Ultrasonic characterization of proteins in complex fluids

Our aim is the study of the physicochemical protein properties (volume, compressibility) as a function of hydration by an ultrasonic technique. For this purpose, we have used a biomimetic medium: reverse micelles, a system where we can control precisely water concentration. The compressibility of a medium is obtained by measuring the density and the ultrasonic celerity. The difficulty is increased in a complex fluid: nanometric size inclusions (micelles) dispersed in an organic solvent. To reach this goal, we have custom-built in our laboratory an apparatus allowing the determination of ultrasonic celerity, in very small volumes, with a relative precision of about 10 ppm. Using the effective medium theory, we have determined the compressibility of these inclusions with a precision better than 1%. With a spherical model for micellar inclusions and with the hydrodynamic radius of micelles obtained by x rays, we have estimated the compressibility of the water within micelles. We have also evidenced a difference in compressibility between the central core water and water bound to the polar surfactant headgroups. Finally by applying the mean field theory, we have obtained an estimate of the compressibility of proteins inserted within the micelles, at various hydration levels.