Summary: 454 J. Phys. Chem. 1995, 99, 454-461
Phase Separation in Multicomponent Aqueous-Protein Solutions
Canwen Liu, Aleksey Lomakin, George M. Thurston? Douglas Hayden2 Ajay Pande?
Jayanti Pande, Olutayo Ogun, Neer Asherie, and George B. Benedek*
Department of Physics and Centerfor Materials Sciences and Engineering, M.I.T.,
Received: July 25, 1994; In Final Form: October 14, I994@
We present measurements of the phase-separation temperature (Tph(4,a))as a function of overall protein
volume fraction (4) and protein composition (a)for ternary aqueous (W) solutions of calf (A) and yw
(B)crystallins. Additionally, we have determined the binodal curve describing coexisting points (#,a1)and
(@,an)in the phase diagram at 20 "C.We propose a mean-field form of the ternary Gibbs free energy
G(#,a,T)which contains three interaction energy parameters: Ewt(A,W), Ewr(B,W),and Ewr(AB),which
determine the magnitude of the quadratic (9)mixing energy contribution to G. Using a lattice model it is
possible to express each of these interaction parameters in termsof the mean individual protein-water, protein-
protein and water-water bond energies. In the limit where the two proteins are not too dissimilar, as applies
in our system, we find quite generally that the ternary solution can be regarded as a binary solution with an
interaction energy dependent upon the initial composition (a)of the solution. We have used this finding to
predict the entire coexistence surface TPh(4,a)and the positions of coexisting points along the binodal curve.
The interaction energy parameters were determined and we show that, within experimental error, this theory
accurately describes the full range of our experimental results.