Carbon-deuterium bonds as probes of protein thermal unfolding

We report a residue-specific characterization of the thermal unfolding mechanism of ferric horse heart cytochrome c using C-D bonds site-specifically incorporated at residues dispersed throughout three different structural elements within the protein. As the temperature increases, Met80 first dissociates from the heme center, and the protein populates a folding intermediate before transitioning to a solvent exposed state. With further increases in temperature, the C-terminal helix frays and then loses structure along with the core of the protein. Interestingly, the data also reveal that the state populated at high temperature retains some structure and possibly represents a molten globule. Elucidation of the detailed unfolding mechanism and the structure of the associated molten globule, both of which represent challenges to conventional techniques, highlights the utility of the C-D technique.