Molecular Thermodynamics for Partitioning of Native and Denatured Proteins in Aqueous Two-Phase Systems

Microcompartmentation in artificial cells: pH-induced conformational changes alter protein localization

We report artificial cells in which protein localization in a primitive synthetic model for the cytoplasm is controlled by pH. Our model cells are giant lipid vesicles (GVs, ca. 5-30 microm diameter) with two coexisting aqueous compartments generated by phase separation of an encapsulated poly(ethylene glycol) (PEG) and dextran solution. Proteins are localized to a microcompartment by partitioning between the phases. We quantified the local concentration of fluorescently labeled human serum albumin (HSA) via confocal fluorescence microscopy. At pH 6.5, the labeled HSA was more concentrated in the dextran-rich phase, but at partially/fully denaturing pH (4.1 or 12) it was localized in the PEG-rich phase. This partitioning behavior is consistent with a more expanded, hydrophobic conformation at low and high pH. Labeled HSA could be relocalized from the PEG-rich into the dextran-rich phase domain by increasing the pH from 4.1 to 6.5 to renature the protein. This approach to controlling protein localization does not require extensive reorganization of the vesicle interior; coexisting PEG-rich and dextran-rich compartments are maintained throughout the experiments. It is also quite general; we demonstrated that several other proteins varying in size and isoelectric point also relocalized within compartmentalized artificial cells in response to external pH change. This work presents stimulus-responsive protein relocalization between compartments in an artificial cell; such experimental models can provide a framework for investigating the consequences of protein localization in cell biology.

Modeling Process Partitioning of Biomolecules in Polymer-Polymer and Polymer-Salt Aqueous Two-Phase Systems (ATPS) using an Extended Excess Gibbs Energy Model

A new local composition model was used to study the phase behavior of polymer-polymer and polymer-salt aqueous two-phase systems. The proposed model has three terms which account for the combinatorial, the long-range and the short-range effects in solution. The Flory-Huggins and the Debye-Hückel models were used for the combinatorial and long-range contributions. The interaction parameters of the model studied in this work can be regressed using a non-liner regression between the experimental data and those obtained from the proposed model. Also, this model was applied for modeling the partitioning of biomolecules in polymer-polymer and polymer-salt aqueous two-phase systems. The results showed that the proposed model has a good accuracy in correlating the partitioning coefficients of biomolecules in aqueous two-phase systems.

Phase behavior of polyampholytes from charged hard-sphere chain model

A molecular thermodynamic theory is developed for polyampholytes from the coarse-grained charged hard-sphere chain model. The phase behavior of polyampholytes with variations in sequence and chain length is satisfactorily predicted by the theory, consistent with simulation results and experimental observations. At a fixed chain length, the phase envelope expands as the sequence of charge distribution becomes less random. With increasing chain length, the phase envelope expands for diblock and random polyampholytes, but shrinks for zwitterionic polyampholytes. The predicted critical temperature, density, and pressure exhibit scaling relations with chain length for all the three (diblock, random, and zwitterionic) polyampholytes.