Long ignored, dipole-dipole interactions shape life

In a finding with far-reaching implications, researchers at the University of Massachusetts Amherst recently announced in the Proceedings of the National Academy of Sciences that evenly charged macromolecules—or molecules, such as proteins or DNA, that contain large numbers of atoms all with the same electrical charge—can self-assemble into very large structures. This discovery challenges our understanding of how some of the basic structures of life are built.

Traditionally, scientists have understood charged polymer chains to be composed of smaller, uniformly charged units. Such chains, called polyelectrolytes, display predictable self-organizing behaviors in water: they repel each other because objects of similar charge don’t like to be near each other. If you add salt to water containing polyelectrolytes, the molecules coil together, because the electrical repulsion of the chains is masked by the salt.

However, “the game is very different when you have dipoles,” says Murugappan Muthukumar, Wilmer D. Barrett Professor of Polymer Science and Engineering at UMass Amherst and lead author of the study.

While many molecules have a positive or negative charge, dipoles have both. This means that polymers composed of dipoles behave very differently from more familiar polyelectrolytes, which have a positive or negative electrical charge: they expand in a salt solution and can form crosslinks with other chains of dipolar polymers, which leads then to the formation of complex polymeric structures.

Di Jia, who completed this research as part of his postdoctoral training at UMass Amherst and is the lead author of the study, says “dipoles can cause polyelectrolytes to behave more like polyzwitterions, which exhibit an “anti-polyelectrolyte effect”. This effect is also a characteristic of traditional chemical polyzwitterions, whose dipoles consist of chemical bonds. Therefore, for the physical polyzwitterion in dilute solutions, the size of the polymer increases with the increased ionic strength, exhibiting a globule-coil transition due to intra-chain dipole interactions.”

Dipolar polymers are capable of forming complex, self-regulating structures that could be used in everything from drug delivery systems to next-generation polymers. “We theorize that these dipole forces in charged macromolecules play an important role in almost all biological assembly processes, such as the spontaneous birth of membraneless organelles,” says Muthukumar.

In addition, these polymers composed of dipoles exhibit an “intermediate” state, called “mesomorphism”. In the mesomorphic state, the polymers are neither widely dispersed nor tightly coiled, but gathered into large, stable, uniform structures that have the ability to “self-poison” or dissolve.

“The significance of the discovery that dipoles drive polymer assembly is immense,” says Muthukumar, “because it sheds new light on one of the fundamental mysteries of life processes,” or how biological materials know self-assemble into coherent elements, stable structures. “The theory changes the paradigm for how we think about these systems and highlights the unrecognized role that dipoles play in the self-assembly of biological materials.”

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Materials provided by University of Massachusetts at Amherst. Note: Content may be edited for style and length.

Sharon D. Cole