Dear Friends, I have some crystals of a small RNA in sg R32 that exhibit some bizzare behaviors and fail to give phasing solutions. I am hoping someone out there might be able to lend some insight. All crystals come out of the same condition and look the same morphologically but, for reasons unknown and at this point uncontrollable, have very different cell dimensions:
Xtal 1: R32 77 x 77 x 80 Ang Xtal 2: R32 77 x 77 x 86 Ang Xtal 3: R32 78 x 78 x 366 Ang Xtal 4: R32 78 x 78 x 460 Ang Xtal 5: P3(1)21 77 x 77 x 85 Ang I've collect >95% complete datasets for each crystal form to resolutions between 2.2-3.0 Ang. Denzo/scalepack outputs look fine in each case. Overall R-sym's are okay (range is 8-13%). The only apparent red flag is the 2nd moment of I calculated in truncate. These values are much larger than those expected for either twinned (1.5) or untwinned (2.0) data: Xtal 1: 3.2 Xtal 2: 2.5 Xtal 3: 4.9 Xtal 4: 3.7 Xtal 5: 4.2 Also, use of the Yeates server to look for partial twinning tells me there are no twin laws for R32, while the P3(1)21 data does not seem to be twinned. I'm guessing crystals 1 & 2 are very similar but still somewhat non-isomorphous, and crystal 5 is also very similar with a breakdown in space group symmetry that would put 3 mols/ASU instead of 1 mol/ASU as in R32. But I do not understand what is going on with crystals 3 & 4. Has anyone out there experienced similar non-integer multiples of a cell dimension for a given crystal? I have covalently attached iodine in some of these crystal forms. But no luck finding any sites by SAD, and I can't help but wonder if this funny c-dimension behavior and/or the high <I^2>/<I>^2 values are indicative of some greater crystal pathology. Thanks in advance, Dan
