The mutual diffusion coefficients for two aqueous ternary systems, both containing a protein, human serum albumin (HSA, component 1), were measured. The first system contained a neutral polymer, polyethylene glycol (PEG, component 2), and the second an "organic solvent", 2-methyl-2,4-pentanediol (MPD, component 3). Both PEG and MPD are used as co-precipitants in HSA crystallization protocols. Measurements were performed at constant protein concentration, with increasing precipitant content. The results obtained for the two systems were discussed and compared. In both cases, the two main diffusion coefficients, relative to the motion of the protein and of the precipitant under their own concentration gradient, can be interpreted in terms of non-specific volume interactions between the solutes. Particularly, it was showed that any possible direct HSA-MPD interaction may not have a significant effect on the values of these two diffusion coefficients. Differences arise between the cross precipitant's diffusion coefficients, relative to the motion of the precipitant under the protein concentration gradient, D(i1) with i = 2, 3. In the case of PEG, the D(21) trend vs. c(2) can be simply interpreted in terms of an "exclude volume" effect. In contrast, in the case of MPD, the D(31)vs. c(3) trend seems to indicate a more complex mechanism of transport. Because the cross precipitant's diffusion coefficient plays an important role in the crystallization process, the implication of the observed difference on the crystallization procedure was also discussed.
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