Probing globular protein self-assembling dynamics by heterodyne transient grating experiments

In this work, we studied the propagation of ultrasonic waves of lysozyme solutions characterized by different degrees of aggregation and networking. The experimental investigation was performed by means of the transient grating (TG) spectroscopy as a function of temperature, which enabled measurement of the ultrasonic acoustic proprieties over a wide time window, ranging from nanoseconds to milliseconds. The fitting of the measured TG signal allowed the extraction of several dynamic properties, here we focused on the speed and the damping rate of sound. The temperature variation induced a series of processes in the lysozyme solutions: Protein folding-unfolding, aggregation and sol–gel transition. Our TG investigation showed how these self-assembling phenomena modulate the sound propagation, affecting both the velocity and the damping rate of the ultrasonic waves. In particular, the damping of ultrasonic acoustic waves proved to be a dynamic property very sensitive to the protein conformational rearrangements and aggregation processes.