The volume and compressibility changes of lysozyme associated with guanidinium chloride and pressure-assisted unfolding

The apparent specific volumes and isentropic compressibilities of hen egg w hite lysozyme were measured in aqueous guanidinium chloride solutions at 25 degrees C by means of a vibrational densimeter and a sing-around ultrasoni c velocimeter. Little transition attributable to a protein unfolding was de tected in the partial specific volume, while the partial specific isentropi c compressibility decreased slightly around the transition region. The pres sure-assisted unfolding was also investigated in aqueous guanidinium chlori de solutions by means of ultraviolet spectroscopy. Assuming a two-state tra nsition model, it was found that the free energy change of unfolding depend s almost Linearly on pressure and the unfolding reaction is accompanied by a small decrease in volume. The compressibility behavior is in conflict wit h the notion that a protein structure is almost completely unfolded by guan idinium chloride and most of the amino acid residues in the protein interio r are exposed to solvent. These results support the current view that globu lar proteins have some residual structures even in the unfolded state induc ed by a strong denaturant. (C) 1999 Academic Press.