Fig. 2.

Logistical corrections to distribution function of disordered regions. (a) Paramagnetic enhancement to nuclear spin relaxation at pH 2.3 for an apoMb variant with a nitroxide spin label at position 77. The histogram shows the experimental intensity ratios (Q = I_para/I_dia) for each residue with an adequately resolved cross peak in the ¹H-¹⁵N HSQC spectrum. An intensity ratio of 1 indicates no effect of the spin label on an amide proton. The superimposed curves show Q values calculated from Eqs. 1–4 with l_p = 20 (green) or fitted to the logistically corrected Eq. 4 (red) with parameters l_p = 2.64 ± 0.04, A = 16.6 ± 0.4, and B = 0.108 ± 0.001. Note that the logistical correction contributes to chain stiffness that removes the direct correspondence between the parameter l_p and the chain's actual persistence length. (b) P(r;|i − j|) calculated by using Eq. 4 with (red lines) and without (green lines) its logistical correction for sequence separations |i − j| of 10 residues (l_c = 38 Å; solid lines) and 20 residues (l_c = 56 Å; dashed lines). Data for overlapping resonances are omitted from the PRE profiles and analysis.