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<title>MCCCS Towhee (MGM Stereochem Enforcer)</title>
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<div align="center"> <font size="5"> <b><font face="Arial, Helvetica, sans-serif"><a name="top"></a>MCCCS
Towhee (MGM Steroechemistry Enforcer)</font></b> </font> </div>
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<td width="697" valign="top"> <b>Overview</b>
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This section covers the MGM Stereochemistry Enforcer improper torsion as implemented into the
towhee_ff_mgmstereo file in the ForceFields directory. This is by no means a complete force field,
it only exists to enforce L or D (or if you prefer R or S) stereochemistry. This is needed because
configurational-bias is easily able to switch stereochemistry of a center during a configuartional-bias
regrowth. This is remedied by added this improper torsion onto the stereochemical center.
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<b>MGM Stereochemistry Enforcer in Towhee</b>
<ul>
There are no atom names for this force field, it is used in combination with another force field.
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<b>Coulombic interactions</b>
<ul>
There are no specific coulombic interactions for this force field as it is used with other force fields.
</ul>
<b>Improper torsions</b>
<ul>
The 'mgmstereo' improper torsion can be combined with other forcefields by listing an improper
torsion with a type of -1. This triggers a special flag in assemble.F to
look for the mgmstereo improper torsion instead of searching the atom types of the actual force
field for the molecule. In order to use this improper you need to define three improper torsions.
If you wanted to define an "L" stereocenter on an amino acid C<sub>alpha</sub> you would define
the following torsions from the C<sub>alpha</sub> atom. You would
list the actual atom number instead of the names, but this gives the idea.
<dt>N H<sub>alpha</sub> C<sub>beta</sub> -1</dt>
<dt>N C<sub>beta</sub> C(=O) -1</dt>
<dt>N C(=O) H<sub>alpha</sub> -1</dt>
<dt>In order to define a "D" stereocenter you simply reverse the atom orders.</dt>
<dt>N C<sub>beta</sub> H<sub>alpha</sub> -1</dt>
<dt>N C(=O) C<sub>beta</sub> -1</dt>
<dt>N H<sub>alpha</sub> C(=O) -1</dt>
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<b>Proteins</b>
<ul>
This is often used with a protein force field. See the improper torsion section for more information.
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<a href="../towhee_capabilities.html">Return to the Towhee Capabilities web page</a>
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<i><font size="2">Send comments to:</font></i> <font size="2">
<a href="mailto:marcus_martin@users.sourceforge.net">Marcus G. Martin</a><br>
<i>Last updated:</i>
<!-- #BeginDate format:Am1 -->September 07, 2005<!-- #EndDate -->
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