Dear Bulletin Board, Prodded by pdb annotators, which are very hesitant to accept coordinate files when their Rfactor does not correspond with our Rfactor, I had a look again into some old data sets, which I suspect are twinned. Below are the results of some twinning tests with the Detwin program (top value: all reflections, lower value: reflections > Nsig*obs (whatever that may mean). The space group is P32, the resolution is 2.3 - 2.6 Å and data are reasonable complete: 95 - 100%.
>From the Detwin analysis, it seems that the crystals are twinned with twin >operator k,h,-l with a twinning fraction of 0.3 for crystal 1, 0.15 for >crystal 2 and 0.4 for crystal 3. Crystal 2 can be refined while ignoring >twinning to get acceptable but not stellar R and Rfree values. However, when I >try to detwin Fobs of e.g. crystal 1 (twinning fraction 0.3), R and Rfree >values stay about the same, whatever twinning fraction I try. At the time, I >used the CNS detwin_perfect protocol to detwin using Fcalcs, which brought the >Rfactors in acceptable range, but I do not feel that was the perfect solution. >Ignoring twinning on e.g. crystal 1 produces an Rfactor of 22% and an Rfree of >29% Do you have any idea what could be going on? Thank you for your help! Herman Crystal 1: operator -h,-k,l Suggests Twinning factor (0.5-H): 0.113 Suggests Twinning factor (0.5-H): 0.147 operator: k,h,-l Suggests Twinning factor (0.5-H): 0.277 Suggests Twinning factor (0.5-H): 0.323 operator -k,-h,-l Suggests Twinning factor (0.5-H): 0.101 Suggests Twinning factor (0.5-H): 0.134 Crystal 2: operator -h,-k,l Suggests Twinning factor (0.5-H): 0.077 Suggests Twinning factor (0.5-H): 0.108 operator: k,h,-l Suggests Twinning factor (0.5-H): 0.126 Suggests Twinning factor (0.5-H): 0.161 operator -k,-h,-l Suggests Twinning factor (0.5-H): 0.072 Suggests Twinning factor (0.5-H): 0.106 Crystal 3: operator -h,-k,l Suggests Twinning factor (0.5-H): 0.123 Suggests Twinning factor (0.5-H): 0.149 operator: k,h,-l Suggests Twinning factor (0.5-H): 0.393 Suggests Twinning factor (0.5-H): 0.433 operator -k,-h,-l Suggests Twinning factor (0.5-H): 0.110 Suggests Twinning factor (0.5-H): 0.133
