Hysteresis during heating and cooling of hyaluronan solutions in water observed by means of ultrasound velocimetry.

Ultrasound velocity was measured in hyaluronan solutions of various compositions at different temperatures. The velocity dependence on hyaluronan concentration at constant temperature (25 °C) was linear both in water and in 0.15 M NaCl regardless of hyaluronan molecular weight, confirming diluted-solution behavior. During cyclic heating and cooling, hysteresis on the temperature dependence of ultrasound velocity was observed in the range 30-55 °C in aqueous solutions for all molecular weights. In NaCl solutions, the hysteresis was suppressed and, in contrast to solutions in water where the velocity demonstrated a local maximum with temperature, the velocity decreased with increasing temperature. These findings were attributed to slow and hydration-linked hyaluronan conformation transitions during cooling in water, which were suppressed by the presence of salt, which makes hyaluronan's coiled conformation more compact.