Hi Andy, There are two main possibilities: (1) With the "addaa" command you can build out from the C-term of a protein, residue by residue. <http://www.cgl.ucsf.edu/chimera/current/docs/UsersGuide/midas/addaa.html
(2) With the "Build Structure" tool you can create a peptide that is not attached to anything, then move it to a reasonable position and combine it with the existing protein. <http://www.cgl.ucsf.edu/chimera/current/docs/ContributedSoftware/editing/edi...
The moving/combining part of #2 can be rather difficult, so if it is a C-terminal truncation, definitely #1 is the way to go. For example, if the last residue in the structure is 309 in chain A of model #0, you could use commands like: addaa ala,310,alpha #0:309.a addaa lys,311,alpha #0:310.a addaa thr,312,alpha #0:311.a ... etc. ... If you need to extend the N-terminal end, however, approach #2 is the only choice. In Build Structure (under Tools... Structure Editing), go to the "Add Atoms" tab and choose the "peptide sequence" option. There you would just enter the sequence in one-letter amino acid codes, specify alpha-helix backbone angles, indicate the new atoms should be put in a "new model" and click Add. You don't want to put the helix in the same model as the protein because then you will not be able to move them separately. Then get the helix into position by activating/deactivating models and using the mouse (deactivating a model freezes it so that only the other models move with the mouse). Getting a reasonable position is nontrivial, unfortunately. Besides moving things interactively with the mouse, another possibility is to build enough atoms to overlap part of the protein structure and then match those overlapping parts, such as with the "match" command. Once the helix is in a reasonable position relative to the protein, you could combine the models into one model and then add a bond. If there were overlapping atoms, you would first need to delete the duplicate atoms. Perhaps you only need a rough representation, and in that case do not need to combine and bond the structures. See these previous posts for details on how to do the combining and bonding: <http://www.cgl.ucsf.edu/pipermail/chimera-users/2008-November/003255.html
<http://www.cgl.ucsf.edu/pipermail/chimera-users/2008-October/003238.html
I hope this helps, Elaine ----- Elaine C. Meng, Ph.D. meng@cgl.ucsf.edu UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab Department of Pharmaceutical Chemistry University of California, San Francisco http://www.cgl.ucsf.edu/home/meng/index.html On Nov 18, 2008, at 12:55 PM, Anindito Sen wrote:
Dear All I was wondering if there is any tool by which can generate an alpha helix, as an extension of a truncated crystal structure docked in the density map. The possibility of the truncated portion of the protein behaving as an alpha helix has been predicted by different groups. The density map shows some empty regions (after the crystal structure has been docked) just good enough for an alpha helix to fit in. Best Andy