> The programs give me solutions with "reasonable" maps, but it is never > possible to refine to achieve Rvalues below 0.40.
This strongly suggests a space group issue. If the systematic absences are compelling, you should try to drop it down to P2 first. Look at your packing in the C222(1). If you change the direction of one of the coils relative to the other, will it break the orthorhombic symmetry and drop it to monoclinic? This may provide a clue to the space group you should try. James On Oct 17, 2011, at 11:09 AM, Napoleão Valadares wrote: > Hi there! > I got crystals from some synthetic peptides I bought, they are 30 residues > long and are supposed to form a coiled coil. I collected various data sets > (home source, Brookhaven and Diamond), including some at the resolution of > 1.65 A, for which the space group appears to be C222 or C2221. The unit cell > is small, 22.67, 88.06, 26.13, and the Matheus Coefficient indicates that's > there's only one helix in the asymmetric unit and a 25% solvent content. > > I have tried A LOT of Molecular Replacement using Phaser and Phenix > AutoMR. I'm using a 80% identity coiled coil helix as search model. The > programs give me solutions with "reasonable" maps, but it is never possible > to refine to achieve Rvalues below 0.40. Additionally, maps from different > solutions look reasonable, so I'm thinking these are all bias. > > I have 5 other synthetic 30 residues peptides (that crystallize in > different space groups and diffract to lower resolutions), including a > SelenoMethionine (SM) derivative (but it does not have enough anomalous > signal, ASU is too big, it is possible that the SM are disordered). I'm stuck > on this since March. > > Regarding the search model, I already tried trimming some or all side > chains and removing 2, 3 or 5 residues on each/both sides. I also tried other > search models. Maybe some "magic" combination of parameters on Phaser or > other programs can help me. > > What is your advice regarding how to proceed with MR? Is there some > program, procedure, parameter, pray or human sacrifice that could help me? > Thank you. > Regards, > Napo
