Fig. 7

Optical tweezers stretching curves of type II procollagen described at low force by entropic elasticity. a The Worm-Like Chain (WLC) model (red; equation (3)) is fit to an example force-extension curve (black dots), giving a persistence length of 32 nm for a molecule of 300 nm contour length, when a maximum force of 5 pN is used for the fit. Inset: a schematic showing procollagen stretching in the optical tweezers and illustrating the extension z and bead offset from trap Δz, from which force is determined. Schematic is not to scale. b The persistence length from fitting the WLC model decreases as the maximum force used in the fitting increases. The error bars show the uncertainty of the fitting parameter