Condensation transition in DNA-polyaminoamide dendrimer fibers studied using optical tweezers

TL;DR

This study investigates the force-induced decondensation transition of DNA-dendrimer fibers using optical tweezers, revealing a first-order phase transition driven by electrostatic interactions and supported by a theoretical model and AFM imaging.

Contribution

It provides the first detailed force measurement analysis of DNA-PAMAM dendrimer fiber decondensation and introduces a simple model explaining the transition mechanism.

Findings

Force-extension curves show a characteristic plateau at ~10 pN.

Hysteresis indicates a first-order phase transition.

Electrostatic forces are key to the condensation process.

Abstract

When mixed together, DNA and polyaminoamide (PAMAM) dendrimers form fibers that condense into a compact structure. We use optical tweezers to pull condensed fibers and investigate the decondensation transition by measuring force-extension curves (FECs). A characteristic plateau force (around 10 pN) and hysteresis between the pulling and relaxation cycles are observed for different dendrimer sizes, indicating the existence of a first-order transition between two phases (condensed and extended) of the fiber. The fact that we can reproduce the same FECs in the absence of additional dendrimers in the buffer medium indicates that dendrimers remain irreversibly bound to the DNA backbone. Upon salt variation FECs change noticeably confirming that electrostatic forces drive the condensation transition. Finally, we propose a simple model for the decondensing transition that qualitatively reproduces the FECs and which is confirmed by AFM images.