Single-molecule studies of biopolymer coacervation

About This Grant

NON-TECHNICAL SUMMARY This project aims to understand how mixtures of charged polymers—called ‘complex coacervates’ can form microscopic droplets with unique properties, such as the ability to encapsulate and deliver drugs or act as adhesives. These materials are increasingly important for applications, such as in personal care products, but the fundamental science of how and why they form remains unclear. This research uses a powerful instrumental approach in which a single molecule of a negatively charged polymer is stretched using a magnetic field, and then exposed to positively charged molecules in solution. This setup allows researchers to directly observe how the molecule responds to its environment, including when it folds into a droplet-like state, helping to uncover the precise conditions under which coacervation occurs. The project also develops new instruments that combine force measurements with 3D fluorescence imaging, providing a rare view of droplet formation at the molecular level. In parallel, the team will build theoretical models and computer simulations to interpret the results. This research serves the national interest by enhancing scientific knowledge about biomaterials, including a range of natural polymers used in technology. This scientific impact is joined by direct support of graduate and undergraduate student training, which will help guarantee a well-trained national STEM workforce. The project further strengthens ties between academic and national laboratories. TECHNICAL SUMMARY This project seeks to develop a mechanistic understanding of complex coacervation through single-molecule magnetic tweezer (MT) manipulation of a tethered polyanion interacting with free polycations in solution. By applying mechanical tension to the polyanion, the system can probe phase separation as a function of force, enabling direct measurement of coacervation transitions. The central hypothesis is that the force-dependent collapse of the polymer into a condensed phase encodes quantitative information about coacervate thermodynamics, including equilibrium conditions, kinetics, and configurational dynamics. Three aims structure the work: (1) development of MT-based instrumentation with active thermal stabilization and confocal fluorescence capabilities, (2) the systematic study of coacervation in biomaterials-relevant systems, including hyaluronic acid and single-stranded RNA interacting with various classes of polycations (polymeric, peptide, and protein), and (3) the development of a theoretical and simulation framework incorporating chain tension into coacervation phase diagrams. Simulations are conducted using coarse-grained molecular dynamics models that incorporate discrete ion effects. The work will reveal the role of polymer configuration, ionic strength, temperature, and polycation architecture in determining phase behavior, and seeks to establish a new physical framework for understanding tension-modulated complex coacervation. This project will provide direct support of graduate and undergraduate student training, which will help guarantee a well-trained national STEM workforce. The project further strengthens ties between academic and national laboratories. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.