On Mon, Nov 21, 2011 at 10:23 AM, Eleanor Dodson <c...@ysbl.york.ac.uk> wrote: > Just a plea for less molecular replacement. > > If you get a new crystal of a known protein with the same cell dimension as > youur old crystal, the most likely scenario is that it has the same group, > and you really should not try MR - use the previous solution as input to do > rigid body refinement, and then > a) the R factor will tell you if this is a reasonable hypothesis (it > usually is..) and > b) you dont have this awful problem of not being able to compare the > solutions.. > > Eleanor
But sometimes (or actually we find quite often) the crystal after soaking & freezing is sufficiently non-isomorphous (we sometimes see up tp 10% changes in cell parameters) that RBR just doesn't work, and then you have to fall back on MR. The solution in that case to avoid problems of generating symmetry-related and/or origin-shifted molecules is to do a limited MR search, e.g. rotating/translating each molecule in the a.u. independently by up to 5 deg & 5 Ang. from the model (which of course has 100% similarity making it a lot easier). Furthermore, because the number of points sampled is much less one can afford to do the more accurate full 6-D search, as opposed to the usual 3-D RF followed by 3-D TF on each RF solution. So now we do this routinely (we don't even bother with a preliminary RBR). I believe the limited 6-D search can be done with Phaser. Cheers -- Ian
