Hi Ian,
Dock Prep now rebuilds  truncated sidechains using data from rotamer libraries (Dunbrack or Richardson):

http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/dockprep/dockprep.html

It automatically locates the truncated sidechains, retrieves rotamer information, and chooses the best rotamer based on clashes, H-bonds, and (by default) backbone conformation.  This may take a while... it sounds like your system is pretty large. It will not add in missing backbone segments or entire residues, however, only sidechains. 

Dock Prep was changed after the last production release (Nov 2007), so I recommend getting a daily build:
http://www.cgl.ucsf.edu/chimera/alpha-downloads.html

Best,
Elaine
-----
Elaine C. Meng, Ph.D.                          meng@cgl.ucsf.edu
UCSF Computer Graphics Lab and Babbitt Lab
Department of Pharmaceutical Chemistry
University of California, San Francisco
                     http://www.cgl.ucsf.edu/home/meng/index.html

On May 1, 2008, at 10:08 PM, Ian Yarbrough wrote:

Hi,
A model that I am working with has several missing or partial side chains, and I'm dealing with about 80 subunits of this protein (organized into 3 chains).  I am trying to repair these, but don't want to go through the task of finding every broken chain.  Is there a way to get chimera to locate the broken ones and repair them all?  I've seen the dock prep feature, but don't want to change them all to alanines.
Thanks for you help.

Ian Yarbrough
Texas A&M University