Overlapping anomeric effects in a sucrose analogue

Area of Science:

• Carbohydrate Chemistry
• Computational Chemistry
• Structural Biology

Background:

• Molecular mechanics (MM3) calculations previously yielded high energies for sucrosyl geometries in oligosaccharides.
• Observed interresidue linkage conformations in disaccharides like maltose and cellobiose showed lower MM3 energy ranges (within 2.8 kcal/mol).
• Discrepancies suggest MM3 may overestimate energies for linkages between pyranose and furanose anomeric centers.

Purpose of the Study:

• To investigate the conformational energies and geometry of a sucrose analogue, tetrahydro-2-[(tetrahydro-2-furanyl)oxy]-2H-pyran.
• To compare the accuracy of ab initio (4-21G) calculations with MM3 for modeling carbohydrate structures.
• To determine if ab initio methods can better reproduce experimentally observed geometries and energy ranges.

Main Methods:

• Utilized ab initio calculations at the 4-21G level.
• Employed the molecular mechanics program MM3.
• Studied the conformational energies and geometry of a specific sucrose analogue.

Main Results:

• Ab initio 4-21G calculations reduced the energy range for observed analogue structures to 2.4 kcal/mol.
• New MM3 calculations increased the analogue's energy range to 7.5 kcal/mol.
• 4-21G calculations better reproduced observed variations in endocyclic C-O bond lengths compared to MM3.

Conclusions:

• Ab initio 4-21G calculations offer improved accuracy for conformational energies of sucrose analogues compared to MM3.
• The 4-21G method provides a more reliable representation of experimentally observed geometries, including bond lengths.
• These findings highlight the importance of selecting appropriate computational methods for accurate carbohydrate modeling.