Diffusion in ternary aqueous systems containing human serum albumin and precipitants of different classes

The mutual diffusion coeffs. for two aq. ternary systems, both contg. 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 "org. solvent", 2-methyl-2,4-pentanediol (MPD, component 3). Both PEG and MPD are used as co-precipitants in HSA crystn. protocols. Measurements were performed at const. protein concn., with increasing precipitant content. The results obtained for the two systems were discussed and compared. In both cases, the two main diffusion coeffs., relative to the motion of the protein and of the precipitant under their own concn. gradient, can be interpreted in terms of non-sp. vol. interactions between the solutes. Particularly, it was shown that any possible direct HSA-MPD interaction may not have a significant effect on the values of these two diffusion coeffs. Differences arise between the cross precipitant's diffusion coeffs., relative to the motion of the precipitant under the protein concn. gradient, Di1 with i = 2, 3. In the case of PEG, the D21 trend vs. c2 can be simply interpreted in terms of an "exclude vol." effect. In contrast, in the case of MPD, the D31vs. c3 trend seems to indicate a more complex mechanism of transport. Because the cross precipitant's diffusion coeff. plays an important role in the crystn. process, the implication of the obsd. difference on the crystn. procedure was also discussed.