Urea-generated free rotating water molecules are active in the protein unfolding process.

The critical urea concentration (C3*) which destabilizes the structure of bovine serum albumin and chymotrypsinogen was determined by UV difference spectroscopy. The increase of the relative content of mobile rotating water molecules in aqueous urea was formerly shown by millimeter spectroscopy [1]. The rise of rotator content at a urea concentration C3 > or = C3* when the bulk water is practically exhausted is suggested as a main driving force of protein unfolding.