The interactions between ribonuclease A and solvent components in aqueous 2-methyl-2,4-pentanediol (MPD) have been investigated by differential refractometry and light scattering at pH 5.8, i.e., conditions similar to those used to crystallize the protein from this solvent system. Application of multicomponent thermodynamic theory shows that, at all solvent compositions up to 50% (v/v) MPD, the protein is preferentially hydrated; i.e., addition of ribonuclease to the mixed solvent leads to an increase in the chemical potential of MPD. This unfavorable thermodynamic interaction leads to phase separation, probably caused by local salting out of the MPD by the charges on the surface of the protein molecule. A parallel examination by circular dichroism (CD) has shown that the CD spectrum of ribonuclease in 50% MPD is indistinguishable from that in dilute buffer.

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