Hi Fulvio, Yes, it can. An increase in cell parameters can indicate a failure of the cooling system or (more likely) radiation damage of the crystal.
For refinement you probably do not need all those 360 frames, unless you collected thin frames and your space group is P1 I suggest you integrate the two sets separately and check how well they merge - does the Rmeas rise a lot? Then it may be worth using only the first data set (unless your data are incomplete). Tim On Thu, May 19, 2011 at 04:16:22PM +0200, fulvio saccoccia wrote: > Ok, so another question in my mind: > the data come from a collection interrupted after the first 200 frames > and restarted after 1 hour to recover the missed 160 frames. > The first and second group of frames, if separately indexed, give cell > parameter values that differ of about 1A for axes and 1 degree in angle. > Can this affect thw wilson plot scaline and as consequence the B > factor? > > > Il giorno gio, 19/05/2011 alle 14.14 +0100, Ian Tickle ha scritto: > > Hi Fulvio > > > > There are 2 different issues here: the Wilson plot scale & B factor on > > the one hand and Wilson statistics on the other. The first are not > > affected by twinning since they depend only on the intensity averages > > in shells. The second refers to the distribution of intensities (i.e. > > the proportion of reflections with intensity less than a specified > > value) within a shell, or to the distribution of normalised > > intensities (Z = I/<I> ignoring symmetry issues for now) over the > > whole dataset. This distribution is different for a twin because > > averaging the components which contribute to the intensity of a > > twinned reflection tends to shift the distribution towards the mean, > > so you get fewer extreme values. > > > > The Wilson B factor is not a 'statistic' in the strict sense, merely a > > derived parameter. I suspect the low value you get has more to do > > with the fact that the resolution is only 3 A, than the fact it's > > twinned. > > > > See here for more mathematically-oriented info: > > > > http://www.ccp4.ac.uk/dist/html/pxmaths/bmg10.html > > > > Cheers > > > > -- Ian > > > > On Thu, May 19, 2011 at 1:45 PM, fulvio saccoccia > > <fulvio.saccoc...@uniroma1.it> wrote: > > > Dear ccp4 users, > > > I have a data set arising from a nearly-perfect pseudo-merohedrally > > > twinned cystal, diffracting up to 3 A. I solved the structure and ready > > > for deposition, but there is still a trouble. > > > The Wilson scaling from raw data gave a B of 3A^2. > > > Initially, I did not seemed too alarming. But I do not know why I have > > > these statistics. > > > > > > Does anyone know why Wilson scaling falls when treating that kind of > > > twinned data? I read that twinned data do not obey twe Wilson statistics > > > but I don't know why. > > > Here the presentation I read: > > > > > > http://bstr521.biostr.washington.edu/PDF/Twinning_2007.pdf > > > > > > Do you know any articles, reviews or book in which this particular > > > aspect of of twinned data is treated in depth, possibly in mathematical > > > manner? > > > > > > Thanks to all > > > > > > Fulvio Saccoccia, PhD student > > > Biochemical Sciences Dept. > > > Sapienza University of Rome > > > -- -- Tim Gruene Institut fuer anorganische Chemie Tammannstr. 4 D-37077 Goettingen phone: +49 (0)551 39 22149 GPG Key ID = A46BEE1A
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