Dear CCP4 community: Sorry if there is a duplicate post. I am a beginner to crystallography and therefore my apologies if this question is too simple.
Basically we obtained several crystal forms of the same molecule, which is a hetero- trimer containing protein A(18kD), protein B(16kD) and a RNA segment(40nt or about 15kD). We have solved the structure of one crystal form(form_1); its information is as follows: space group = P 42 22; unit cell = 126.514 126.514 76.766 90.00 90.00 90.00 resolution = 2.7A; Rwork = 0.25, Rfree = 0.265; solvent content = 60%; Number of molecules per Asymmetric Unit = 1; Data redundancy = 5; Data Completeness = 94%; I am now trying to solve the structure of form_2 crystal using molecular replacement. So far the information I know about form_2 is as follows: space group = I 422 or I 41 22; unit cell = 180.096 180.096 152.530 90.00 90.00 90.00 (unit cell is about 4 times the size of form_1) resolution = 3.3A; (which is low) Number of molecules per Asymmetric Unit = 2 or 3; Data redundancy = 4; Data Completeness = 92%; There is no twinning(as shown by Sfcheck); As shown in "Analyse Data for MR", the first peak is 100 and second is 68.92; I am not sure if this indicates translation in a Asymmetric Unit; The problem is, I can not get a good solution by MR using Phaser (both I422 and I4122 are tried). When I searched for 3 molecules per Asymmetric Unit, Phaser did not give solutions at all. When I used 2/ASU instead, I was able to get some solutions, with typical statistics as follows: RFZ=14, TFZ=35.9, PAK=0, LLG=1036; However, these solutions had high R values(like Rwork=0.59 and Rfree=0.58), which indicated that they are not solutions at all. Still, I tried refinement using refmac5, but R values did not go down even after 50 rounds; sometimes they even increased after refinement. Besides, the RMS values bond length, bond angle and chiral center were all 0 as show by refmac5. I tried limiting resolution range to 15-4A in Phaser, which did not help either. Now I am completely stuck. Could anyone give me some advice? I know this situation is very strange, because I am using the SAME molecule for MR but can not can a solution. Thanks a lot, P.S. 1) Both form_1 and form_2 crystals were grown using Selenomethionine-containing samples. There are 3 Sel_Met in protein A and 1 in protein B. 2) A 10-aa internal segment of protein B is missing in the solved structure, which may indicate high flexibility. Chao
