> 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.
Can't the crystal be flash-cooled at high pressure? The inventors of the Crystal Harp in Zurich use a machine that does this automatically for cryo e.m., which many universities already have. On Fri, Sep 23, 2011 at 4:40 PM, Leif Hanson <leif.han...@gmail.com> wrote: > 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 > -- patr...@douglas.co.uk Douglas Instruments Ltd. Douglas House, East Garston, Hungerford, Berkshire, RG17 7HD, UK Directors: Peter Baldock, Patrick Shaw Stewart http://www.douglas.co.uk Tel: 44 (0) 148-864-9090 US toll-free 1-877-225-2034 Regd. England 2177994, VAT Reg. GB 480 7371 36
