Dear Michele, Your 'pseudo-translation' is too close to an origin, it cannot be real. The problem is probably your model, it wasn't derived from an NMR structure by any chance? It would seem to me that your structure would be very suitable for ACHIMBOLDO.
Best wishes, George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-22582 On Mon, 25 Jan 2010, Michele Lunelli wrote: > Dear all, > > I am trying to solve a structure at 2.05 A resolution by molecular > replacement. The space group > seems to be P21, with unit cell dimension 52.63, 29.43, 104.970 and beta = > 95.60. > Only one copy of the protein should be present in the asymmetric unit, with > 58% of solvent content. > The search model used for MR is a truncated construct of the same protein, > comprising more that 60% > of the residues. However, no convincing MR solution is found (I used phaser, > molrep, epmr and also > mr.bump). No solutions refine to R and Rfree lower than 51-52%. > > The CCP4 documentation about twinning states that "Monoclinic with na + nc ~ > a or na + nc ~ c can be > twinned". This is not clear to me, but I have c = 2a, and therefore n = 2/3. > Nevertheless all the tests run by ctruncate (and sfcheck) exclude twinning. > The observed cumulative > distribution for |L| almost overlap the expected untwinned, and the observed > cumulative intensity > distribution is not sigmoidal at all (actually it is growing faster that the > theoretical). Also the > acentric and centric moments exclude twinning, for example the acentric: > <E> = 0.858 (Expected value = 0.886, Perfect Twin = 0.94) > <E**3> = 1.442 (Expected value = 1.329, Perfect Twin = 1.175) > <E**4> = 2.438 (Expected value = 2, Perfect Twin = 1.5) > > Both ctruncate and sfcheck found a pseudo-translation vector: > ctruncate (0.050, 0.000, 0.957), ratio 0.23 > sfcheck (0.954, 0.000, 0.040), ratio 0.218 > However a second copy cannot be present in the asymmetric unit (there would > be 16% of solvent > content). Since the protein is expected to form a coiled-coil, I think that > the detected > pseudo-translation arises from the helices. > Alternatively, it is possible that the space group is wrong? And if so, how > can I figure out the > correct one? > > > Thank you in advance, > Michele >
