Dear Professor, Thank you very much for suggestion.
With kind regards B.Vijayakumar On Mon, Oct 29, 2012 at 5:13 PM, Peter Keller <pkel...@globalphasing.com>wrote: > 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 > -- *B.Vijayakumar Research Scholar, CAS in Crystallography and Biophysics, University of Madras, Guindy campus,Chennai-25 INDIA Mobile: +919791929209*
