An ab Initio Study of the Geometry and Rotational Barrier of 4-Phenylimidazole

The molecular design of several synthetic artificial enzymes, which mimic the action of the serine protease alpha-chymotrypsin, incorporates the phenylimidazole molecular fragment to play the role of the His-57 residue in the native enzyme active site. Study of these artificial enzymes by molecular modeling techniques requires accurate torsional force field parameters for the phenylimidazole interring bond. This, in turn, requires accurate characterization of the barrier to rotation around this bond. Previous semiempirical calculations of this rotational barrier have neglected geometry optimization of the molecule at the points along the rotational pathway. The 4-phenylimidazole rotational barrier (5.6 kcal mol(-1)) presented here was obtained by full ab initio geometry optimization at the 3-21G level at each of the points along the rotational pathway.