[Chimera-users] Measuring SASA of ligand

Elaine Meng meng at cgl.ucsf.edu
Wed Nov 9 13:40:32 PST 2016

Hi George,
That earlier post is probably not that relevant to your situation.  I’ll try to outline what you might try.  First you would need to figure out command-line specifications that work on your trajectories, for:

(1) the ligand - sometimes “ligand” will work, but not always, as that might also include other stuff you don’t want like sulfate ions.  Just check with “sel ligand” and see if that is the only thing that gets selected.  Otherwise you might need to use its residue name or number, e.g. :HEM or :128

(2) the protein monomer in one trajectory - “protein” might work but you should check to make sure that it selects everything you want and nothing else.  If not, you may need to use a residue number range instead.

(3) the protein dimer in the other trajectory - again “protein” might work

Once you’ve figured out those things, let’s call them spec1, spec2, and spec3, you can try out the buriedArea command on some single frame of the trajectory.  For the first trajectory, command:

measure buriedArea spec1 spec2
… for example:
measure buriedArea ligand protein


… and for the second trajectory, measure buriedArea spec1 spec3.  If the command works (you see plausible results in Reply Log), then you could put the command in a per-frame script in MD Movie to have it run at every frame.  The results in the Reply Log will include the area of the two things together and each individually, as well as the resulting buried area, and two sets of results, one for SES (solvent-excluded surface) and one for SAS (solvent-accessible surface).  You may have used a per-frame script before for H-bond calculations.  


One problem is that sometimes the surface calculation may fail, especially during dynamics.  You may not be able to get results for every frame, so it is probably good to have the script echo frame number into the Reply Log so you can be sure about which results you are getting.  In other words, the per-frame command script could include a command like:

echo frame # <FRAME>

I hope this helps,
Elaine C. Meng, Ph.D.                       
UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab
Department of Pharmaceutical Chemistry
University of California, San Francisco

On Nov 9, 2016, at 12:10 PM, George Tzotzos <gtzotzos at me.com> wrote:

> I’m dealing with two separate trajectories involving:
> 1. Ligand A in complex with the monomeric subunit of a given protein A
> 2. the same ligand in complex with the dimer of protein A
> I would like to measure the buried (or solvent accessible) surface area of ligand A in both cases.
> I referred to a relevant posting in the list (http://www.cgl.ucsf.edu/pipermail/chimera-users/2016-June/012472.html) but I’m not sure how this could apply in the above case. 
> I’d be grateful for any suggestion on how to deal with this type of measurement.
> Many thanks in advance for any suggestions
> George
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