[gmx-users] N-terminal hydrogen problem

[Anton Feenstra]

Paul Barrett wrote:
> Please can anyone help me see where I am going wrong!
> 
> -I have 2 proteins I want to simulate together.
> -either simulated by itself is fine
> -when I simulate the two proteins together the N-terminal of the second
> protein always gives this error: 
> 
> Fatal error: Atom H1 in residue VAL 299 not found in rtp database
>              while sorting atoms. Maybe different protonation state.
>              Remove this hydrogen or choose a different protonation
> state.
>              Option -ignh will ignore all hydrogens in the input.
> 
> Which is understandable because H1 doesnt exist in [VAL] in the .rtp.
> The problem is that this H1 comes from protonate which makes the
> n-terminal different to the .rtp.

Do I understand you correctly, that you first used protonate (Gromacs
tool) to add hydrogens, and subsequently used pdb2gmx? That may be one
source of problems; pdb2gmx is designed to add hydrogens according to
the definitions in the forcefield you choose (i.e. the corresponding
.rtp file).

> HOWEVER: the N-terminal of the same protein when simulated alone works
> fine. Also in the combined simulation the N-terminal MET of the other
> protein is fine, even though it contains H1, H2 and H3 which do not
> exist in [MET] in .rtp
> 
> Maybe Im failing to signal to GROMACS that the second protein starts at
> residue 299 so it is being intolerant...

Adding a 'TER' line in your pdb file doesn't help, unfortunately. This
is a shortcoming of pdb2gmx that should be fixed. In the meantime you
will need to add explicit chain identifiers, e.g. 'A' for your first
chain and 'B' for the second. Easiest is to use editconf, it has an
option to add a chain identifier. You can either first label the two
separate pdb files and then merge them, or label the merged file, and
then use an editor to replace the 'A' label with 'B' for the second
protein.

> The N terminals of the two proteins in the .gro file are included
> below.
> 
> Can anyone help me make GROMACS deal with the 2nd N-terminal in the same
> way that it deals with the 1st?



-- 
Groetjes,

Anton
  _____________ _______________________________________________________
|             |                                                       |
|  _   _  ___,| K. Anton Feenstra                                     |
| / \ / \'| | | Dept. of Pharmacochem. - Vrije Universiteit Amsterdam |
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|             | Feenstra at chem.vu.nl - www.chem.vu.nl/~feenstra/       |
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