A single -phage DNA molecule is held in an optical trap at ( )

PHYSICS · Multi-part · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Biophysics & Medical Physics · Macromolecules, membranes and cell architecture

Forces, Machines and Signals, Ch The elastic polymer: DNA from random walk to worm-like chain

Problem

A single λ\lambda-phage DNA molecule is held in an optical trap at T=298 KT=298\,\mathrm{K} (kBT=4.11 pN nmk_BT=4.11\,\mathrm{pN\,nm}). At an applied force f=1.0 pNf = 1.0\,\mathrm{pN}, the measured relative extension is z/L=0.86z/L = 0.86. ((a)) Using the high-force asymptote of the worm-like chain, 1−z/L=12kBT/(fξp)1-z/L = \tfrac12\sqrt{k_BT/(f\xi_p)}, translate this single data point into a persistence length ξp\xi_p. ((b)) Check that f=1.0 pNf=1.0\,\mathrm{pN} lies well above the crossover force kBT/ξpk_BT/\xi_p implied by your answer, so that using the high-force asymptote (rather than the full Marko-Siggia interpolation) was justified. Then extract ξp\xi_p from the same point with the full Marko-Siggia formula, and compare both values with the accepted value for double-stranded DNA.

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