Fig. 6.

Residue-specific hydrogen exchange of PP32. (A) Heteronuclear single quantum coherence (HSQC) spectra after PP32 is exchanged from H₂O to D₂O buffer for various times: 21 min (black), 19 h (blue), 10 d (yellow), and 69 d (red). (B) Exponential decays of peak heights of representative amide protons with very different exchange rates. The lines show single-exponential fits to the data. (C) Protection factors (k_ex/k_int) for main-chain amides with quantifiable protection. The β-sheet residues are red-filled circles; non–β-sheet residues are black circles. The solid curve results from fitting a cosine-modulated Gaussian distribution to the data (Materials and Methods). The N terminus shows uniformly low protection. The region with highest protection factors is toward the C terminus. (D) Mapping of protection factors onto the structure of PP32. For residues with main-chain amides that have quantifiable protection and are not exposed to solvent, Cα’s are shown in spheres, and colored from white to blue, with blue having the highest protection factors.