Professor Omar A. Saleh
Materials Dept. and BMSE Program, UCSB

Single-molecule meets scaling: The low-force elasticity of single-stranded DNA

Abstract:

The elasticity of a polymer depends sensitively on the structural properties it acquires from both local interactions (between neighboring monomers) and global interactions (between well-separated monomers). Single-molecule manipulation experiments exploit this link: by measuring the extension of single polymers stretched with a known force, they directly measure polymer elasticity; thus, these techniques have been used to determine the structural properties of a wide variety of biologically- and technologically-relevant polymers. However, single-molecule data is typically compared to `ideal' models that account for the polymer's local characteristics, but ignore global interactions; this approach contradicts the classic scaling theories (due to Flory, de Gennes, etc.) which indicate that global interactions must be included to correctly describe a polymer's self-avoiding random walk structure.
Here, I will discuss our recent work which reconciles single-molecule approaches and scaling theory. We show that the forces used in typical single-molecule experiments are so large as to `turn off' global interactions, enabling application of the ideal models. Using a low-force experimental technique to probe the elasticity of single-stranded DNA, we recover the effects of global interactions: we measure a non-linear elastic regime predicted by the `tensile-blob' model of a self-avoiding chain. We exploit our experimental access to this regime to quantify the importance of (screened) electrostatic interactions to the structure of the charged DNA; the results shed light on some long-standing questions of the physics of charged polymers.