I would guess that coiled-coil structures might be difficult to solve by MR because of multiple false solutions in a repetitive structure.
There's a lot to be said for experimental phases Phil On 25 Jan 2010, at 17:50, 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
