Figure 2.

Force-extension curves for single-pectin molecules reveal a two-step transition. (A) The shape of the curves obtained from short and long molecules clearly deviates from the shape expected for a FJC. The curves reveal two enthalpic extensions that occur at ≈300 and ≈800–900 pN. (B) High-resolution plot of the force-extension relationship for a single pectin molecule. The extension, x, was normalized: x_norm = 0.45x/l₁^c, where 0.45 nm corresponds to length of the residue vector O₁O₄^chair in the relaxed state of the polymer (chair ⁴C₁; see Table 1), and l₁^c the contour length of the molecule determined from the FJC fit to the nonnormalized data before the first transition (l₁^c = 69.97 nm for this recording). Thin lines are the local fits of the FJC model modified to include the extensibility of the monomers (segment elasticity S_E; 10, 16, 20). l₁ = 0.45 nm (by definition); S_E1 was found to be 17,000 pN/nm; l₂ was determined to be 0.492 nm (it corresponds to O₁O₄^boat); S_E2 was determined to be 24,000 pN/nm; l₃ was found to be 0.541 nm (it corresponds to O₁O₄^inv-chair), whereas S_E3 was found to be 83,000 pN/nm. The length of the Kuhn segment l_K was determined from the low force region fit to be 1.81 nm. (Inset) Structure of the α-d-galactouronic acid (the pectin monomer). Note that in contrast to α-d-glucopyranose that has the C4—O4 bond in the equatorial position, α-d-galactopyranuronic acid has the C4—O4 bond in the axial position.