Dear Tim and B. Vijay, On Mon, 2012-10-29 at 12:02 +0100, Tim Gruene wrote: > Dear B. Vijay, > > for single-wavelength (as opposed to Laue) X-ray crystallographic data > collection it is in general helpful to mount your crystal in an > arbitrary orientation.
Well, this depends on the sample, and how you are going to solve the structure if you don't already know it for the crystal form that you have (i.e. MR, SAD, MAD etc). The overall oscillation range required for completeness depends on the orientation, and if your sample is radiation sensitive then achieving (near) completeness early can be helpful. > If you happen to mount it such that a symmetry > axis is parallel to the rotation axis, you may not be able to collect > fully complete data. .... and also if the symmetry axis is a screw axis you won't have observed any systematic absences. So it may be a good idea to tilt the symmetry axis away from the rotation axis a bit. But (in some cases) not too far: apart from the rotation range issue, if you have one cell axis much longer than the others, putting it close to the rotation axis will reduce spot overlap, which can also be helpful. These factors (as well as others such as anisotropy of the sample) fight against each other, and the best compromise depends on the sample, the wavelength and the instrumentation that you are using. A truly arbitrary orientation risks getting it badly wrong. If you are unlucky you may then be unable to process the images and/or solve the structure (or at least have severe problems). > Indexing routines figure out the orientation of your crystal. After > integrating all reflections, the orientation is refined (depending on > the integration program you use). > > For anomalous data you may want to collect in inverse beam mode which > makes sure you collect Bijvoet pairs close in time and thus reduce the > effect of radiation damage. As drawback you risk possible systematic > errors in the Bijvoet pairs, but I am not sure this is a major > drawback for MX crystals. If you can adjust the orientation so that Bijvoet pairs are on the same image, this can help here. > > I recomend you take a look a Zbigniew Dauter's article > "Data-collection strategies", Acta Cryst D55 (1999) p. 1703-1717 > doi:10.1107/S0907444999008367 This is of course excellent advice. Regards, Peter. > > Best, > Tim > > > On 10/27/2012 07:58 AM, Vijayakumar.B wrote: > > Dear CCP4BB users, > > > > > > I have some basic questions in the data collection. Please give me > > some ideas to get clear in this part. > > > > > > 1) Why orientation of the crystal is importance? > > > > > > 2) If we mounted the crystal in arbitrary, what it leads? > > > > > > 3) How to find out crystal misseting angels in the data > > collection if we mounted arbitrary? > > > > > > 4) What should we make clear before collecting anomalous signal > > data ? > > > > > > Thanks in advance. > > > > > > With regards > > > > B. Vijay > > > > > -- Peter Keller Tel.: +44 (0)1223 353033 Global Phasing Ltd., Fax.: +44 (0)1223 366889 Sheraton House, Castle Park, Cambridge CB3 0AX United Kingdom
