Radiation Damage, Small Protein Crystals and the Cornell ERL
Colin Nave
Daresbury Laboratory
A recent paper (Nave & Hill; J. Synchrotron Rad. (2005) 12, 299-303) examined the possibility of reduced radiation damage for small protein crystals (10 microns and below in size) under the conditions where the photo-electron could escape the sample. The conclusion was that higher energy radiation (e.g. 40keV) could offer an advantage as the photo-electron path length will be greater than the crystal size and less energy would be deposited in the crystal. These calculations have now been extended to include the effects of energy deposited due to Compton scattering and the energy difference between the incident photon and the emitted photo-electron. This provides an estimate for the optimum wavelength for collecting data from a protein crystal of a given size and composition. The size and likely perfection of the micro-crystals means that a small parallel beam of x-rays will be required. Any additional divergence in the diffracted beam will produce larger diffraction spots on the detector and a poorer signal to background ratio. If the crystal is perfect, the divergence of the diffracted beam can be derived from the diffraction broadening produced by a slit corresponding to the crystal size. Under these circumstances it emerges that illuminating the crystal with a highly coherent beam could give an advantage. The requirement for a source of high energy coherent x-rays should be fulfilled very well by a machine such as the Cornell ERL.
Another way of reducing radiation damage from a protein crystal is to collect data with a very short pulsed x-ray source under conditions where a single image can be obtained before subsequent radiation damage occurs. This requires a beam with a sufficient flux density so that a single useful diffraction pattern can be obtained from each pulse. A reasonably broad bandpass for the radiation will be required so that enough diffraction data is obtained from each image to allow scaling between different crystals. Ideally the incident radiation should have a stable spectral profile. The spectral bandwidth and stability requirements are not well matched to the radiation from x-ray free electron lasers such as the LCLS. A single ERL pulse could have the required spectral profile but insufficient flux density unless the crystal is large or the unit cell size small. A comparison of this short pulse approach with the approach of using higher energy coherent x-rays will be made.
