The reason your code did not work is that "ribbonData" is recomputed whenever the molecule representation is changed (thereby wiping out your changes). The attribute that actually controls the atoms being used for ribbon representation is in "ribbonResidueClass" (the name made sense when I wrote the code :-). Attached is a script that changes the guide atom for all residues to "C". Please let me know if you have any questions or suggestions.

Conrad Hello Conrad, Thank you for your answer and the script, I'll try that right away. Thanks again Patrick