| Popis: |
The MNDO and AM1 molecular orbital methods and the molecular mechanics MMP method have been applied to the E and A conform-ers of the 3-OH flavanoids, (+)-catechin, (−)-epicatechin, 3-O-acetyl-(+)-catechin, and 3-O-acetyl-(−)-epicatechin. The gradient norms for MNDO and AM1 were reduced in the precise mode. The MNDO method best represents the relative trend in energies for catechin and epicatechin that allows estimation of the equilibrium ratio of equatorial to axial conformers as determined by 1H-NMR However, the AMI method generates the most reasonable molecular structures especially in terms of optimizing the pyran ring torsional angles. The AMI method shows a dependency of the energy on the pyran ring C3-O2-H rotational angle that is not detected with MNDO. AMI also predicts a favored structure for the A conformer of (+)-catechin and the E conformer of (−)-epicatechin with the pyran 3-OH rotated into the pyran ring, oriented toward the pyran ring oxygen atom. The AMI method predicts that 3-O-acetyl-(−)-epicatechin structure is slightly weighted toward the axial conformer. The relative energies of the E and A conformers for 3-O-acetyl-(+)-catechin are nearly the same. These relative energies and torsional angles are compared to MMP and literature MM2 findings. |
