Collaborative Research: Fundamental Characterizations of Hydrophilic Macroions - Hydration Shell Thicknesses, Counterion Distribution and Intermolecular Distances

Charged macromolecules (macroions) including biomolecules such as proteins and DNAs contain many charged (ionic) groups of the same kind, balanced by small ions of opposite charge (called counterions). Investigators from the University of Akron and the University of Chicago will employ state-of-the-art techniques – analytical ultracentrifugation and Anomalous Small- and Wide-Angle X-ray Scattering at the NSF’s ChemMatCARS beamline of the Advanced Photon Source at Argonne National Laboratory – to explore the fundamental properties of model macroions in dilute aqueous solution. The researchers will try to determine the thickness of water layers around the macroions, the amount and distribution of counterions surrounding macroions, and the distances between the macroions when they attract with each other, aiming for understand how to interpret and predict the macroions’ interesting but poorly understood solution behaviors. The project will provide research and training opportunities for postdoctoral associates, graduate and undergraduate students, and broaden their experience through reciprocal visits, collaborative experiments conducted at both sites. The team will also organize workshops for faculty members, students, and high school teachers, as well as workforce-development activities through the educational programs of the NSF’s ChemMatCARS beamline. This project will investigate the fundamental properties of macroions – soluble, charged macromolecules with 1-6 nm in size. The disparity in size and charge between macroions and their counterions leads to moderate counterion association around the macroions and the resulting counterion-mediated attraction. These interactions give rise to solution behaviors that are distinct from those observed in systems of simple ions or colloidal particles. The team will pursue the following critical determinations using advanced techniques: (1) the hydration-shell thicknesses of macroions using analytical ultracentrifugation (AUC); (2) the spatial distributions of counterions surrounding macroions using Anomalous Small- and Wide-Angle X-ray Scattering at the NSF’s ChemMatCARS beamline of the Advanced Photon Source at Argonne National Laboratory; (3) the intermolecular distances within macroion dimers formed through counterion-mediated attraction by using AUC, and correlate these changes with the self-assembly, molecular recognition, and macrophase transitions of macroions. The team aims to advance the fundamental understanding of macroionic solutions to a level at which their phase transitions can be more reliably interpreted and predicted by the quantitative determination of those key molecular-level parameters. The project will provide research and training opportunities for postdoctoral associates, graduate and undergraduate students, and broaden their experience through reciprocal visits, collaborative experiments conducted at both sites. The team will organize workshops for faculty members, students, and high school teachers, as well as workforce-development activities through the educational programs of the NSF’s ChemMatCARS beamline. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.