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A mutated variant of 20 amino acid miniprotein Trp-cage (TC5b), called TC5c (Asp9 replaced by Asn9), was designed to demonstrate the effect of a salt bridge. As a result of strong electrostatic interaction, the distance distribution between Asp9 and Arg16 exhibited a larger probability in the range of the salt bridge for TC5b compared to TC5c. The probability of α-helix formation for residues 3–8, as well as for residues 11–14, was high for TC5b. The salt bridge formation between Asp9 and Arg16 in TC5b was indicated by (a) a strong correlation of their distance of separation with the subtended angle with the centre and (b) a step decrease in the distance between Gly11O and Arg16H at 12 ns. Replica exchange molecular dynamics simulation at different temperatures in the range of 270–590 K indicated that the average distance between Asp9 and Arg16, end-to-end distance, root mean square deviation with respect to a reference NMR structure of TC5b did not change significantly with temperature below 370 K for TC...

A coarse-grain (CG) algorithm is developed for molecular dynamics (MD) simulation of proteins in which the polypeptide backbone as well as side chains were mapped into spherical interaction centres. The force field parameters for non-bonded interaction between these centres (beads) were evaluated from the interaction of all atoms corresponding to these beads using pairwise additivity. The validity of CG algorithm was demonstrated by comparing the potential energy, radius of gyration (RG), end-to-end distance and root-mean-square deviation with those obtained by all-atom (AA) simulation for a small protein molecule Trp-cage as well as for lysozyme in solution. The CG simulation for lysozyme in solution requires less than 1/50 CPU-time compared to that for the AA method. Also, the CG method converges to equilibrium potential energy much faster than AA simulation. CG simulation for lysozyme adsorbed on silica surface showed that the molecule is more unfolded with a less compact tertiary structure compared to...

The effect of steam sterilization (dwell@121 degrees C) on the apparent viscosity of solutions of carbomer 940P, guar gum, hydroxyethylcellulose (HEC), and xanthan gum was studied. Guar gum and carbomer 940P could be steam sterilized for 30 min without any change in the apparent viscosity. Steam sterilization of HEC and xanthan gum resulted in a substantial decrease in the apparent viscosity and the desirable shear-thinning rheology. The addition of sodium chloride to either polymer prior to steam sterilization diminished the effect of sterilization on the rheology. The apparent viscosity of xanthan solutions was not significantly affected if the solution was made isotonic with sodium chloride (0.15 M). The addition of sodium chloride to xanthan gum after steam sterilization resulted in full recovery of the initial rheological properties. Carbomers, guar gum, and xanthan (in 0.15 M NaCI) are candidates for use in pharmaceutical solutions/suspensions that are subjected to steam sterilization.

A statistical thermodynamic model for the prediction of precipitation curves of globular proteins using nonionic polymers has been proposed. The model accounts for protein-polymer, polymer-solvent, electrostatic, and hydrophobic interactions as well as the entropy of mixing and employs simplifying assumptions such as spherical globular protein molecule with uniform surface properties and linear, homogeneous polymer uniform with respect to molecular weight. The proposed model can only be employed to predict precipitation curves of charged proteins at sufficiently high ionic strengths since it does not account for electrostatic protein-protein interactions due to overlap of electrical double layers. The model predictions of precipitation curves of human serum albumin (HSA) at the isoelectric point using polyethylene glycol (PEG) for different initial protein concentrations and molecular weights of PEG agreed well with the experimental data. Higher polymer concentrations were found to be required to...

A model for concentration of proteins from dilute solutions in a continuous foam fractionation column is proposed. This model accounts for (1) kinetics of adsorption of proteins in the liquid pool as well as in the foam, (2) liquid drainage from thin films due to Plateau border suction and disjoining pressure, (3) gravity drainage of liquid from Plateau borders, and (4) bubble coalescence in the foam. Protein enrichment and recovery were found to increase as the liquid pool height increased, eventually attaining constant values (corresponding to adsorption equilibrium), thus demonstrating the strong dependence of protein separation on adsorption kinetics. Higher enrichments and lower recoveries were obtained for smaller gas velocities, larger bubble sizes, and higher feed flow rates. Enrichments as well as recoveries were higher at lower feed concentrations. Coalescence was found to lead to higher enrichments and lower recoveries, this dependence being stronger for larger inlet bubble sizes.

A model for the hydrodynamics of coalescing foam bed is proposed which accounts for liquid drainage through thin films and plateau borders, bubble coalescence, and mixing of liquid between thin films and plateau borders. Foam bed is assumed to consist of equal sized dodecahedral bubbles at any cross section, and the variation of bubble size with foam height due to coalescence is described through phenomenological models. This model is employed to investigate the effect of coalescence on enrichment and recovery for the nonanol-water system in a continuous foam fractionation column for different inlet bubble sizes, superficial gas velocities, and inlet concentrations. Coalescence is found to yield higher enrichments and lower recoveries, possibly because of the predominant effect of increased liquid drainage due to larger bubble sizes resulting in a significant decrease in liquid holdup with foam height. Larger bubble sizes and lower superficial gas velocities were found to result in higher enrichm...