Stick-release pattern in stretching single condensed polyelectrolyte toroids

The loop-by-loop unfolding of a toroidal PE (polyethylene) that produces the stick-release pattern in force-extension curve (FEC) was investigated. One end of the condensed PE chains was fixed and the other end was pulled outward along the x-axis using a spring force with constant pulling speed V = 0.0005. About 20 independent toroids were prepared and stretching was performed. The extension x is the distance between two chain ends. The data have been passed through a low-pass filter to attenuate the high-frequency noise. This tooth-like structure repeatedly appears in the FEC and becomes shaper and sharper as the extension increases, known as stick-release pattern. It was observed that the major radius R₀ decreases early until the release of the first loop. It abruptly increases at the moment when a loop is pulled out of the toroid, and then decreases with the extension. The results give deep insight of the elastic response and the structural transition of condensed DNA molecules being stretched.