Topology-Controlled Relaxation Dynamics of Single Branched Polymers.

In this work, we report the synthesis and direct observation of branched DNA polymers using single molecule techniques. Polymer topology plays a major role in determining the properties of advanced materials, yet understanding the dynamics of these complex macromolecules has been challenging. Here, we study the conformational relaxation dynamics of single surface-tethered comb polymers from high stretch in a microfluidic device. Our results show that the molecular topology of individual branched polymers plays a direct role on the relaxation dynamics of polymers with complex architectures. Macromolecular DNA combs are first synthesized using a hybrid enzymatic-synthetic approach, wherein chemically modified DNA branches and DNA backbones are generated in separate polymerase chain reactions, followed by a "graft-onto" reaction via strain-promoted [3 + 2] azide-alkyne cycloaddition. This method allows for the synthesis of branched polymers with nearly monodisperse backbone and branch molecular weights. Single molecule fluorescence microscopy is then used to directly visualize branched polymers, such that the backbone and side branches can be tracked independently using single- or dual-color fluorescence labeling. Using this approach, we characterize the molecular properties of branched polymers, including apparent contour length and branch grafting distributions. Finally, we study the relaxation dynamics of single comb polymers from high stretch following the cessation of fluid flow, and we find that polymer relaxation depends on branch grafting density and position of branch point along the main chain backbone. Overall, this work effectively extends single polymer dynamics to branched polymers, which allows for dynamic, molecular-scale observation of polymers with complex topologies.