Figure 3: Rotational fluctuations in the FOMT are constrained by the torsional stiffness of DNA only.
From: Freely orbiting magnetic tweezers to directly monitor changes in the twist of nucleic acids
Data in panels a–f were obtained using a 0.35μm radius bead and a 3.4-kb dsDNA tether in the FOMT geometry. (a) Histogram of the (x,y) fluctuations (see Supplementary Fig. S13 for additional plots of the same trace). The histogram reveals essentially uniform coverage of the positions on the circle. (b) Rotational fluctuations of the bead determined from the (x,y) positions. (c) Histogram of the rotational fluctuations. The red line is a Gaussian fit with σθ=223°. (d) The energy landscape implied by the rotational fluctuation density from (b). The difference between the landscape implied by the rotational fluctuations and the harmonic approximation (with kθ=kBT/σθ2=0.27 pN.nm/rad) is much smaller than the thermal energy kBT. Data are offset for clarity such that θ0=0. (e) Temporal autocorrelation of the rotational fluctuations. The red line is an exponential fit with the trap stiffness kθ=0.27 pN.nm/rad and the friction coefficient γ=1.82 pN.nm.s implying a characteristic time τc=γ/kθ=6.75 s. (f) Power spectrum of the rotational fluctuations. The red line is a Lorentzian fit with the corner frequency fc=0.024 Hz, in excellent agreement with the value expected from the temporal autocorrelation using the relationship fc=1/(2πτc). (g) Effective DNA torsional stiffness as a function of applied stretching force determined from equilibrium fluctuations (equation 4). Data are the mean and s.e.m. from at least five independent measurements in PBS+ buffer. MTT6 and fluorescence polarization anisotropy11,12,13 data are shown for comparison. Predictions from the first- and third-order Moroz–Nelson model are shown as dashed and solid lines, respectively.
