Ultrasonic Absorption by Solvent-solvent Interactions and Proton Transfer in Aqueous Solutions of Peptides and Small Proteins

Ultrasonic absorption values in aqueous olutions of myoglobin, apomyoglobin, a-lactalbumin, bacitracin, and the Cand N-terminal CNBr cleavage fragments of myoglobin were determined by the cylindrical resonator technique at temperatures of 4 øC and 20 øC in the frequency range 0.5-7 MHz. A proposed mechanism of absorption in solutions of proteins and peptides is perturbation by the ultrasound of the equilibrium between protein bound and free water, producing a distribution of relaxation processes occurring within the frequency range of the investigation. The hypothesis supported by the dependence ofthe ultrasonic absorption on solute surface area and molecular weight. Another mechanism of absorption is that due to proton transfer which occurs noticeably in the presence of phosphate ions. Those proteins {myoglobin and a4actalbumin} with significant globular structure appear to undergo additional relaxation processes at frequencies above 7 MHz.