Submitted to: Molecular Structure
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 27, 1999
Publication Date: N/A
Interpretive Summary: The understanding of molecular shapes and reactivity is important to a wide range of practical problems relating to the biosynthesis and utilization of agricultural molecules. The energy of a molecule is an important indicator of its stability and reactivity, and energy increases when the molecule is distorted. The experimental determination of the energy of formation of molecules of cellulose, starch and sugar, for example, is complicated by the fact that they occur as solids, and it is important to know the energy of the isolated molecule. The present work shows that the energy of some simple relatives of these molecules can be calculated by several theoretical methods and similar answers obtained, regardless of method. The energies of the simple relatives of sugar are considerably more stable than the simple relative of cellulose, the major molecule in plant cell walls, including cotton fiber. This is due to a special feature of the sugar molecule which has a longer sequence of alternating carbon and oxygen atoms than in the relative of cellulose. A similar relative of an imaginary molecule had even less stability because it did not have this alternating sequence of carbon and oxygen atoms. These specific conclusions apply only to the simple relatives, but are important to improving the ability to build useful computer models of these molecules. This information is of use to scientists trying to understand molecular structure and energies.
Technical Abstract: Heats of formation were calculated with the bond- and group-enthalpy method for analogs of 13 disaccharides to learn the relative stabilities for various configurations about the carbon atoms connected to the linkage oxygen atom. The analogs were based on tetrahydropyran and, in the case of the sucrose analogs, tetrahydrofuran. The molecular mechanics program MM3 calculates these values as an option. The method was also used with RHF/6-31G* and B3LYP/6-31G* quantum mechanics (QM) theory. Values for the isomers had 15-17 kcal (all energies herein are molar) ranges by the different methods, with the analogs of the three trehaloses and sucrose having the lowest values. Values for the isomeric analogs of non-reducing sugars with two anomeric centers are about -150 kcal, about 8 kcal lower than for the analogs of the reducing dimers (nigerose, maltose, laminarabiose, cellobiose and glucopyranosyl-galactose) by all three methods. The analogs of three non-glycosidic pseudo-disaccharides had heats of formation within 1.0 kcal by each of the three methods. They were about 8 kcal higher than for the molecules that contain one glycosidic sequence. The relative heats of formation by QM were similar to their corresponding relative electronic energies. Compared to the B3LYP results, RHF theory overestimated the stability of all molecules by about 2.85 kcal. The MM3 values were close to the B3LYP numbers, with the largest discrepancy for the cellobiose analog, 1.76 kcal. The stabilization, as embodied in the group enthalpy increment for anomeric centers, is another manifestation of the anomeric effect.