FIGURE 2. Single-molecule AFM measurements of the mechanical properties of the N2B and PEVK regions of titin.

a, Top inset: the arrow points to the location of the N213 region in the I-band. Force–extension curve of a protein chimaera containing the cardiac N2B unique sequence flanked on either side by three I27 domains (I27₃-N2B-I27₃), bottom insert. A Levenberg–Marquardt fit of the WLC equation (thin line) to the force–extension curve before the first I27 unfolding event measured the contour length, L_c, and persistence length, p, of N2B. b, Frequency histogram of persistence-length values. A narrow distribution is found, centred at 0.66 nm. c, Top inset: the arrow points to the location of the PEVK region in the I-band. Force–extension curve of a protein chimaera containing human cardiac PEVK domains alternating with Ig I27 domains, (I27-PEVK)₃, bottom insert. As in a, we used Levenberg–Marquardt fits of the WLC equation to measure L_c and p of the PEVK region (thin line). d, Frequency histogram of persistence-length values measured for the PEVK domain. A relatively broad distribution is seen (p = 0.4–2.5 nm; average value, 0.91 nm).