Hydrophilic Macroions in dilute aqueous solution - electrostatic interaction regulated self-assembly, chiral recognition and phase transitions

Between traditional simple ions and large colloidal particles, there exists a transitional stage – macroionic solutions. In this regime the charged solutes have solution behavior fundamentally different from the above two categories. The best model macroions cover a broad range of materials, including structurally well-defined molecular clusters, metal-organic cages, small nanoparticles, uranyl peroxide clusters and globule proteins, with accurate and tunable charges. Such 1-5-nm-size macroions can selectively attract counterions around them, lead to moderate counterion association around them. ASAXS technique can accurately determine the amount and distance of the counterions to the macroion. Furthermore, it creates counterion-mediated attraction and therefore induces strongly attract the like-charged macroions with each other. This lproduces interesting solution behaviors – for both microphase (self-assembly) and macrophase phase separations. In dilute solutions, they tend to reversibly self-assemble into single-layered 2-D nanosheets, which eventually bend and form stable, hollow, spherical “blackberry” structures with their sizes accurately tunable by solvent content, macroionic charge density or pH. The blackberry structure represents a universal, free energy favored state of soluble macroions with moderate charge, and mimics some biological processes, such as the virus capsid shell formation. When the 2-D sheets are too tough to bend, they stay as standalone in solution, leading to the macrophase separations into hydrogel and coacervate phases, both are common for charged polymers but very unique for inorganic molecules.

The macroions are perfect models to understand some fundamental biological behaviors such as the self-recognition and chiral selection of biological assemblies. Inorganic macroions can achieve the level of self-recognition similar to biomolecules in dilute solution, even among those with identical size, shape and charge, or enantiomers. The metal-organic cage-based macroions containing amino acid components can be used for clarifying the chiral discrimination phenomenon among amino acids, which is connected to the homochirality feature of our lives.