[gmx-users] Inconsistent charges in OPLS-AA residues ASPH and GLUH

[Oliver Stueker]

Hi,

while having a closer look at the topologies of aspartic- and glutamic-acid
in OPLS-AA, I noticed that the charges of the carboxylic oxygen atoms are
not consistent in the protonated forms of those residues:

from ffoplsaa.rtp of gromacs 4.0.7:
[ ASPH ]
[ atoms ]
     N    opls_238   -0.500     0
    H    opls_241    0.300     0
    CA    opls_224B   0.140     1
   HA    opls_140    0.060     1
    CB    opls_136   -0.120     2
  HB1    opls_140    0.060     2
   HB2    opls_140    0.060     2
   CG    opls_267    0.520     3
   *OD1*    opls_269   *-0.530*     3
  *OD2*    opls_268   *-0.440*     4
   HD2    opls_270    0.450     4
    C    opls_235    0.500     5
     O    opls_236   -0.500     5
[ ... ]
[ GLUH ]
[ atoms ]
     N    opls_238   -0.500     1
    H    opls_241    0.300     1
    CA    opls_224B   0.140     1
   HA    opls_140    0.060     1
    CB    opls_136   -0.120     2
  HB1    opls_140    0.060     2
   HB2    opls_140    0.060     2
   CG    opls_136   -0.120     3
   HG1    opls_140    0.060     3
  HG2    opls_140    0.060     3
    CD    opls_267    0.520     4
  *OE1*    opls_269   *-0.440*     4
   *OE2*    opls_268   *-0.530*     5
  HE2    opls_270    0.450     5
     C    opls_235    0.500     6
    O    opls_236   -0.500     6

in case of ASPH atom OD1 has a charge of -0.530 while it's equivalent in
GLUH (OE1) has a charge of -0.440 and vice versa for OD2 and OE2.

While fast browsing over the OPLS-AA papers, I couldn't find a listing of
the original charges to confirm which of the cases is the correct one.
For all other forcefields for gromacs that have topologies for the
protonated forms of aspartic- and glutamic-acid (gromos, ffencads and
charmm27.ff (in GMX 4.5) the charges of OD1 and OD2 in ASPH match to the
corresponding ones in GLUH.

I found this in the ffoplsaa.rtp of gromacs 4.0.7 and as far as I can see
it's currently the same in the branches release-4-0-patches and
release-4-5-patches.

Cheers,
Oliver
-- 
Oliver Stueker
NRC - National Institute for Nanotechnology /
University of Alberta, Canada
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