The assignments for resolved backbone residues are labeled with the residue number. (a) Chemical shift perturbations of the 15N-labeled NTD of FKBP38 upon deletion of the N-terminal extension (residues 1–32) were analyzed by overlaying 2D 1H-15N HSQC spectra of the NTD (red) and FKBD38 (black). (b) The 15N and 1H chemical shift changes were combined using the equation, Δδ = ((Δδ 1H)2 + (0.154* Δδ 15N)2)1/2). A threshold value at 0.025 ppm is indicated by a horizontal dotted line. (c) The residues of the perturbed chemical shifts attributed to deletion of the N-terminal extension are indicated by residue numbers (left) or highlighted in green on the surface representation (right). For simplicity, only the combined weighted chemical shifts higher than 0.025 ppm are shown. (d) ±1 kT/e electrostatic potential of NTD generated using APBS PyMOL plotted on the solvent accessible surface (blue, positively charged; red, negatively charged; white, neutral), in approximately the same orientation as that of the molecules in Fig. 3c (left). Positively charged residues K62 and K119 are labeled, where residue K62 might be involved in the interaction with the negatively charged residues in N-terminal extension.