| Abstract
| - The first multicomponent diffusion data ever determined in protein−polymer systems arepresented for the system lysozyme(1)−PEG 400(2)−water. Although there are no specific interactionsbetween protein and polymer, the cross-term diffusion coefficient D21, that links the PEG flow to theprotein concentration gradient, is up to 35 times the main-term diffusion coefficient of the protein. Thisobservation can only be due to a “crowding effect” and not to specific interactions, such as electrostaticones. The exclusion effect is also qualitatively confirmed by the measured counter-flow associated withthe protein motion. On the base of a hard core potential, our recent predictive equations are used topredict diffusion coefficients in this ternary system, and a good agreement with the experimental D21 isobtained. The PEG concentration dependence of the main-term diffusion coefficient of the protein cannotbe interpreted exclusively by the excluded volume effect. Some dielectric effect or aggregation phenomenamust be invoked to completely describe diffusive behavior in protein−PEG systems. A strong dielectricconstant decrease and an anomalous pH dependence on PEG concentration in this system have beenobserved. We have extended to this nonelectrolyte system a recent procedure for extracting thermodynamicdata from ternary diffusion coefficients that uses the Onsager reciprocal relations and the coupling of Dijand second virial coefficient data. Thus we obtained the change of the lysozyme chemical potential withincreasing 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.
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