[gmx-users] ACE patch for CHARMM27

[Krzysztof Kuczera]

  Dear Justin,
Thank you very much for your quick response, which put me on the right 
track.
Of course, I have some more questions and comments.

1. Putting residues in .rtp means adding ACE to the peptide sequence.
This is not the CHARMM convention, where all blocking groups are handled
through patches to existing residues. The advantage of the .rtp route is 
that this makes the treatment consistent with the other implemented 
force fields.

2. I believe the residues from the email need a little tweaking for 
consistency with CHARMM topology/parameters.

[ ACE ] needs an extra improper and a CMAP term:
[ impropers ]
+N CH3 CA +HN
[ cmap ]
C +N +CA +C

and [ NAC ] needs a CMAP term
[ cmap ]
-N  -CA  -C  N

3. Why won't the .n.tdb approach work?
The other reason I went with the .n.tdb file was that the amide (CT2) and
N-methylamide (CT3) C-terminals are already implemented in the .c.tdb
(CT3 is equivalent to the emailed NAC, with CMAP included). Belatedly,
I see that the authors of CT3 also had problems with missing bonds, and 
added
one in explicitly - I will try that and check my .top file for energy terms.

Thanks
Krzysztof

On 9/17/10 5:14 PM, Justin A. Lemkul wrote:
>
>
> Krzysztof Kuczera wrote:
>>  Hi GMXers,
>>
>> I am interested in simulating blocked peptides with CHARMM (and other
>> force fields), using gromacs-4.5, and would appreciate some help from
>> more experienced GROMACS users on details.
>>
>> To test things out I made a stab at creating an ACE N-terminal patch 
>> - this exists in OPLSAA, AMBER and CHARMM, so I added this to 
>> charmm27.ff/aminoacids.n.tdb:
>>
>> [ ACE ]
>> [ add ]
>>  1     2     CY   N    CA   C
>>        C    12.01100    0.51
>>  1     2     OY   CY   N    CA
>>        O    15.99900   -0.51
>>  1     1     CAY  CY   OY   N
>>        CT3  12.01100  -0.27
>>  3     4     HY   CAY  CY   N
>>        HA    1.00800   0.09
>> [ impropers ]
>>  CY CAY N OY
>>  N  CY  CA HN
>> [ cmap ]
>>  CY  N  CA  C  +N
>>
>>
>> Then built the top file with:
>> pdb2gmx -f -inter ala5_alpha.pdb -p ala5.top -o ala5.gro -ff charmm27 
>> -water tip3p -nochargegrp
>>
>> selecting ACE/CT2 as my terminal types
>>
>>
>> The ala5.gro geometry looks fine, but in the ala5.top file  there is 
>> no bond between atoms 2 and 7 (CY==OY in the patch), so the OY oxygen 
>> starts to drift off in EM and MD. All other bonds are generated.
>>
>> Can you tell me what is wrong here? Maybe somebody has already made a
>> patch that works?
>>
>
> Nowhere in the .tdb entry is such a bond specified, nor can it be.  
> The correct approach is to define .rtp entries, i.e.:
>
> http://lists.gromacs.org/pipermail/gmx-users/2010-July/052270.html
>
> -Justin
>
>> I attach the ala5_alpha.pdb file
>>
>> Krzysztof
>>
>


-- 
Krzysztof Kuczera
Departments of Chemistry and Molecular Biosciences
The University of Kansas
2010 Malott Hall
Lawrence, KS 66045
Tel: 785-864-5060 Fax: 785-864-5396 email: kkuczera at ku.edu
http://oolung.chem.ku.edu/~kuczera/home.html