Nonlocal interactions stabilize compact folding intermediates in reduced unfolded bovine pancreatic trypsin inhibitor.

To further our understanding of the protein folding process, it is desirable to examine the structural intermediates (equilibrium and kinetic) that are populated between the statistical coil state and the folded molecule. X-ray crystallography and NMR structural studies are unable to determine long-range distances in proteins under denaturing solution conditions. Nonradiative (Förster) energy transfer, however, has been shown to be a spectroscopic ruler for the measurement of distance distributions and diffusion between selected sites in proteins under a range of different solution conditions. The distributions of distances between a donor probe at the N-terminal residue and an acceptor attached to one of the four lysine residues (15, 26, 41, 46) of reduced and unfolded (in 6 M guanidine hydrochloride and 20 mM dithiothreitol) bovine pancreatic trypsin inhibitor (BPTI) were measured as a function of temperature. Even in strong denaturant and reducing agent, BPTI does not exist as a statistical coil polypeptide. It appears that nonlocal (long-range) interactions are already beginning to "fold" the protein toward a more compact, native conformation. As the temperature is increased under these conditions, hydrophobic interactions lead to an even more compact structure consistent with the predictions of phase diagrams for globular proteins.