<lurk mode>
The answer I think is yes, but partially. Colin Nave published a very nice paper on this, "A description of Imperfections in Protein Crystals" in Acta Cryst D54, 848-853 (1998). The 'real' mosaicity is a convolution of the variation in cell dimensions within crystallites or domains making up a crystal (observed with X-ray topography), the misalignment of those domains (measured with very parallel beams and fine phi slicing) and finally the volume variation. The measured mosaicity is typically something else entirely as often, even for many cryocooled samples, the mosaicity of the crystal is masked by the contribution of the geometrical and spectral effects of the instrumentation and beam. This is especially the case on a lab source. </lurk mode> Cheers, Eddie Edward Snell Ph.D. Assistant Prof. Department of Structural Biology, SUNY Buffalo, Hauptman-Woodward Medical Research Institute 700 Ellicott Street, Buffalo, NY 14203-1102 Phone: (716) 898 8631 Fax: (716) 898 8660 Email: [EMAIL PROTECTED] Telepathy: 42.2 GHz Heisenberg was probably here! ________________________________ From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On Behalf Of Jacob Keller Sent: Tuesday, September 09, 2008 5:24 PM To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] Are cell parameter shifts real? In thinking about one or two responses, I am wondering whether mosaicity could be the result of having a distribution of each of the cell parameters within one crystal? Is that what mosaicity really is? That is, if axis a has a broad distribution of lengths, due to being caught in the act of changing to the lower-temp state, the spots will be streaky along a*, and so for the others, resulting in an overall "mosaicity" value which encompasses all of these distributions, being used mainly as a data-processing expedient? The actual physical phenomenon of mosaicity has always puzzled me, but perhaps this is an answer? Jacob ******************************************* Jacob Pearson Keller Northwestern University Medical Scientist Training Program Dallos Laboratory F. Searle 1-240 2240 Campus Drive Evanston IL 60208 lab: 847.491.2438 cel: 773.608.9185 email: [EMAIL PROTECTED] ******************************************* ----- Original Message ----- From: Jim Pflugrath <mailto:[EMAIL PROTECTED]> To: CCP4BB@JISCMAIL.AC.UK Sent: Tuesday, September 09, 2008 3:54 PM Subject: Re: [ccp4bb] Are cell parameter shifts real? How would you tell the difference between a unit cell shift and a wavelength shift when collecting diffraction data at a synchrotron beamline? Well, all the cell length would scale by the wavelength, so that would be one hint that the wavelength changed. If a got longer and c got shorter, then it would be less likely to be a wavelength shift. The crystal-to-detector distance can easily change if the crystal rotation device (i.e. goniometer) rotates the crystal without keeping it at the exact same crystal-to-detector distance. This could easily happen if the crystal is not centered at the rotation point or if more crystal volume rotated into the beam during your experiment. However in a typical experiment the crystal is not going to move by 5 mm and remain in the X-ray beam, so you would not expect your distance to change by 5 mm. Jim ________________________________ From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On Behalf Of Jacob Keller Sent: Tuesday, September 09, 2008 3:08 PM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] Are cell parameter shifts real? ... The bottom line questions are: 1. given that there really are cases of cell shifts, and that there are also probably experimental artifactual changes, how is one to decide what to do? 2. Can there be (or what is?) a plausible mechanism for these shifts? Jacob Keller
