[gmx-developers] Fwd: Re: using replace/delete syntax of termini database for specbond

[Roland Schulz]

On Wed, Aug 25, 2010 at 10:18 AM, Pär Bjelkmar <bjelkmar at cbr.su.se> wrote:

>
> It's something similar to what CHARMM does. They use a lot of such special
> linkages to combine existing "residues" into new ones, all protonation
> states for example but also some much more complicated things like
> "building" the DPPC lipid from Palmitate (PALM) and "PCGL", using two
> linkages (EST1 and EST2), called patches in CHARMM.
>
> Yes. It is similar to the CHARMM patches. But I tried to modify how patches
work within CHARMM to a a format which matches the current syntax of
terminal databases and specbond, with the hope to make it easy to
implement.

> And this is also, I guess(?), why you, Roland, used the trick with the
> modified psfgen program (NAMD/VMD software) to build GROMACS topologies of
> your lignin and cellulose molecules: it could build the molecules modularly
> using the patches.
>
>  Yes. While cellulose is easy to describe with GROMACS topogies lignin is
more difficult. Lignin has 4 different linkages and can be branched. Thus a
residue can be modified at 2 or 3 sites by either one of the 4 linkages.
Thus the monomer can be modified in 4^3+4^2 different ways (a few less
because some are exclusive but other linkages also exist). Generating those
~80 versions of a monomer by hand is of course not very nice. Thus we
decided to use the modified psfgen. Now we would like to try virtual sites
with lignin and thus we look at this issue again.

> Regarding the CHARMM port: this modularity is why it's kind-of hard to port
> the CHARMM ff "completely".
>
> Would my proposed extension (using the tdb syntax similar to patches) help
with porting the parts of the CHARMM ff which remains to be ported?

Roland

>
> /Pär Bjelkmar
>
> Message: 1
>
> Date: Tue, 24 Aug 2010 19:44:07 -0400
>
> From: Roland Schulz <roland at utk.edu>
>
> Subject: [gmx-developers] using replace/delete syntax of termini
>
> database for specbond
>
> To: Discussion list for GROMACS development
>
> <gmx-developers at gromacs.org>
>
> Message-ID:
>
> <AANLkTim5=wX060wcv29XxuApPXdhEK8DLYnxg9dn23a_ at mail.gmail.com>
>
> Content-Type: text/plain; charset="iso-8859-1"
>
>
> Hi,
>
>
> it would be very nice if it would be possible to use the replace/delete/add
>
> features of the termini (tdb) files also in combination with specbond. This
>
> would make it simpler to specify some non-standard linkages.
>
>
> As an example: Instead of fully specifying the CYS2 it would be nice to be
>
> able to say:
>
> [ CYS2 ]
>
> [ replace ]
>
>   CB    opls_214    12.011 0.0975
>
>   SG    opls_203   32.06500 -0.2175
>
> [ delete ]
>
>   HG
>
>
> For this example the saving would be very limited. But with a large number
>
> of possible linkages this would give a considerable saving in specifying
> the
>
> topology for possible combinations of linkages. The residue used as basis
>
> (CYS) for the modified residue (CYS2) could come from the beginning of the
>
> name or could be specified explicitly. This modified residue could be used
>
> by a specbond line (which wouldn't change from the current line using
>
> CYS2). It seems this feature  could be implemented by a very limited patch
>
> because it would use only features already implemented for termini. But I
>
> don't understand the termini code so I can't judge this myself. Also I'm
> not
>
> sure whether it might be a problem with the charge groups.
>
>
> Would this be something easy and useful to add to pdb2gmx?
>
>
> Roland
>
>
> --
>
> ORNL/UT Center for Molecular Biophysics cmb.ornl.gov
>
> 865-241-1537, ORNL PO BOX 2008 MS6309
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-- 
ORNL/UT Center for Molecular Biophysics cmb.ornl.gov
865-241-1537, ORNL PO BOX 2008 MS6309
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