Dark progression reveals slow timescales for radiation damage between T = 180 and 240 K

Can radiation damage to protein crystals be `outrun' by collecting a structural data set before damage is manifested? Recent experiments using ultra‐intense pulses from a free‐electron laser show that the answer is yes. Here, evidence is presented that significant reductions in global damage at temperatures above 200 K may be possible using conventional X‐ray sources and current or soon‐to‐be available detectors. Specifically, `dark progression' (an increase in damage with time after the X‐rays have been turned off) was observed at temperatures between 180 and 240 K and on timescales from 200 to 1200 s. This allowed estimation of the temperature‐dependent timescale for damage. The rate of dark progression is consistent with an Arrhenius law with an activation energy of 14 kJ mol −1 . This is comparable to the activation energy for the solvent‐coupled diffusive damage processes responsible for the rapid increase in radiation sensitivity as crystals are warmed above the glass transition near 200 K. Analysis suggests that at T = 300 K data‐collection times of the order of 1 s (and longer at lower temperatures) may allow significant reductions in global radiation damage, facilitating structure solution on crystals with liquid solvent. No dark progression was observed below T = 180 K, indicating that no important damage process is slowed through this timescale window in this temperature range.