Hi, It is not obvious to me if there is a simple way to do this in Chimera. Given a PDB file containing a number of chains of identical molecules arranged into a helix, but without exact helical symmetry, is there some way to measure the rotation between adjacent chains? I imagine that I can use MatchMaker, and then convert the resulting rotation matrix to the angle about the helical axis... Regards, Ed

Hi Ed, If PDB models #1 and #2 are fit to a filament map you can use Chimera command measure rotation #1 #2 which will produce output in the Reply Log (menu Favorites / Reply Log) like Position of 1mwk (#2) relative to 1mwk (#1) coordinates: Matrix rotation and translation 0.89907669 -0.42653054 -0.09865501 -14.80260678 0.38452015 0.87709721 -0.28782762 9.59677669 0.20929731 0.22084426 0.95258724 -47.13790770 Axis 0.50580078 -0.30621414 0.80647286 Axis point 19.41814624 18.14474471 -0.00000000 Rotation angle (degrees) 30.18795070 Shift along axis -48.44128198 This specifies the rotation in PDB #1 coordinates, not the coordinates of the map these two molecules have been fit into. It is described as a 3x3 rotation matrix and a translation (4th column of above matrix). And it is equivalently described as an axis, point on the axis, rotation angle and shift along the axis. The command will also show in the graphics window the axis of rotation for this latter specification. Unfortunately there is not an option to express this rotation in the map coordinate system which is probably what you are interested in. Here's a way to do that though. Fit the first molecule in the map. Then write out that PDB file using the map coordinates using Chimera menu File / Save PDB... with the "Relative to model mymap.mrc" option enabled. Now close that fit molecule and open the saved PDB. It has the same coordinate system as the map. Open a second copy of that PDB and fit it in another position in the map. Then use the "measure rotation #1 #2" command (can be shortened to "me rot #1 #2"). I'll see if I can easily add an option to the measure rotation command to report the result in the map coordinate system. Tom