[How many molten globules states exist?]

The problem of a variety of denatured forms of the protein molecule under equilibrium conditions is considered. The experimental conditions are described at which the protein molecule can exist in various non-native states. The history of the discovery of a universal intermediate molten globule state and the current status of research in this field are briefly outlined. Particular emphasis is placed on the fact that the molten globule state is a thermodynamic state of the protein molecule that is separated from both the native and the completely unfolded state by "all-or-none" transitions, i.e., intramolecular analogs of the 1st-order phase transitions. It is also shown that the molten globule state is not the only intermediate state observed for a particular protein under equilibrium conditions. The main structural features of the protein molecule in various denatured conformations are described. How many molten globule states there exist? A molten globule, a precursor of the molten globule, a highly structured molten globule: are these particular conformational states or different forms of the unique intermediate state? Or different forms of the native protein molecule with different degrees of disorder? Or differently structured forms of the unfolded polypeptide chain? This review is an attempt to answer these questions.