Thermodynamic nonideality in macromolecular solutions

Abstract

Second virial coefficients and hence covolumes for self-interaction of five proteins, viz. ribonuclease, ovalbumin, bovine serum albumin, catalase and a-crystallin, have been determined by analyzing the concentration dependence of the partition coefficient obtained from frontal chromatographic studies on either Fractogel TSK HW55 or porous glass beads. The resulting estimates of the effective radii essentially duplicate their Stokes counterparts and thereby provide further justification for assuming the approximate identity of the thermodynamic and hydrodynamic radii of hydrated globular proteins. Gel chromatographic evaluation of second virial coefficients for protein/dextran systems has led to elimination of the sphere/sphere model as a valid thermodynamic description of the space-filling effects in protein/polymer mixtures, since it does not predict the observed independence of covolume, expressed per unit mass of polymer, upon size of the polymer. This requirement is met by the sphere/ rod model [Edmond, E. & Ogston, A. G. (1968) Biochem. J. 109, 569-5761 and also by the sphere/flexiblesegment model [Hermans, J. (1982) J . Chenz. Phys. 77, 2193. Furthermore, similar studies of the effect of solute radius on covolume for interaction with dextran T70 attest to the adequacy of either model for predicting the thermodynamic nonideality arising from the inclusion of dextrans in protein solutions, and also provide the relevant calibration of the model.Studies of the effects of inert solutes on macromolecular interactions have led to the prediction and demonstration that self-association is enhanced by high concentrations of such solutes [l -61. In addition, advantage may be taken of the space-filling effects of inert solutes to detect enzyme isomerization [7], be it a substrate-induced [8-111 or a pre-existing [12 -141 phenomenon. Such thermodynamic nonideality of proteins/enzymes may be expressed quantitatively in terms of composition-dependent activity coefficients based on statistical-mechanical application of the excluded volume concept [15 -181. A quantity central to the prediction of thermodynamic nonideality is the covolume, i.e. the minimal volume in which two molecules can be located. In calculations of covolume, globular proteins have been regarded either as spheres or as ellipsoids of revolution [19, 201, whereas solutes such as poly(ethy1ene glycol) and dextran have been modelled as long rods [15] or as flexible segmented chains [17, 181. The major problem encountered in experimental studies of thermodynamic nonideality is thus not a lack of theoretical expressions for application but rather a lack of information on the magnitudes of parameters for substitution therein.In most studies of thermodynamic nonideality in protein solutions the stance has been taken that the Stokes radius of a globular protein should provide a reasonable estimate of its effective hydrated radius for use in covolume calculations [6 -12, 21, 221; however, this approximation is open to criticism on the grounds that a thermodynam...

Biochemical effects of molecular crowding

“…For example, in the case of eno lase, which reversibly associates as monomer−dimer, increase in the dimer fraction with increasing sucrose concentration has been observed [78]. The fact that low molecular weight species can also participate in the effects of excluded volume is demonstrated by a three fold increase of dimerization constant for α chy motrypsin in the presence of 0.15 M sucrose [77]. Similar effects have been observed also for DNA-protein interac tions [81,82].…”

Conformational Sampling of Peptides in the Presence of Protein Crowders from AA/CG-Multiscale Simulations

“…The focus on excluded volume effects is a result of the experimental and theoretical studies of crowding that have been carried out in the past. Typically, inert, polar and non-interacting polymers such as Ficoll 70 and Dextran 70 (in the experiments) and their theoretical counterparts (via hard sphere approximation) have been used as the crowding agents 29–31 . These agents have very poor attractive interactions with the proteins, with some exceptions such as in the case of polyethylene glycol (PEG) 32,33 .…”

A generalized approach to thermodynamic properties of biomolecules for use in bioseparation process design

“…However, concentration dependence of retention appears to be a nonnegligible parameter in SEC when chromatographic operation is done at high concentration of biomolecules [22][23][24]. Molecular size is usually approximated simply by molecular weight.…”