Crystallization is a fundamental and ubiquitous process that is well understood in the case of atoms or small molecules, but its outcome is still hard to predict in the case of nanoparticles or macromolecular complexes. Controlling the organization of virus nanoparticles into a variety of 3D supramolecular architectures is often done by multivalent ions and is of great interest for biomedical applications such as drug or gene delivery and biosensing, as well as for bionanomaterials and catalysis. In this paper, we show that slow dialysis, over several hours, of wild-type Simian Virus 40 (wt SV40) nanoparticle solution against salt solutions containing MgCl2, with or without added NaCl, results in wt SV40 nanoparticles arranged in a body cubic center crystal structure with Im3m space group, as a thermodynamic product, in coexistence with soluble wt SV40 nanoparticles. The nanoparticle crystals formed above a critical MgCl2 concentrations. Reentrant melting and resolubilization of the virus nanoparticles took place when the MgCl2 concentrations passed a second threshold. Using synchrotron solution X-ray scattering we determined the structures and the mass fraction of the soluble and crystal phases as a function of MgCl2 and NaCl concentrations. A thermodynamic model, which balances the chemical potentials of the Mg2+ ions in each of the possible states, explains our observations. The model reveals the mechanism of both the crystallization and the reentrant melting and resolubilization and shows that counterion entropy is the main driving force for both processes.
Bibliographical noteFunding Information:
We thank Daniel Harries for very helpful discussions. We acknowledge the European Synchrotron Radiation Facility (ESRF) beamline ID02 (T. Narayanan and his team), Desy synchrotron at Hamburg, beamline P12 (D. Svergun and his team), and Soleil synchrotron, Swing beamline (J. Perez and his team), for provision of synchrotron radiation facilities and for assistance in using the beamlines. We thank Ofra Moshel for her help with conducting the electrophoretic mobility measurements. R.A. acknowledges support from the Kaye−Einstein Fellowship Foundation. U.R. and R.A. acknowledge financial support from the Israel Science Foundation (grant 656/17), US−Israel Binational Science Foundation (grant 2016311), the FTA-Hybrid Nanomaterials Program of the Planning and Budgeting Committee of the Israel Council of Higher Education, and the NIH (award number 1R01AI118933).
© 2017 American Chemical Society.
- bridging interactions
- like charge attraction
- reentrant condensation
- small-angle X-ray scattering