|French, Alfred - Al|
Submitted to: Carbohydrate Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2005
Publication Date: 4/5/2005
Citation: Lii, J., Chen, K., Johnson, G.P., French, A.D., Allinger, N.L. 2005. The External-Anomeric Torsional Effect. Carbohydrate Research. 340(5):853-862. Interpretive Summary: Because the various properties of cotton depend to a large extent on the shape of the cellulose molecule, it is important to know that molecule’s shape properties. One approach is to define the range of shapes and the most likely shape by computerized molecular modeling. The most practical approach to modeling is to use empirical force fields but it has been difficult to devise widely applicable force fields that also reproduce the known shape properties of molecules related to cellulose. The present paper makes an in-depth study of small molecules related to cellulose with electronic structure theory, also called quantum mechanics. This time-consuming analysis was based on fundamental constants such as the speed of light, electron charge and mass, and the assignment of electrons into atomic orbitals. Differences between this work and existing empirical studies of the same molecules showed that the empirical method was failing to account for a transfer of electrons that occurs when the small molecules take certain shapes. The empirical method was modified to account for this partial electron transfer, allowing it to account for the experimentally observed shapes. This work is primarily of interest to those studying various carbohydrate materials by molecular modeling and those who are developing software that incorporates empirical force fields.
Technical Abstract: The rotational barrier for a methyl group at the end of an anomeric system is sometimes lower than we might have anticipated. Thus, in the trans-trans conformation of dimethoxymethane, the barrier to methyl rotation is calculated (B3LYP/6-311++G(2d,2p)) to be 2.22 kcal/mol, just slightly smaller than the corresponding barrier to rotation of the methyl group in methyl propyl ether of 2.32 kcal/mol. However, if the methyl being rotated in dimethoxymethane is placed into a gauche conformation, that rotational barrier is reduced to 1.52 kcal/mol. This substantial (0.80 kcal/mol relative to methyl propyl ether) reduction in barrier height in the latter case is attributed mainly to the change in the bond order of the C-O bond to which the methyl is attached, as a function of conformation, which in turn is a result of the anomeric effect. We have called this barrier lowering the External-Anomeric Torsional Effect. This effect is apparently widespread in carbohydrates, and it results in the changing of conformational energies by up to about 2 kcal/mol. If polysaccharide potential surfaces are to be accurately mapped by molecular mechanics, this effect clearly needs to be accounted for.