A sequence alignment of mammalian cytochromes c with yeast iso-1-cytochrome c (y-cyt- c ) shows that the yeast protein contains five extra N-terminal residues. We have been interested in understanding the question: What is the role of these five extra N-terminal residues in folding and stability of the protein? To answer this question we have prepared five deletants of y-cyt- c by sequentially removing these extra residues. During our studies on the wild type (WT) protein and its deletants, we observed that the amount of secondary structure in the guanidinium chloride (GdmCl)-induced denatured (D) state of each protein is different from that of the heat-induced denatured (H) state. This finding is confirmed by the observation of an additional cooperative transition curve of optical properties between H and D states on the addition of different concentrations of GdmCl to the already heat denatured WT y-cyt- c and its deletants at pH 6.0 and 68°C. For each protein, analysis of transition curves representing processes, native (N) state ↔ D state, N state ↔ H state, and H state ↔ D state, was done to obtain Gibbs free energy changes associated with all the three processes. This analysis showed that, for each protein, thermodynamic cycle accommodates Gibbs free energies associated with transitions between N and D states, N and H states, and H and D states, the characteristics required for a thermodynamic function. All these experimental observations have been supported by our molecular dynamics simulation studies.