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ABSTRACT: Conformational transitions of oxidized and reduced cytochrome c at various solvent conditions are summa-rized. Sorbitol stabilizes and NaCl destabilizes native cytochrome c structure against the acid denaturation. In the process of heating, NaCl more strongly stabilizes molten globular cytochtome c state than sorbitol in secondary structure region, but in heme region, sorbitol is stronger stabilizer of cytochrome c molten globular state than NaCl. In the presence of strong inorganic acids H 2 SO 4 and HClO 4 and their salts two low spin and one high-spin heme conformers occur. The un-stable pentacoordinated conformer is formed in 8M H 2 SO 4 . Cytochrome c creates complexes with polyanions poly(vynilsulfate) and poly(4-styrene-sulfonate), which are able to mimic the effect of cytochrome c natural redox partner on its structure, as well as to prevent its the aggregation in aggregation prone conditions. Cytochrome c forms complexes with gold nanoparticles covered by glutation, which enhance stability in heme region at room temperature. Cytochrome c (cyt c) is one of the most studied proteins with respect to its conformational transitions. This protein is an essential redox biomacromolecule found in the mitochon-dria. It functions as an electron transporter in the energy-yielding respiratory chain and participates in a process of apoptosis. Its prosthetic group (heme) is covalently bound to the polypeptide through thioether linkages with two cysteine residues. The iron in the heme coordinates two axial ligands, histidine (His18) and methionine (Met80), so that it adopts a low-spin configuration in both its ferrous and ferric forms at physiological conditions (Fig. 1). Acidification of a salt-free solution of ferricyt c to pH 2.0 leads to the unfolding of the protein associated with a con-version of the low to the high-spin state. The structure of unfolded protein is an extended coil having a dimension greater than that of a random coil owing to electrostatic re-pulsions among the positively charged lysine and arginine residues. Upon addition of salts to the acid unfolded cyto-chrome c the protein cooperatively folds to a compact struc-ture with a molten globule (MG) character. Molten globular conformation of cyt c has the secondary structure content similar to that of the native conformation, but less packed tertiary structure.