conformational stability of cytochrome c probed by optical spectroscopy over the last 50 years cytochrome c has been used as a model system for studying electron transfer and protein folding processes recently convincing evidence has been provided that this protein is also involved in other biological processes such as the apoptosis and synuclein aggregation numerous lines of evidence suggest that the diversity of the functional properties of cytochrome c is linked to its conformational plasticity this chapter introduces circular dichroism and absorption spectroscopy as an ideal tool to explore this proteins conformational in solution besides assisting in distinguishing different conformations and in obtaining the equilibrium thermodynamics of the transitions between them the two spectroscopies can also be used to explore details of hemeprotein interaction for example the influence of the external electric field on the prosthetic heme group