AW: [ccp4bb] truncate ignoranceIn 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 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
