Ultrasonic absorption in myoglobin and other globular proteins.

The ultrasonic absorption of myoglobin has been measured by the resonance and pulse-echo techniques as a function of pH. The absorption at a given frequency can be separated into pH-dependent and pH-independent contributions. Like other proteins, two peaks at pH 3 and 11.5 are observed which can be accounted for a proton-transfer reactions of the side-groups. In addition, the absorption undergoes a large increase within a small range of 0.2 pH unit at pH around 4, when denaturation of myoglobin occurs, indicating that the absorption is sensitive to the overall protein conformation. To elucidate the origin of the pH-independent component, the absorptions of several other globular proteins at neutral pH are also measured. The absorptions of these proteins exhibit a linear correlation with their isothermal compressibilities, suggesting that the pH-independent component is related to volume fluctuations of protein molecules. The activation energy of 4 kcal/mol found for this relaxation is consistent with such an interpretation.