Two time scales for self and collective diffusion near the critical point in a simple patchy model for proteins with floating bonds.

Using dynamic Monte Carlo and Brownian dynamics, we investigate a floating bond model in which particles can bind through mobile bonds. The maximum number of bonds (here fixed to 4) can be tuned by appropriately choosing the repulsive, nonadditive interactions among bonds and particles. We compute the static and dynamic structure factor (intermediate scattering function) in the vicinity of the gas-liquid critical point. The static structure exhibits a weak tetrahedral network character. The intermediate scattering function shows a temporal decay deviating from a single exponential, which can be described by a double exponential decay where the two time scales differ approximately by one order of magnitude. This time scale separation is robust over a range of wave numbers. The analysis of clusters in real space indicates the formation of noncompact clusters and shows a considerable stretch in the instantaneous size distribution when approaching the critical point. The average time evolution of the largest subcluster of given initial clusters with 10 or more particles also shows a double exponential decay. The observation of two time scales in the intermediate scattering function at low packing fractions is consistent with similar findings in globular protein solutions with trivalent metal ions that act as bonds between proteins.

Hydration and interactions in protein solutions containing concentrated electrolytes studied by small-angle scattering.

During protein crystallization and purification, proteins are commonly found in concentrated salt solutions. The exact interplay of the hydration shell, the salt ions, and protein-protein interactions under these conditions is far from being understood on a fundamental level, despite the obvious practical relevance. We have studied a model globular protein (bovine serum albumin, BSA) in concentrated salt solutions by small-angle neutron scattering (SANS). The data are also compared to previous studies using SAXS. The SANS results for dilute protein solutions give an averaged volume of BSA of 91,700 Å(3), which is about 37% smaller than that determined by SAXS. The difference in volume corresponds to the contribution of a hydration shell with a hydration level of 0.30 g g(-1) protein. The forward intensity I(0) determined from Guinier analysis is used to determine the second virial coefficient, A(2), which describes the overall protein interactions in solution. It is found that A(2) follows the reverse order of the Hofmeister series, i.e. (NH(4))(2)SO(4) < Na(2)SO(4) < NaOAc < NaCl < NaNO(3) < NaSCN. The dimensionless second virial coefficient B(2), corrected for the particle volume and molecular weight, has been calculated using different approaches, and shows that B(2) with corrections for hydration and the non-spherical shape of the protein describes the interactions better than those determined from the bare protein. SANS data are further analyzed in the full q-range using liquid theoretical approaches, which gives results consistent with the A(2) analysis and the experimental structure factor.

The Genetic Structure of Pseudoperonospora cubensis Populations.

Pseudoperonospora cubensis is a destructive foliar pathogen of economically important cucurbitaceous crops in the United States and worldwide. In this study, we investigated the genetic structure of 465 P. cubensis isolates from three continents, 13 countries, 19 states of the United States, and five host species using five nuclear and two mitochondrial loci. Bayesian clustering resolved six genetic clusters and suggested some population structure by geographic origin and host, because some clusters occurred more or less frequently in particular categories. All of the genetic clusters were present in the sampling from North America and Europe. Differences in cluster occurrence were observed by country and state. Isolates from cucumber had different cluster composition and lower genetic diversity than isolates from other cucurbits. Because genetic structuring was detected, isolates that represent the genetic variation in P. cubensis should be used when developing diagnostic tools, fungicides, and resistant host varieties. Although this study provides an initial map of global population structure of P. cubensis, future genotyping of isolates could reveal population structure within specific geographic regions, across a wider range of hosts, or during different time points during the growing season.

Use of microcalorimetry in monitoring stability studies. Example: vitamin A esters.

Vitamin A is very sensitive to chemical degradation caused by oxygen, light, heat, and other stress factors. If light and oxygen are excluded, the dominant degradation reaction for vitamin A derivatives is heat-induced formation of kitols, that is, dimers or higher oligomers. In this study vitamin A esters were used as model systems to evaluate microcalorimetry as a tool for monitoring the stability of heat sensitive substances. To obtain more knowledge about the model reaction, analytical investigations (supercritical fluid chromatography) were also performed. Because analytical and microcalorimetry data were consistent, a quantitative description of the kinetics and thermodynamics of the kitol formation reaction could be obtained. Aside from the academic motivation, this is important for practical purposes such as shelf life stability of vitamin A in feed, food, and pharmaceutical products. The vitamin A stability of a given sample can easily be predicted from the initial heat flow in a simple microcalorimetry experiment. Compared to conventional stability tests, this offers savings of money and time.

Temperature-, electric field- and solute-induced percolation in water-in-oil microemulsions.

We report investigations on the percolation of the aqueous phase in water-in-oil microemulsions, comparing systems stabilized by ionic AOT and non-ionic Igepal amphiphiles. First, we briefly review the opposite effect of temperature on the two systems and compare electric conductivity with viscosity data. In the second part, we show that percolation can be induced by high electric fields resulting in a shift of the percolation curve. The electric field measurements allow to investigate the dynamics of clustering of the water droplets to form a network of percolating channels. We examine the slow build-up and the fast decay of the percolating structure, monitoring simultaneously electric conductivity and electric birefringence. In the third part we discuss the effect of some solutes on the percolation curve, especially of small molecules which act as protein denaturants and of native and denatured proteins like methemoglobin, chymotrypsin and gelatin. The spectroscopic determination of the dimerization of hemin, released from denatured hemoglobin, reflects the incorporation of the hemin monomers in the surfactant monolayer. In the gelatin system time resolved electric birefringence shows that even at low concentrations it is the macromolecule which determines the structure of the aqueous domain. In the appendix, a simple estimate of the intrinsic Kerr-constant is given for microemulsion droplets deformed in an electric field.