New theory upends what we know about how charged macromolecules self-assemble

In a discovery with wide-ranging implications, researchers at the University of Massachusetts-Amherst recently announced in the Proceedings of the National Academy of Sciences that uniformly charged macromolecules that contain a large number of atoms all with the same electrical charge can self. Our understanding of how some of life's basic structures are built has been upended.

Scientists used to think of charged polymer chains as being composed of smaller, uniformly charged units. The chains will repel each other because similar objects don't like to be close to each other. If you add salt to the water it will screen the electrical repulsion of the polyelectrolytes.

The game is different when you have dipoles, says Murugappan Muthukumar, the study's senior author, a professor at the University of Massachusetts-Amherst.

The dipoles have both positive and negative charges. The more familiar polyelectrolytes have either a positive or negative electrical charge and can form cross-links with other dipole polymer chains.

The lead author of the study says that dipoles can make polyelectrolytes behave more like polyzwitterions. The traditional chemical polyzwitterions have chemical bonds that cause this effect. Increasing ionic strength leads to a globule-to-coil transition in the polyzwitterion.

It's possible to use dipolar polymers in everything from drug-delivery systems to next- generation polymers. Muthukumar theorizes that charged macromolecules play a significant role in almost all biological assembly processes.

There's an "in-between" state called "mesomorphism." In the mesomorphic state, the polymers congregate into large, stable structures that have the ability to self-poison.

The discovery that dipoles drive the assembly of polymers throws new light on one of the fundamental mysteries of life's processes. The paradigm of how we think about these systems has been changed by the theory.

More information: Di Jia et al, Dipole-driven interlude of mesomorphism in polyelectrolyte solutions, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2204163119 Journal information: Proceedings of the National Academy of Sciences