The conformational preferences of the threo and erythro diastereomeric forms of a guaiacyl β-O-4
dimer have been investigated by molecular modeling using the CHARMM force field. Many low
energy conformations have been identified for each diastereomer, showing that β-O-4 dimers can
adopt a large variety of shapes. A consistent structural model has emerged that indicates different
conformational behavior for the threo and erythro forms, corresponding to a preferential extended
overall shape for the threo form. All the low energy conformers are stabilized by intramolecular
H-bonds. In particular, the highly directional H-bond between the α or γ hydroxyl hydrogen and
the aromatic methoxy oxygen governs the B aromatic ring orientation. However, it has appeared
that the conformational preferences are predominantly governed by local steric interactions
rather than by differences in the H-bonding pattern. From the satisfactory agreement between
computed data (geometries and the Boltzmann distribution of the low energy conformers) and the
experimental data reported in the literature (X-ray crystal structures and 3JHαHβ NMR coupling
constant), the CHARMM force field has been validated for the study of β-O-4 structures. Clearly,
the molecular modeling calculations have led to an improved rationalization of the conformational
data collected by experimental techniques.
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