Unit-cell volume change as a metric of radiation damage in crystals of macromolecules

The use of third-generation synchrotron sources has led to renewed interest in the effect that ionizing radiation has on crystalline biological materials. Simple criteria have been sought to study the effects systematically. The unit-cell volume of protein crystals shows a linear increase with absorbed dose and has therefore been proposed to be such a measure. This paper demonstrates that the increase is sample dependent, and thus it might not be a useful indicator when comparing different samples. For individual samples, however, the increase can be used to quantify ambient temperature and dose-rate effects. In this study, highly absorbing cubic crystals of holoferritin have been used to accurately determine how cell volume changes with absorbed dose. The experiments show that, for this protein, a dose-rate effect exists and that trapped radicals can be mobilized at ca 180 K.