Julio A. Deiber

Hydration, charge, size, and shape characteristics of peptides from their CZE analyses.

A CZE model is presented for peptide characterization on the basis of well-established physicochemical equations. The effective mobility is used as basic data in the model to estimate relevant peptide properties such as, for instance, hydration, net and total electrical charge numbers, hydrodynamic size and shape, particle average orientation, and pH-microenvironment from the charge regulation phenomenon. Therefore 102 experimental effective mobilities of different peptides are studied and discussed in relation to previous work. An equation for the estimation of peptide hydration as a function of ionizing, polar, and non-polar amino acid residues is included in the model. It is also shown that the shape-orientation factor of peptides may be either lower or higher than one, and its value depends on a complex interplay among total charge number, molar mass, hydration, and amino acid sequence.

Comparison between two- and one-field models for natural convection in porous media

Abstract The two-field model (2-F) for natural convection in porous media is studied in relation to the one-field model (1-F), which is the result of the local thermal equilibrium assumption. These models are used to evaluate heat transfer through a porous medium of relatively high permeability contained in a vertical annulus. The conceptual differences between 2- and 1-F models are shown within the context of the theory of mixtures of continuum mechanics. Criteria are generated to determine when the 1-F model can be applied in practical situations as a good approximation, and without introducing errors in the evaluation of the temperature field and wall heat fluxes. This study includes a comparison between the Nusselt numbers obtained from these two models, and also the analysis of local differences between fluid and solid temperatures within the porous cavity. Numerical calculations are carried out for variable porosity, which is modeled either with exponential decaying and damped oscillating functions involving normal distance from the annulus walls. Different correlations for the heat transfer coefficient between solid and fluid phases are analyzed in relation to the 2-F model.

Estimation of global structural and transport properties of peptides through the modeling of their CZE mobility data

Peptide electrophoretic mobility data are interpreted through a physicochemical CZE model, providing estimates of the equivalent hydrodynamic radius, hydration, effective and total charge numbers, actual ionizing pK, pH-near molecule and electrical permittivity of peptide domain, among other basic properties. In this study, they are used to estimate some peptide global structural properties proposed, providing thus a distinction among different peptides. Therefore, the solvent drag on the peptide is obtained through a characteristic friction power coefficient of the number of amino acid residues, defined from the global chain conformation in solution. As modeling of the effective electrophoretic mobility of peptides is carried out in terms of particle hydrodynamic size and shape coupled to hydration and effective charge, a packing dimension related to chain conformation within the peptide domain may be defined. In addition, the effective and total charge number fractions of peptides provide some clues on the interpretation of chain conformations within the framework of scaling laws. Furthermore, the model estimates transport properties, such as sedimentation, friction and diffusion coefficients. As the relative numbers of ionizing, polar and non-polar amino acid residues vary in peptides, their global structural properties defined here change appreciably. Needs for further research are also discussed.

Analysis of the interplay among charge, hydration and shape of proteins through the modeling of their CZE mobility data

Electrophorectic mobility data of four proteins are analyzed and interpreted through a physicochemical CZE model, which provides estimates of quantities like equivalent hydrodynamic radius (size), effective charge number, shape orientation factor, hydration, actual pK values of ionizing groups, and pH near molecule, among others. Protein friction coefficients are simulated through the creeping flow theory of prolate spheroidal particles. The modeling of the effective electrophoretic mobility of proteins requires consideration of hydrodynamic size and shape coupled to hydration and effective charge. The model proposed predicts native protein hydrations within the range of values obtained experimentally from other techniques. Therefore, this model provides consistently other physicochemical properties such as average friction and diffusion coefficients and packing fractal dimension. As the pH varies from native conditions to those that are denaturing the protein, hydration and packing fractal dimension change substantially. Needs for further research are also discussed and proposed.

FLOW OF NEWTONIAN FLUIDS THROUGH SINUSOID ALLY CONSTRICTED TUBES. NUMERICAL AND EXPERIMENTAL RESULTS

Numerical and experimental predictions of pressure drops in the flow of Newtonian fluids through sinusoidally constricted tubes (SCT) are carried out. The numerical evaluations analyzed in this wor...

Slow two-phase flow through a sinusoidal channel

Abstract The two-phase flow through a symmetric sinusoidal channel is studied by means of a regular perturbation analysis, where the small parameter is defined as the ratio between the amplitude of variation of the channel wall and the average thickness of the non-wetting phase. Results are valid for Reynolds numbers of the same order of magnitude as that of the expansion parameter. It is thus found that the fluid-fluid interface presents a wavy shape characterized by an amplitude and a phase-shift with respect to the fixed solid-fluid interface. Instabilities of the two-phase flow can arise for large values of the viscosity, flow rate and phase thickness ratios. Results are expected to be a first step towards the understanding of the hydrodynamics of trickle bed reactors, where several flow regimes are possible.

A two-field model for natural convection in a porous annulus at high Rayleigh numbers

This work studies the natural convection in a vertical porous annulus for Rayleigh numbers up to 8×104. A model is deduced within a simple context of the mixture theory in which the following effects are included: (1) The solid and the fluid may be at different temperatures depending upon the flow regime and the permeability of the porous matrix. (2) Density, heat conductivity and viscosity of the fluid depend on temperature. (3) The porosity of the rigid solid matrix varies near the cavity walls. (4) Heat conduction in the fluid phase is accompanied with thermal dispersion. (5) Forchheimer inertial, Brinkman viscous and convective terms are considered in the momentum balance of the fluid phase. The resulting model is used to evaluate the heat transfer in a porous medium, the matrix of which is composed of equal spheres of well-specified diameters. Inner and outer walls of concentric cylinders are maintained at different temperatures and the two horizontal walls are adiabiatic. Numerical predictions of the Nusselt number are compared with available experimental data. For high Rayleigh and Darcy numbers (porous media of high permeability) all these effects have to be considered in the mathematical model. Thermal dispersion has a significant effect on the heat transfer through the solid–fluid composite.

Global conformations of proteins as predicted from the modeling of their CZE mobility data.

Estimations of protein global conformations in well‐specified physicochemical microenvironments are obtained through global structural parameters defined from polypeptide‐scale analyses. For this purpose protein electrophoretic mobility data must be interpreted through a physicochemical CZE model to obtain estimates of protein equivalent hydrodynamic radius, effective and total charge numbers, hydration, actual ionizing pK and pH‐near molecule. The electrical permittivity of protein domain is also required. In this framework, the solvent drag on proteins is obtained via the characteristic friction power coefficient associated with the number of amino acid residues defining the global chain conformation in solution. Also, the packing dimension related to the spatial distribution of amino acid residues within the protein domain is evaluated and discussed. These scaling coefficients together with the effective and total charge number fractions of proteins provide relevant interpretations of protein global conformations mainly from collapsed globule to hybrid chain regimes. Also, protein transport properties may be estimated within this framework. In this regard, the central role played by the friction power coefficient in the evaluation of these properties is highlighted.

Estimation of Molecular Weight Distribution of Elastomers and Polymer Melts through Dynamic Rheometry

This work presents a rheometric molecular weight distribution technique (RMWD) to estimate molecular parameters of low and high poly dispersity samples of elastomers and polymer melts. Samples of polystyrene, polybutadiene, polyethylene and styrene-butadiene rubbers are considered in this study. Important conclusions are obtained concerning the elimination of experimental information with glassy modes that are not relevant for the RMWD technique. A discussion on the calibration of this technique is pre sented involving an interplay with size exclusion chromatography and capil lary rheometry. Estimation of molecular weights Mw and Mn, and polydisper sity Po through rheometry is a promising technique, which when comple mented with rheological theories of polymer melts, may give additional infor mation into the chemical architecture of polymer chains. Correlations between rheological and molecular parameters are also presented.

Free damped oscillations of the linear viscoelastic material

This work shows that the complex dynamic viscosity η* (ω) can be evaluated from experimental data of free damped oscillations (FDO) when upper and lower angular displacements measured in a plate-plate geometry are analyzed by Fourier series. We deduce the equation required to evaluate experimentally the dynamic functions of linear viscoelastic materials in FDO. Two loss tangents can be defined in FDO, which are not necessarily equal to tan δ(ω) of the forced oscillation test.