This thread caught my attention several days ago and I now have enough time to add my two cents worth. These are my own biases and probably do not reflect the views of my friends and colleagues at various neutron facilities.
With respect to the size of crystals for neutron diffraction, a good rule of thumb is that there should be at least 10exp24 uniformly ordered unit cells in a D2O exchanged crystal to have successful diffraction on par with rotating anode data measured on a crystal with a tenth the volume. Several data sets have been measured from smaller crystals, and perdeuteration lowers the volume needed to extract useful information. Most of the neutron data has a resolution cutoff of 1.8 to 2.0Å, which permits unambiguous placement of deuterons and solvent molecules, especially when completing dual refinement of X-ray and neutron data from the same crystal. There have been a limited number of low temperature neutron diffraction experiments for several reasons. First, of the available neutron beamlines for macromolecular data measurement there are only one or two with open flow cryostats available, limiting the locations for standard macromolecular cryocrystallography. Second, there are a tremendous number of important structures that can be done at room temperature. It is difficult to justify the time needed for low temperature work to experimental review panels when crystals are available to resolve a knotty enzyme mechanism problem. Third, the size of crystal needed for successful neutron diffraction is right at the limit of the size of crystal that can be successfully flash-cooled without inducing excess mosaicity. Most neutron beamlines use some form of quasi-Laue data collection strategy. Mosaicities in excess of 0.5º render most crystals unusable for neutron data measurement. Remember that a lot of uniform unit cells are needed to get a usable diffraction signal from neutrons. Often a large flash-cooled cooled crystal appears to have low mosaicity when exposed to 0.5mm x-ray beam. However, when placed in the 3mm neutron beam, limited streaky low-resolution diffraction appears. It is difficult to judge the quality of flash-cooled neutron diffraction sized crystal without placing it in the neutron beam. Returning to point 2 it is difficult justify the time needed on fishing expedition. So far the only large crystals I have been able to flash-cool that met the demands of size and crystal perfection had very low solvent content or were grown in high levels of cryoprotectant. That said, several critical problems cry out for low temp neutron studies so there is every reason to persevere. I would be pleased to answer any questions off-line for those of you with more interest in neutron cryocrystallography. Finally with respect to radiation damage, Benno Schoenborn has had a myoglobin crystal in sealed capillary that he has used as a “standard candle” for testing neutron beamlines. There has been no discernable degradation of the crystal in all the years he has used it. The neutrons used for neutron diffraction are ‘cold’ neutrons, usually with energies of 1 – 10 meV. Damage could come from activated nuclei, but these are usually very limited on a molar basis within the crystal. As can be seen with Benno’s myoglobin crystal, 30 years of iron activation has yet to produce a measurable defect. Leif Hanson University of Toledo
