right now I'm getting stuck on things like adding hydrogens (e.g. I would like a -COOH and not -COO group)
Hi, We may make some changes to make this easier in the future, but for now here is the procedure: (a) change the atom type of the oxygen to be protonated to O3. This could be done by Ctrl-clicking to select that oxygen and then using the command: setattr a idatmType O3 sel (b) add hydrogens with command "addh" or the AddH tool (c) if this is in a "standard residue" (amino acid ASP or GLU) and you are also going to assign charges in Chimera, you will also need to adjust the names of the atoms so they will receive the correct Amber partial charge values. If the residue is ASP, you will need to make sure that the sidechain oxygen without the hydrogen is named OD1, the one with the hydrogen is named OD2, and the hydrogen itself is named HD2. If GLU, it is analogous except the names contain E instead of D. Show atom labels for that residue to see the current names (e.g. to show atom labels for residue 9 in chain A, use command: lab :9.a). To rename an atom, you could select it and use a command something like: setattr a name OD1 sel Once the names are correct, charge assignment will give you the correct Amber values for residue ASH (protonated ASP) or GLH (protonated GLU). I hope this helps, ELaine ---------- Elaine C. Meng, Ph.D. UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab Department of Pharmaceutical Chemistry University of California, San Francisco