Ultrasonic absorption mechanisms in aqueous solutions of bovine hemoglobin

In order to study further the principal loss mechanisms of ultrasonic energy in biological media, the absorption and velocity were determined in aqueous solutions of hemoglobin, largely at 10°, over the frequency range 1-50 MHz. A distribution of relaxation processes is necessary to characterize the absorption spectra. Interaction of the acoustic wave with the hydration layer of the macromolecule, and not direct interaction with macromolecular configuration, appears as a dominant mechanism. The ultrasonic absorption titration curves exhibit maxima around pH 2-4 and pH 11-13, in addition to possessing a broad peak in the pH range 5-9. The peaks in the absorption titration and the similar ones observed for bovine serum albumin (J. Phys. Chem., 73, 4256 (1969)), are attributed to the proton-transfer reaction occurring between particular amino acid side chain groups and the solvent. The broad peak is partially attributed to the proton transfer resulting from the imidazolium function of the histidine residue.