and buffer salt. We correlate these observed trends in the rate constant with related trends in the solubility and surface charge density of the crystals. We conclude that the rate constant for supersaturation decay is inversely proportional to the protein solubility.     

The origin of the attraction between like charged hydrophobic and hydrophilic walls confining a near-critical binary aqueous mixture with ions

The effect of ionic solute on a near-critical binary aqueous mixture confined between charged walls with different adsorption preferences is considered within a simple density functional theory. For the near-critical system containing small amounts of ions, a Landau-type functional is derived on the basis of the assumption that the correlation, ξ, and the Debye screening length, κ(-1), are both much larger than the molecular size. The corresponding approximate Euler-Lagrange equations are solved analytically for ions insoluble in the organic solvent. A nontrivial concentration profile of the solvent is found near the charged hydrophobic wall as a result of the competition between the short-range attraction of the organic solvent and the electrostatic attraction of the hydrated ions. An excess of water may be present near the hydrophobic surface for some range of the surface charge and ξκ. As a result, the effective potential between the hydrophilic and the hydrophobic surface can be repulsive far from the critical point, then attractive and again repulsive when the critical temperature is approached, in agreement with a recent experiment (Nellen et al 2011 Soft Matter 7 5360).     

Colloidal monolayer trapped near a charged wall: a synchrotron x-ray diffraction study

Using x-ray diffraction from microfluidic channel arrays, we have determined concentration profiles of charge-stabilized silica colloids (radius 60+/-2 nm) confined between two like-charged dielectric walls at a few hundred nanometer distance. In solutions of very low ionic strength, strongly repulsive Coulomb interactions drive the colloids toward the central region between the walls. The addition of a small quantity of salt ions (0.2 mM) causes a dense colloidal monolayer to be trapped near the walls.     

The coulomb glory effect in collisions of antiprotons with heavy ions

The Coulomb glory effect in the back scattering of antiprotons with energies of from 100 eV to 3 keV from a bare nucleus of uranium and from uranium ions with closed shells is considered in terms of nonrelativistic and relativistic quantum theory. The appearance of Coulomb glory in collisions with multiply charged ions is caused by nucleus charge screening by filled electron shells. In scattering from a bare nucleus, the effect appears because of the screening properties of the vacuum polarization potential.     

Enhancement of lysozyme crystallization under ultrasound field

With the increasing demand for biopharmaceuticals, a method to crystallize biomolecule products with high quality, high yield and uniform size distribution as well as regular crystal habit is needed. In this work, ultrasound was used as a nucleation accelerator to decrease the energy barrier for lysozyme crystal formation. Crystallization experiments on egg-white lysozyme were carried out with and without ultrasound. The effect of ultrasound on induction time, metastable zone width, crystal size and morphology and process yield was investigated in detail. The nucleation-promoting effect produced by ultrasound is illustrated by the reduction of metastable zone width and induction time. By inducing faster nucleation, ultrasound leads to protein crystals grow at lower supersaturation levels with shorter induction time. It was found that ultrasound could result in uniform size distribution of the product due to the preventing of aggregation. However, long time continuous application of ultrasound could result in smaller particle size. Hence, ultrasonic-stop method was found to be a more appropriate strategy to enhance the crystallization process of proteins such as lysozyme.     

Molecular orbital study of excited state geometry of substituted benzenes

The liquid-like structure of a polystyrene latex solution of particle size 109 nm and its dependence on the ionic strength of added sodium chloride is investigated via the static light scattering structure factor S(Q). The experimental S(Q)s are compared with structure factors derived in the rescaled mean spherical approximation (RMSA) of Hansen and Hayter. After adjustment of the macromolecular charge Z _M such that the neat (salt free) latex solution is best represented by the RMSA, the experimental structure factors obtained after addition of salt can adequately be accounted for in terms of the RMSA by merely readjusting the ionic strength in the calculations. Our investigation indicates that the RMSA and the underlying screened Coulomb potential are suited for a detailed simulation of the structure and structural changes due to a variation of the ionic strength of liquid-like ordered solutions.     

Melting of strongly coupled,negatively charged,two-dimensional dust cluster: The role of charge fluctuation amplitude

Structural and melting characteristics are investigated for negatively charged dust particles in the presence of a two-dimensional electrostatic parabolic confinement potential. For a restricted number of dust particles that are subject to the permanent flow of electrons and ions, numerical simulation is conducted taking into account the random charge fluctuation. The amplitude of the charge fluctuation affects the ground-state configuration and melting characteristics of a finite number of particles interacting through Coulomb potential. The melting temperature decreases when the amplitude of the charge fluctuation increases as a result of particles' strong repulsive interaction.     

Secular Motion Frequencies of ~9Be~+ Ions and ~(40)Ca~+ Ions in Bi-component Coulomb Crystals

We obtain bi-component Coulomb crystals using laser-cooled ~(40)Ca~+ ions to sympathetically cool ~9Be~+ ions in a linear Paul trap. The shell structures of the bi-component Coulomb crystals are investigated. The secular motion frequencies of the two different ions are determined and compared with those in the single-component Coulomb crystals. In the radial direction, the resonant motion frequencies of the two ionic species shift toward each other due to the strong motion coupling in the ion trap. In the axial direction, the motion frequency of the laser-cooled~(40)Ca~+is impervious to the sympathetically cooled ~9Be~+ ions because the spatially separation of the two different ionic species leads to the weak motion coupling in the axial direction.     

Specific ion effects: Role of salt and buffer in protonation of cytochrome c

Changes in background salt and buffer are known to influence the properties of proteins. The reasons have remained obscure. The challenge posed by many such problems is this. Can physical chemistry contribute any predictive quantitative insights to what is in effect the simplest macromolecular solution behavior? Or must all remain specific? Our thesis is that it can. For definiteness we consider here as an illustrative example: surface pH and protonation equilibria of cytochrome c. We demonstrate an important role for ionic dispersion forces, missing from previous theoretical treatments. Unlike charge interactions these are different for each ionic species, and act between a protein and both salt and buffer ions. The charge of proteins depends not only on pH, ionic charge, and salt concentration. Taking ionic dispersion forces into account goes some way towards explaining the dependence on ionic species. We demonstrate why the addition of buffer can have profound effects, including reversal of the salt dependence of the protein charge.     

Elucidating the mechanism of nucleation near the gas-liquid spinodal

We have constructed a multidimensional free energy surface of nucleation of the liquid phase from the parent supercooled and supersaturated vapor phase near the gas-liquid spinodal. In particular, we remove the Becker-Doring constraint of having only one growing cluster in the system. Close to the spinodal, the free energy, as a function of the size of the largest cluster, develops surprisingly a minimum at a subcritical cluster size. It is this minimum at intermediate size that is found to be responsible for the barrier towards further growth of the nucleus at large supersaturation. An alternative free energy pathway involving the participation of many subcritical clusters is found near the spinodal where the growth of the nucleus is promoted by a coalescence mechanism. The growth of the stable phase becomes collective and spatially diffuse, and the significance of a "critical nucleus" is lost for deeper quenches.     

Analysis of polarizabilities of alkali halides using Narayan-Ramaseshan repulsive potential

The repulsive potential in ionic crystals recently proposed by Narayan and Ramaseshan (NR) can be expressed as the sum of the contributions from the individual ions. In the present paper we show that using this repulsive potential it is possible to divide the polarizability arising from the relative displacement of ions into its ionic constituents. NR have also derived the ionic radii in alkali halides which we have used to estimate the electronic polarizabilities of ions with the help of polarizability-radius cube relation. The electronic polarizabilities of alkali and halogen ions thus evaluated show a good agreement with those deduced from the experimental refraction data.     

Coagulation of Charged Aerosols

We consider the coagulation of an aerosol embedded in a stationary atmosphere of bipolar ions. Particles respond to the ionic environment by developing an instantaneous charge the fluctuations of which may produce attraction or repulsion between the particles. The governing parameter is the charge asymmetry factor which quantifies the relative charging efficiency of positive and negative ions. We use a Monte Carlo method to solve the coagulation equation in the free-molecule regime. We perform simulations for conditions ranging from symmetric and nearly symmetric environments (e.g. flames, ionizers), which result in particles that are on the average neutral to highly asymmetric conditions (low-pressure plasmas), which produce a substantial non-zero net charge. In symmetric ionic atmospheres we find that electrostatic interactions are unimportant and particles grow as if in the absence of charging ions. In asymmetric bipolar atmospheres, electrostatic interactions between particles are repulsive, the mean particle size grows logarithmically in time and the resulting size distributions are significantly narrower than the classical self preserving distributions.     

Using GPU parallelization to perform realistic simulations of the LPCTrap experiments

We performed classical molecular dynamics (MD) simulations in order to search the conditions for efficient sympathetic cooling of highly charged ions (HCIs) in a linear Paul trap. Small two-component ion Coulomb crystals consisting of laser-cooled ions and HCIs were characterized by the results of the MD simulations. We found that the spatial distribution is determined by not only the charge-to-mass ratio but also the space charge effect. Moreover, the simulation results suggest that the temperature of HCIs do not necessarily decrease with increasing the number of laser-cooled ions in the cases of linear ion crystals. We also determined the cooling limit of sympathetically cooled ¹⁶⁵Ho¹⁴⁺ ions in small linear ion Coulomb crystals. The present results show that sub-milli-Kelvin temperatures of at least 10 Ho¹⁴⁺ ions will be achieved by sympathetic cooling with a single laser-cooled Be⁺.     

Molecular dynamics simulation of homogeneous nucleation of KBr cluster

We perform molecular dynamics simulations to study the homogeneous nucleation in the freezing of molten potassium bromide clusters. The nucleation rates tend to decrease with increasing cluster size and temperature. The solid-liquid interfacial free energy σ_sl of 42.4-52.3 mJ/m² is close to the values predicted by Turnbull's relation and comparable to the experimental observation by Buckle and Ubbelohde. It is interesting to find that there is no cluster size effect on the critical nucleus size. Critical nucleus sizes inferred from classical nucleation theory are of 6.5-20.7 K⁺Br⁻ ionic pairs in the temperature range of 400-600 K. The critical nucleus size at bulk MD freezing temperature obtained by extrapolation is about 45 K⁺Br⁻ ionic pairs, which is comparable to the experimental value of NaCl.     

Compressive energy of ions in ionic crystals

The possibility of writing the repulsive energy in the Born model of binary ionic crystals as a sum of two separate contributions from the two ions has been investigated. Such an approach leads to two identities, one connecting the lattice spacings of a family of ionic crystals and the other connecting their compressibilities. These identities have been tested on the alkali halide crystals over a range of pressures. The agreement is found to be quite satisfactory. Some further predictions with respect to crystals which exist as two polymorphs have also been tested. In all cases, the deviations of the experimental values from the exact identities can be traced to the fact that second neighbour repulsions in the crystals have been neglected. It is hence concluded that individual compressive energies for ions in ionic crystals is a very attractive possibility.     

Generation of collimated beams of relativistic ions in laser-plasma interactions

A method is proposed for generating collimated beams of fast ions in laser-plasma interactions. Two-dimensional and three-dimensional particle-in-cell simulations show that the ponderomotive force expels electrons from the plasma region irradiated by a laser pulse. The ions with unneutralized electric charge that remain in this region are accelerated by Coulomb repulsive forces. The ions are focused by tailoring the target and also as a result of pinching in the magnetic field produced by the electric current of fast ions.     

Nuclear quadrupolar relaxation in ionic crystals

The nuclear quadrupolar relaxation time of alkali halide crystals is investigated by the Heitler-London approximation. We use the Slater determinant which is constructed by the Hartree-Fock wave functions of the free ions. The charge distribution around an ion is not spherically symmetrical because of mutual overlap of the atomic wave functions of nearest-neighbor ions. Moreover, the charge distribution around an ion changes its shape during thermal vibrations of the ions, because the degree of the overlap depends upon the distance apart of the ions. Therefore, the quadrupolar interaction in alkali halide lattices is the resultant of a combination of the thermal vibration with the charge overlap. As illustrations of the theory, we have computed the quadrupolar relaxation time of the ³⁹K and the ³⁵Cl nuclei in the KCl crystal, and the ²³Na nucleus in the NaCl crystal. The theoretical results are in good agreement with those of experiments. We have also computed the ratio of the quadrupolar relaxation time of the metal nucleus to that of the halogen nucleus for some alkali halide crystals. After computing the same ratio by the covalent model and the deformation model, we have compared these results with the available experimental data. Finally, by using the same overlap model as that mentioned previously, we have developed a formula which gives the chemical shift of ionic crystals.     

高压下氢化锂晶体中的离子间力与结合能公式的原子分子设计

本文提出一种利用材料Hugoniot数据研究高压下离子晶体中微观离子状态及其离子间排斥作用势的新方法。对氢化锂晶体进行研究时,我们得到Li~+和H~-离子之间排斥作用势函数,结果表明文献〔1〕中提出的离子压缩效应具有客观性。     

The mechanism of the appearance of an electromotive force on heating of SmS single crystals

We analyze the experimental variation of the concentration of conduction electrons in semiconducting SmS single crystals with increasing temperature within a shallow-impurity model. It is shown that the appearance of an electromotive force is due to accumulation of the critical concentration of free electrons, which results in screening of the Coulomb potential of Sm²⁺ impurity ions that are responsible for the creation of donor levels with an activation energy of 0.045 eV in the band gap of SmS single crystals.     

Screening effect of impurities in metals: a possible explanation of the process of cold nuclear fusion

Summary The screening length of the deuterium ion by surrounding electrons in a palladium metal lattice, as estimated using two approaches—viz. the Thomas-Fermi screening theory and the Debye screening theory for plasmas in metal—is found to be less than the interatomic separation of ordinary hydrogen molecules. This has important implications for the possibility of cold nuclear fusion at room temperature, since slight fluctuations in equilibrium conditions may drive the deuterons to fuse together. The relative magnitudes of screening length for the cold nuclear fusion regime and classical hot nuclear regimes (inertial and magnetic confinement) reveal that in the former a comparatively smaller amount of energy is needed to overcome the repulsive Coulomb barrier between two deuterium ions.