Broadband acoustic spectra of aqueous solutions of human serum albumin

Between 0.2 and 2000 MHz, sonic absorption spectra of solutions of albumin in water and in aqueous phosphate buffer systems have been measured at different temperatures (15-35 degreesC), pH values (1.8-12.3), and protein concentrations (0.2-4%, w/w). Broad absorption spectra, indicating the presence of various relaxation processes, have been found. Depending on the albumin concentration, the spectra can be analytically represented by a sum of the asymptotic high-frequency contribution and up to four Debye-type relaxation terms. Other relaxation models, including one based on a continuous distribution of relaxation times, also appropriately describe the measured spectra. Low-frequency relaxation processes with relaxation times between about 30 and 300 ns appear to be due to cooperative conformational changes of the polypeptide chain. High-frequency processes with relaxation times between 0.3 and 3 ns seem to reflect elementary reactions of the albumin molecule that are associated with changes in the hydration properties.