Equilibrium titrations of acid-induced unfolding–refolding and salt-induced molten globule of cytochrome c by FT-IR spectroscopy

Despite extensive investigations on the acid-unfolded and acid/salt-induced molten globule(-like) states of cytochrome c using variety of techniques, structural features of the acid-unfolded state in terms of residual secondary structures and the structural transition between the acid-unfolded and acid/salt-refolded states have not been fully characterized beyond the circular dichroism (CD) spectroscopy. It is unusual that secondary structure(s) of the unfolded state leading to the molten globule state, an important protein folding intermediate, as determined by CD was not fully corroborated by independent experimental method(s). In this study, we carried out an equilibrium titration of acid-induced unfolding and subsequent acid- and salt-induced refolding of cytochrome c using Fourier transform infrared spectroscopy. The spectral profiles of the equilibrium titration reveal new structural details about the acid-unfolded state and the structural transition associated with the acid/salt-refolded molten globule(-like) states of cytochrome c.