Submitted to: Carbohydrate Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/4/2004
Publication Date: 1/13/2005
Citation: Appell, M.D., Willett, J.L., Momany, F.A. 2005. Dft study of alpha- and beta-d-mannopyranose at the b3lyp/6-311++g** level. Carbohydrate Research. 340:459-468. Interpretive Summary: Carbohydrates are sources of energy, structural materials, and essential to bioactivity. Understanding of molecular structural features and energetics of carbohydrates is of primary importance in identifying the potential utilization of these abundant natural, renewable resources. Structural modifications of carbohydrates have resulted in modifications of carbohydrate properties. For example, starch cellulose are carbohydrate polysaccharides with differing properties related to the linkage of their glucose components. Mannose and glucose are both carbohydrate monosaccharides but differ in configuration of an important substituent, and these structural differences give them different physical properties. Our research was carried out using fast, parallel computers to investigate the electronic structure of mannose. This work has allowed us to compare the electronic structures of mannose to glucose. We identified important structural relationships unique to mannose. This work provides a more detailed understanding of carbohydrate interactions, and provides information to accelerate the design of useful biodegradable polymers.
Technical Abstract: Thirty-five conformations of a- and B-D-mannopyranose, the C-2 substituted epimer of glucopyranose, were geometry optimized using the density functional (B3LYP) and basis set (6-311++G**). Full geometry optimization was performed on the hydroxymethyl rotamers (gg/gt/tg) and an analytical hessian program was used to calculate the harmonic vibrational frequencies, zero point energy, enthalpy, and entropy. The lowest energy conformation investigated is the B-tg in the 4C1 chair conformation. The in vacuo calculations showed little energetic preference for either the a- and B-anomer for mannopyranose in the 4C1 chair conformation. Results are compared to similar glucopyranose calculations in vacuo where the a-anomer is less than 1 kcal/mol lower in electronic energy than the B-anomer. In the case of the generally higher energy 1C4 chair conformations, one low energy, low entropy B-gg-1C4 chair conformation was identified that is within less than 1.5 kcal/mol of the lowest energy 4C1 conformation of mannopyranose. Other 1C4 chair conformations in our investigation are less than 6-10 kcal/mol higher in overall energy. Many of the 3.0B, B3,0, 1,4B, and B1,4 boat forms passed through transitions without barriers to 1S3, 5S1, 1S5 skew forms with energies between less than 3.5-8.5 kcal/mol higher in energy than the lowest energy conformation of mannopyranose. Boat forms were found that remained stable upon gradient optimization. As with glucopyranose, the orientation and interaction of the hydroxyl groups make a significant contribution to the conformation/energy relationship in vacuo.