The adiabatic compressibility of several globular proteins has been measured by using an ultrasonic technique in the frequency range 0.5 to 10 MHz. The contributions to the measured compressibility from the protein matrix and from surface processes involving ionization of side chains and solvation effects are discussed. The internal protein compressibility is very low, indicating the existence of "dynamic domains" which are tentatively assigned to secondary structure elements.

A differential method for the measurement of sound velocity in solutions is described. The instrument uses two identical acoustic resonators and a newly developed electronic measuring system. The sound velocity is determined with a relative accuracy of ≈3×10-6 over the entire frequency range 0.5-10 MHz. The instrument has been used to determine the compressibility of protein solutions and to study their velocity dispersion.