Section 9.1.12.1. Historical perspective

All crystals irradiated with X-rays absorb at least a fraction of the radiation, resulting in damage to the sample (Henderson, 1990). The energy from the absorbed photons may initially result in the disruption of chemical bonds, before being eventually dissipated as thermal energy. For well ordered small-molecule crystals the lattice is close packed and the effects arising from the absorbed photons are restricted to the immediate environment of the absorption event, so-called primary damage. Only when a substantial fraction of the crystal has been affected do cooperative effects set in.

In contrast, roughly 50% of a macromolecular crystal is disordered aqueous solvent (Matthews, 1968). At room temperature this allows a secondary mechanism of radiation damage, resulting from diffusion of radicals and ions produced at the primary absorption site that affects chemical moieties at positions remote from this site. The details of this process remain poorly understood but are related to the extremely damaging effects of X-rays on biological tissue. A consequence of this damage is that degradation of the crystal order continues even after the irradiation is stopped or interrupted. For collection of data at room temperature from protein crystals mounted in capillaries, secondary damage contributes significantly to the rate of deterioration of the diffraction pattern. One of the gains of the early applications of SR was that it allowed recording of data to proceed ahead of the effects of secondary damage, increasing the effective, if not the absolute, lifetime of the crystal in the X-ray beam. An experiment often required several crystals, all of which showed the effects of temporal decay in their recorded intensities, which needed to be merged to provide complete data.