Sorry for the confusion. By tubulin monomer I mean an alpha-beta dimer. A tubulin trimer would be three tubulins of six chains A-B-A-B-A-B, each two belonging to a tubulin monomer.

My question is, if the first monomer has chains A and B, I want the copy/combine model to have different chain ID letters from A and B, for example C and D, and so on for the third, and fourth, etc.

How can I do that?

Regards.