A positron annihilation lifetime study of protein hydration - evidence for a glass transition

Abstract
Positron annihilation lifetimes have been determined for lysozyme at 298 K over the hydration range 0.007 - 0.395 h (g water / g protein) and at hydration levels of 0.15 [MATH] and 0.19 h over the temperature range 120 - 350 K. The results indicate that lysozyme undergoes a glass-like, dynamical transition with a transition temperature which decreases with increasing hydration. This behavior is similar to that found with TSDC techniques and suggests that the transitions previously observed at low hydration (0.07 - 0.2 h) at 298 K and those observed at 180 - 220 K in fully hydrated proteins are related. A second transition is observed at 160 - 170 K which may be due to protein-bound water.