MOLTEN GLOBULE OF HUMAN ALPHA-LACTALBUMIN - HYDRATION, DENSITY, AND COMPRESSIBILITY OF THE INTERIOR

Specific partial volume, partial compressibility, and sound absorption changes induced by the native-to-molten globule state (acid) transiti on of the human alpha-lactalbumin were measured by means of densitomet ric and ultrasonic techniques and interpreted in terms of the protein molecule phase transition and interphase water transfer. The molten gl obule is a highly hydrated state containing about 270 water molecules inside. Intrinsic mass density of the hydrated (swollen) interior of t he protein molecule is 5% smaller and the intrinsic compressibility co efficient 2 times higher than those in the native molecule. The, obtai ned intrinsic compressibility falls into the range of values character istic of highly associated liquids. Water inside the molten globule in terior occupies less volume and is less compressible than in solvent p hase. The acoustic relaxation was found to increase indicating an appe arance of pressure-dependent processes. The commonly used approach to the calculation of the volume fluctuations of protein molecules, based on the well-known relation between the volume fluctuations and compre ssibility, is of limited applicability to the highly hydrated molten g lobule state because a large, if not predominant, part of the fluctuat ions may be determined by the process of water exchange between the mo lten globule and bulk solvent.