E0034

B2 or not B2? Seth Fraden, Joshua Bloustine, Complex Fluids Group, Brandeis Univ. Waltham, MA 02454, www.elsie.brandeis.edu.

We study the liquid-liquid phase transition in suspensions of the protein lysozyme as a function of added poly(ethylene) glycol (PEG). This is an interesting system because over half of all proteins crystallized were done in the presence of PEG. PEG is thought to not interact with protein, but entropic effects cause even an inert polymer to produce an effective "depletion" attraction between the proteins that can be strong enough to induce phase transitions in the protein/polymer mixture. Our initial experiments revealed two unexpected results. First, our light scattering results indicate that for the case of lysozyme/polymer mixtures, adding polymer produces a repulsive interaction between proteins, which is opposite to the depletion prediction. Second, we do not observe the predicted correlation between the second virial coefficient (B2) and phase behavior.