SEMIEMPIRICAL MOLECULAR-ORBITAL CALCULATIONS ON THE ROLE OF HYDROGEN-BONDING IN THE STRUCTURE OF BILIRUBIN DIANION

Semiempirical molecular orbital methods were utilized in the computation of a fully optimized structure of bilirubin dianion. The structure obtained from the theoretical calculations agreed well with the structure determined from X-ray crystallographic studies. The intramolecular hydrogen bond distance parameters (distance) corresponded with the experimentally determined distances, but the calculated values demonstrated interesting differences between the two hamiltonians used in the computations. Calculations based on the PM3 hamiltonian placed the hydrogens substantially closer to the acceptor oxygen than the corresponding calculations based on the AM1 hamiltonian. The heavy atoms forming the hydrogen bond were also much closer in the PM3 results. Conformational studies were carried out using the AM1 hamiltonian while rotating the central C9-C10 bond, which causes severe distortion to the ''ridge tile'' structure found in the experimentally determined structures. The hydrogen bonds exhibited remarkable tenacity during rotation of the C9-C10 bond, resisting breaking until the molecule has rotated to the point where the molecule will invert creating the other enantiomeric form.