Glass transition in thaumatin crystals revealed through temperature‐dependent radiation‐sensitivity measurements

The temperature‐dependence of radiation damage to thaumatin crystals between T = 300 and 100 K is reported. The amount of damage for a given dose decreases sharply as the temperature decreases from 300 to 220 K and then decreases more gradually on further cooling below the protein‐solvent glass transition. Two regimes of temperature‐activated behavior were observed. At temperatures above ∼200 K the activation energy of 18.0 kJ mol −1 indicates that radiation damage is dominated by diffusive motions in the protein and solvent. At temperatures below ∼200 K the activation energy is only 1.00 kJ mol −1 , which is of the order of the thermal energy. Similar activation energies describe the temperature‐dependence of radiation damage to a variety of solvent‐free small‐molecule organic crystals over the temperature range T  = 300–80 K. It is suggested that radiation damage in this regime is vibrationally assisted and that the freezing‐out of amino‐acid scale vibrations contributes to the very weak temperature‐dependence of radiation damage below ∼80 K. Analysis using the radiation‐damage model of Blake and Phillips [Blake & Phillips (1962), Biological Effects of Ionizing Radiation at the Molecular Level , pp. 183–191] indicates that large‐scale conformational and molecular motions are frozen out below T = 200 K but become increasingly prevalent and make an increasing contribution to damage at higher temperatures. Possible alternative mechanisms for radiation damage involving the formation of hydrogen‐gas bubbles are discussed and discounted. These results have implications for mechanistic studies of proteins and for studies of the protein glass transition. They also suggest that data collection at T  ≃ 220 K may provide a viable alternative for structure determination when cooling‐induced disorder at T = 100 is excessive.