Mechanism of Protein−Poly(ethylene glycol) Interaction from a Diffusive Point of View

Abstract

The first multicomponent diffusion data ever determined in protein−polymer systems are presented for the system lysozyme(1)−PEG 400(2)−water. Although there are no specific interactions between protein and polymer, the cross-term diffusion coefficient D 21, that links the PEG flow to the protein concentration gradient, is up to 35 times the main-term diffusion coefficient of the protein. This observation can only be due to a “crowding effect” and not to specific interactions, such as electrostatic ones. The exclusion effect is also qualitatively confirmed by the measured counter-flow associated with the protein motion. On the base of a hard core potential, our recent predictive equations are used to predict diffusion coefficients in this ternary system, and a good agreement with the experimental D 21 is obtained. The PEG concentration dependence of the main-term diffusion coefficient of the protein cannot be interpreted exclusively by the excluded volume effect. Some dielectric effect or aggregation phenomena must be invoked to completely describe diffusive behavior in protein−PEG systems. A strong dielectric constant decrease and an anomalous pH dependence on PEG concentration in this system have been observed. We have extended to this nonelectrolyte system a recent procedure for extracting thermodynamic data from ternary diffusion coefficients that uses the Onsager reciprocal relations and the coupling of D ij and second virial coefficient data. Thus we obtained the change of the lysozyme chemical potential with increasing PEG concentration. We emphasize that it is incorrect to neglect the nonideality of PEG−water systems, as was done in some previous preferential solvation analyses.

Quaternary Diffusion Coefficients in a Protein−Polymer−Salt−Water System Determined by Rayleigh Interferometry

“…23,30,31 However in the lysozyme-polymer case, the observed decrease in (D 11 ) V (12) is more gradual and can be largely attributed to the corresponding increase of solution viscosity, though dielectric effects on the lysozyme-chloride electrostatic interactions may also play an important role. 33 Figure 1 also shows that the behavior of the quaternary data is consistent with the ternary data. However, since electrostatic screening is the dominant effect, the quaternary (D 11 ) V values are closer to the corresponding (D 11 ) V (13) values than to the (D 11 ) V (12) values within our experimental concentration domain.…”

“…Moreover, since the lysozyme concentration is small, µ 32 /µ 33 in eq 8 is essentially equal to µ 32 (23) /µ 33 (33) . We also note that µ 13 (13) /µ 33 (13) is approximately equal to the molality-based chemical-potential ratio, 28 which is directly related to the thermodynamic interaction between two different solutes according to preferential-interaction theory.…”

“…23,[29][30][31][32][33][34] Diffusion measurements at 25°C on lysozymesalt-water 31,32 and lysozyme-poly(ethylene glycol)-water 33,34 by optical interferometry have shown that cross-diffusion terms…”

Cross-diffusion and pattern formation in reaction–diffusion systems

“…Examples of systems that exhibit cross-diffusion include strong electrolytes, [23][24][25] micelles, 26,27 or microemulsions, 28 and systems containing molecules of significantly different sizes, for example protein-salt. [29][30][31] Similar phenomena also occur in biological systems, but since the transport process is driven by an input of energy, as, for example, in bacterial chemotaxis [32][33][34] or in predator-prey systems, [35][36][37][38] these are not true diffusion processes. Nonetheless, the mathematical description of biological or ecological cross-diffusion is the same as in physicochemical systems, as we discuss below.…”

Effect of Macromolecular Polydispersity on Diffusion Coefficients Measured by Rayleigh Interferometry

“…For instance, the 2 × 2 matrix of four diffusion coefficients has been reported for several ternary systems containing a macromolecular solute. [19][20][21][22][23][24][25][26][27][28][29] The Gosting diffusiometer operating in the Rayleigh optical configuration has generated the most accurate multicomponent diffusion data. [19][20][21][22][23] However, polydispersity of a macromolecular solute adds complexity to the interpretation of the diffusion data.…”