The Torsion Library: Semiautomated Improvement of Torsion Rules with SMARTScompare

The Torsion Library
is a collection of torsion motifs associated
with angle distributions, derived from crystallographic databases.
It is used in strain assessment, conformer generation, and geometry
optimization. A hierarchical structure of expert curated SMARTS defines
the chemical environments of rotatable bonds and associates these
with preferred angles. SMARTS can be very complex and full of implications,
which make them difficult to maintain manually. Recent developments
in automatically comparing SMARTS patterns can be applied to the Torsion
Library to ensure its correctness. We specifically discuss the implementation
and the limits of such a procedure in the context of torsion motifs
and show several examples of how the Torsion Library benefits from
this. All automated changes are validated manually and then shown
to have an effect on the angle distributions by correcting matching
behavior. The corrected Torsion Library itself is available including
both PDB as well as CSD histograms in the Supporting Information and
can be used to evaluate rotatable bonds at https://torsions.zbh.uni-hamburg.de.