FIGURE 4.

Measurement of torsion-elasticity constant of a single kinesin-1 molecule. Power spectral density of angular fluctuations is from Fig. 3 C, trace i; solid line is a Levenberg-Marquadt least-squares weighted fit to the data with the Lorentzian: where S(f) is the power at frequency f, f_c is the corner frequency, k_B is the Boltzmann constant, T is the absolute temperature, and γ is the drag coefficient. The torsion elasticity constant is calculated as κ = 2π f_cγ. (Inset) Fit residuals. Solid lines represent expected standard deviation (1/n^1/2, where n is number of points, and each point is an average of 20 data/fit points).