Ultrasonic absorption by solvent–solute interactions and proton transfer in aqueous solutions of peptides and small proteins

Abstract

Ultrasonic absorption values in aqueous solutions of myoglobin, apomyoglobin, α-lactalbumin, bacitracin, and the C- and N-terminal CNBr cleavage fragments of myoglobin were determined by the cylindrical resonator technique at temperatures of 4 °C and 20 °C 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 is supported by the dependence of the 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 α-lactalbumin) with significant globular structure appear to undergo additional relaxation processes at frequencies above 7 MHz.