Ab initio computational study of beta-cellobiose conformers using B3LYP/6-311++G**

Area of Science:

• Computational Chemistry
• Molecular Modeling
• Carbohydrate Chemistry

Background:

• Beta-cellobiose conformation is crucial for understanding its properties.
• Existing crystallographic and NMR data suggest specific dihedral angles for stable conformers.
• Previous computational studies have yielded conflicting results regarding beta-cellobiose stability.

Purpose of the Study:

• To investigate the molecular structure and energetics of beta-cellobiose conformers using high-level computational methods.
• To compare the stability of different beta-cellobiose conformations, including those not previously reported.
• To reconcile discrepancies between theoretical predictions and experimental observations of beta-cellobiose structure.

Main Methods:

• Gradient geometry optimization of 27 beta-cellobiose conformers in vacuo.
• Utilized B3LYP density functionals and the 6-311++G** basis set for calculations.
• Explored conformationally dependent geometry changes, energies, and hydrogen-bonding networks.

Main Results:

• Energetically favorable 'flipped' conformations with dihedral angles (phi, psi) ~ (180°, 0°) were identified, being ~2.5 kcal/mol more stable than 'experimentally accepted' forms.
• Vibrational free energy analysis also favored 'flipped' forms over those with experimentally observed dihedral angles.
• The 'normal' conformations exhibit a wider range of bridging dihedral angles due to phi dihedral variance, suggesting a flatter energy surface.

Conclusions:

• The study identifies novel, energetically superior 'flipped' conformations of beta-cellobiose.
• Discrepancies with experimental data may be explained by the influence of polar solvent effects on stabilizing 'normal' conformations.
• These high-level DFT/ab initio calculations provide new insights into beta-cellobiose molecular structure and stability.