|French, Alfred - Al|
Submitted to: Journal of Physical Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2002
Publication Date: 5/20/2002
Citation: French, A.D., Johnson, G.P., Kelterer, A., Dowd, M.K., Cramer, C.J. 2002. Quantum mechanics studies of the intrinsic comformation of trehalose. Journal of Physical Chemistry. 106(19):4988-4997.
Interpretive Summary: The carbohydrate molecule called trehalose is of substantial interest to several aspects of agriculture. It is implicated in resistance to drought and frost in plants, and it is the energy storage sugar for some insects. It is found in the honeydew that is the cause of sticky cotton that makes it difficult and expensive to spin cotton fiber into yarn. Understanding the reason that trehalose confers these properties requires an understanding of its molecular shape and its interactions with water. The previous theoretical studies of trehalose were based on mechanical models. Those methods gave very different results so that approach could not be trusted. Therefore, we used electronic structure theory, or quantum mechanics, a much more time-consuming method. It showed that it was not necessary for water to change the shape of the molecule to explain the experimental findings. The methods will be of interest for studying the shapes of other small carbohydrate molecules, so that their properties can be better understood. Therefore, the information is of particular value to scientists who are trying to understand the properties of carbohydrate materials.
Technical Abstract: To resolve a controversy over the intrinsic shape of the disaccharide alpha,alpha-trehalose and the magnitude of solvent effects, energy surfaces have been computed at different levels of molecular orbital and density functional theory. All quantum mechanical (QM) levels agreed that the gauche linkage conformations observed by experimental crystallographic and solution studies are 5-7 kcal/mol lower in energy than the trans shape. This is quite different than the findings of the two most recent classical force field studies. In those projects, the trans shape was preferred by 3.3 kcal/mol, and it was inferred that a strong solvent effect was responsible for the gauche experimental conformations. In the QM work, a strong solvent effect was not needed because the trans shapes already had higher energy. A QM continuum model of aqueous solvation had only small effects on the torsional energy surface. The best rationalization of 24 values of the linkage torsion angles from small-molecule crystal structure was provided by HF/6-311++G**//B3LYP/6-31G* theory, a level that under-predicts the strength of hydrogen bonds.