Hello list, I received an answer concerning this topic but via the chimera menus which I tested in the first place and works fine. Actually I want to do this from the python shell, so here is my repost: ----------------------------------------------------------------------- I'd like to change the guide atom in res.ribbonData to use C instead of CA as an example. Here is what I did: for r in mol.residues: for a in r.atoms: if a.name == 'C': r.ribbonData.guide = a # Set C as guide Atom if r.isHelix: # Update the display r.ribbonDisplay = True r.ribbonDrawMode = chimera.Residue.Ribbon_Round This does not seem to redisplay/recompute the helix through C Atoms, the helix still goes through CA but not C atoms. I am not confident with the way I altered the guide Atom or maybe is it another problem like updating the ribbon or so... If someone can help... Thanks to all PL