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Dear Andy, You've mentioned that MR doesn't work in any of the space groups you've tried, but there are different scenarios that you might be able to recognise from where the MR fails. 1. You might have the wrong protein, e.g. crystallised from a contaminant. In that case, I would expect to see no significant signal in either the rotation or translation searches. 2. You might have the right protein, but there could be some pathology (including some relatively obscure things like reticular merohedral twinning, where twin-related reflections interleave and you incorrectly infer long cell dimensions). In this case, I would still expect to see a strong signal for the rotation search, given the nearly-perfect sequence identity, but then there would be no signal in the translation search (or alternatively an unrealistic number of strong translation peaks). 3. There might be some unexpectedly large conformational change, but you probably have a reasonable idea of what kinds of motions occur in your system. Good luck, and have a happy Christmas regardless! Randy Read > On 19 Dec 2016, at 08:43, Andrew Lovering <[log in to unmask]> wrote: > > Dear All, > > I have just collected data on a mutant of a protein that should be facile to solve by molrep (one residue/320 changed, approx 2Ang resolution) but is proving problematic. Data merging stats look good. > > The spacegroup is monoclinic, P21, the cell: > > a=39.47 b=157.36 c=74.9 beta=98.26 > > I spotted the relevant monoclinic twin laws on ccp4 twinning page and all relate multiples of axes a and c with one another (na +nc etc) but in the above case it would appear b~ = 4a > > There are other datasets, all index in this way, some hint at issues by indexing with the alternate a=74 b=157 c=79 (where a and c “swap” with a doubled, and thus our b=4a turns into b=2c) > > I would appreciate any advice on how to progress! Be nice to solve it pre-xmas..... > > Best & thanks in advance, > Andy ------ Randy J. Read Department of Haematology, University of Cambridge Cambridge Institute for Medical Research Tel: + 44 1223 336500 Wellcome Trust/MRC Building Fax: + 44 1223 336827 Hills Road E-mail: [log in to unmask] Cambridge CB2 0XY, U.K. www-structmed.cimr.cam.ac.uk