ULTRASONIC INVESTIGATION OF CONFORMAL CHANGES OF BOVINE SERUM ALBUMIN IN AQUEOUS SOLUTION

The excess ultrasonic absorption and the speed of sound were measured in aqueous solutions of bovine serum albumin (a globular protein which undergoes marked configurational change with pH) at 20° over the frequency range 0.3 to 163 MHz and over the pH range 2.3 to 11.8. A sharp increase in the excess absorption is found outside the range 4.3 < pH < 10.5. The effect is reversible throughout this range and is more pronounced at the lower frequencies. The increase in the absorption below pH 4.3 appears to be correlated with the intermediate N-F′ transition discussed by Foster and the change above pH 10.5 is thought to correspond with expansion of the molecule. At neutral pH, the ultrasonic absorption spectrum and the velocity dispersion are indicative of a broad distribution of relaxation processes. The magnitude of the ultrasonic absorption over the range 4.3 < pH < 10.5 is attributed to the perturbation of the solute-solvent equilibrium by the sound wave. Based on data at 20°, between 2.4 and 50 MHz, the frequency spectrum of the absorption increase at pH 3.5 over that at pH 7.0 may be described by a single relaxation process whose characteristic frequency is 2.2 MHz. Based on measurements of the velocity of sound at pH 7.0, the bulk modulus of BSA has been found to be 3.86 × 109 n/m2.