Conformational Dynamics of the Trp-Cage Miniprotein at Its Folding Temperature

The folding temperature of the tip-cage mini-protein was determined to be in the range 311-317 K depending on the method used. Our study is focused on determining the structure and dynamics of the polypeptide chain close to its unfolding or melting temperature. At T = 305 K, Trp6-Arg16 and Trp6-Pro12 long-range interactions are observed, and at T = 313 K, only the Trp6-Arg16 interactions remain, while all of mentioned interactions are observed in the native state of the protein. Partial (at T = 305 K) and complete (at T = 313 K) melting of the N-terminal alpha-helix is observed, manifested by the appearance of minor sets of signals in NMR spectra. Our key findings are: (i) conformational phase transition (melting point) could be described as a cooperative breaking of the Trp6-Pro12 long-range hydrophobic interaction and the melting of the N-terminal alpha-helix; (ii) many ROE signals corresponding to local or short-range interactions vanish rapidly with temperature increase; however, long-range interaction such as Trp6-Arg16 remains until 313 K. The presence of the native long-range interaction at 313 K makes that conformational ensemble resemble a very diffuse native state structure, but it is not a simple mixture of the folded and unfolded states, as could be expected on the basis of the common two-state folding mechanism.