Fig. 3.
Bandwidth of an extended polypeptide. (A) Brownian dynamics computes the length of a 30 nm long polypeptide placed under a 10 pN sinusoidal load of varying frequency, f, using the measured value of Dpol = 1,300 nm2/s. The figure shows polypeptide responses at two different frequencies (25 Hz, and 250 Hz). (B) We use lock-in detection to measure the in-phase (ϕ0, triangles) and the out-of phase (ϕ90, circles) components of the elastic response of the polypeptide as a function of the load frequency, f. The solid lines (inset), correspond to the real (triangles) and imaginary (circles) parts of the mechanical impedance of a Kelvin-Voigt model (inset), fitted to the data. From ϕ0 and ϕ90 we calculate power as a function of frequency for the polypeptide (triangles). The solid line fits the Kelvin-Voigt model to the data returning values of Dpol = 1,313 nm2/s and kpol = 1.31 pN/nm. The arrow shows the -3 dB point, giving a bandwidth of 66 Hz. These measurements show that the elasticity of a polypeptide-based material is band-limited to a few hundred Hertz, in good agreement with broadly observed animal behavior.