A collaborative team from the University of Vienna and the University of Edinburgh has demonstrated a powerful method for controlling the shape of closed, ring-shaped polymers by adjusting electric charge via pH. Published in Physical Review Letters, their study shows how tuning the polymer’s ionization shifts the balance between two coiling modes—twist (local rotation) and writhe (large-scale folding)—resulting in reversible and programmable conformational changes.
Using a model in which each monomer acts as a weak acid, the team conducted simulations and analytical theory to observe how increasing charge (at higher pH) transforms polymers from compact, writhe-rich shapes toward more extended, twist-dominated conformations. At low supercoiling levels, transitions are smooth, but at higher supercoiling, the polymer can split into coexisting domains of twist- and writhe-rich regions—a novel topologically constrained form of microphase separation.
To predict these behaviors, the researchers developed a Landau-type mean-field theoretical model that accurately captures both continuous and abrupt transitions depending on charge and supercoiling.
The ability to control polymer topology via pH has broad implications: materials can be designed with tunable mechanical and transport properties (e.g., elasticity, viscosity, permeability) by encoding shape via twist–writhe balance rather than chemical composition alone. The researchers suggest future implementation with synthetic DNA rings bearing pH-sensitive side chains—now feasible, thanks to advancements in nucleotide chemistry—to create switchable, shape-adaptive scaffolds for applications such as microfluidics, where local chemical signals could alter polymer form and flow behavior.
This work paves the way to a new generation of topology-tunable, shape-adaptive materials whose function is programmable via external stimuli rather than static molecular design.