1. The viscometer/rheometer described in last year's report has been used to measure the concentration-dependent viscosity of three monoclonal antibodies in buffers containing various concentrations of NaCl and arginine. Theoretical models aimed at providing a unified description of concentration-dependent viscosity and light scattering are under development (A. Grupi, S. Yadav &A. Minton). 2. Utilizing the radial distribution functions of hard sphere and square well fluids computed and reported last year, the concentration-dependent colligative properties of these fluids have been calculated and compared to those observed experimentally and previously interpreted in the context of approximate models invoking both specific associations and nonspecific intermolecular interactions. Initial results of this comparison is that experimental data previously interpreted as indicating the presence of specific associations cannot be modeled by simple square well attractive interactions. (T. Hoppe, A. Minton) 3. The coarse-grained representation of electrostatic interactions reported last year has been used to compute the second virial coefficient of two proteins as a function of pH. The second virial coefficient of these proteins has been measured as a function of pH via concentration-dependent light scattering. Comparison of computed and experimental results indicates that electrostatic interaction due to fixed charges alone is not sufficient to account for net attractive protein-protein interaction close to the isoelectric point, and we are currently exploring various models for the short-range attractive interaction, including hydrophobic interactions and counterion polarization (T. Hoppe, D. Wu)
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