Molecular thermodynamics for partitioning of native and denatured proteinsin aqueous two-phase systems

A molecular-thermodynamic analysis of protein partitioning in an aqueous tw o-phase system shows that the partition coefficient for a native (globular) protein is very much different from that for a denatured (linear) protein; while the former is weakly dependent on protein molecular weight, the latt er depends strongly on molecular weight. The native protein and the denatur ed protein are represented, respectively, by a spherical macroion and by a linear flexible polyion. On the basis of McMillan-Mayer solution theory, th e interactions between particles are represented by a continuum-averaged po tential of mean force containing hard-sphere repulsion, the effect of penet ration or hydration forces, electrostatic interactions, osmotic attraction, and specific interactions. Phase diagrams are calculated for polymer-polym er and for polymer-salt aqueous two-phase-forming systems in good agreement with experiment. Coupled with measured data for obtaining model parameters , partition coefficients are calculated for a native protein, lysozyme. Cal culated partition coefficients for a denatured protein are compared with ex perimental partitioning data for short peptides. Calculated results are rem arkably similar to those observed.