(129c) Bridging the Gap between Scattering Results from Simulations and Experiments for Polyampholytes

Understanding the stability of polyampholyte conformations and their phase behavior is a stepping stone for comprehensive studies on protein solution properties. Recent small angle X-ray scattering (SAXS) measurements of polyampholytes have provided detailed accounts of their sequence-conformation relationship. Though the radius of gyration from simulation trajectories of implicit solvent models are internally consistent with their scattering predictions, the shape that is reflected in the Kratky plots from these models are qualitatively dissimilar to experiments. Implicit solvent scattering profiles also fail to show any sensitivity to the primary sequence of the polyampholyte. This naturally raises questions about the impact of solute-water scattering correlations that are present in experimental measurements, but absent in implicit solvent calculations. Motivated by the molecular quasichemical theory of solutions, we consider the first hydration layer as part of the polyampholyte in the scattering calculations. Comparison of the hydrated polyampholyte and the implicit solvent model reveals the critical role of closely associated water molecules in influencing the scattering. We present results for a collection of polyampholyte sequences and make direct comparisons with experimental data. Overall, our results show the care needed in both experiment and theory in subtracting the effect of the solvent in analyzing scattering data.