[chimerax-users] Align command; designating the positions to align

[Elaine Meng]

Hi Phil,
Your syntax is fine if you really meant to specify all atoms of the residues.  Most likely reasons for the error are:

(1) the residue types are not identical, so they contain different numbers of atoms (e.g. tryptophan has many more atoms than alanine)
(2) one structure or the other is missing some residues from the coordinates (e.g. a flexible loop was not resolved from density)
(3) one structure or the other has multiple chains so you are getting residues with those numbers from more than one chain (e.g. chain A has residues 11-323 but so does chain B)

Another possible problem is that there is no underscore in the "toAtoms" option keyword.  You can truncate it to "to" if you like.

<https://rbvi.ucsf.edu/chimerax/docs/user/commands/align.html>

Assuming it is the first one, the solution is to specify matching only backbone atoms, or only alpha-carbons.  Whichever you would like the RMSD to reflect. You would use an @ symbol to specify atom names.  So it may be something like

align #1:11-323 at n,ca,c,o to #2:24-336 at n,ca,c,o

...for all backbone atoms, or for alpha-carbons only:

align #1:11-323 at ca to #2:24-336 at ca

<https://rbvi.ucsf.edu/chimerax/docs/user/commands/atomspec.html>
 
I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Jul 18, 2022, at 2:22 PM, McClean, Phillip via ChimeraX-users <chimerax-users at cgl.ucsf.edu> wrote:
> 
> Hi,
> 
> I have been trying to properly code the align command.  I would like to use the command to determine the RMSD value between corresponding sections of two highly homologous proteins.  Here are the details:
> 
> Model 1 is to be aligned to Model 2; each model only has one chain
> Model 1 positions (residues=amino acids) 11-323
> Model 2 positions ( residues=amino acids) 24-336
>  
> When I pair each of the amino acids that are being aligned, everyone of the pairs has the same number of atoms.
> 
> Here is what I thought would work:
> 
> align #1:11-323 to_atoms #2:24-336
> 
> Here is the error message:
> 
> Unequal number of atoms to pair, 2483 to 2371
> 
> I am still trying to understand the nomenclature for understanding residues and atoms.
> 
> Thanks for any help you can provide.
> 
> Phil McClean