Anomeric effect energy functions

When two functional groups are borne by the same atom, they interact to cause either stabilization or destabilization. This is the geminal effect.20 If the substituents are both strong o-acceptors and n-donors [for example a gem-dimethoxy-C(OMe)2-], a stabilization occurs.21 This is easy to understand. Oxygen, being electronegative, is a o-acceptor and o co is low in energy and is 71-donor thanks to its lone pair. Therefore, we have here a double anomeric effect ... 

Furthermore, a questionable approximation of the same entropy and volume of a and e isomers is usually assumed. Obviously, the energy of the anomeric effect, AE(AE2), depends on the quality of the method used for determination of AEpp. The extra function AE(AE2) may differ, depending on whether AG or AE° has to be matched. Furthermore, if Eq. 3 applies for an equilibrium in a solvent, the extra term AE(AE2) also includes a contribution of the solvent effect due to its neglect or an incomplete representation in energy AEpp. 

To reproduce an exo-anomeric effect, separate energy functions, Eqs. [3] and [4], were incorporated for a- and p-anomers. The parameters for these equations were derived by fitting to the rotational energy curves for dimethox-ymethane computed with nonoptimized geometries at the HF/4-31G ab initio level.Dimethoxymethane is the simplest chemical model system for a gly-cosidic linkage. 

In order to reproduce an exo-anomeric effect, separate rotational energy functions were incorporated for a- and anomers (equations 4 and 5). ... 

The CFF-PEF force field for carbohydrates proposed by Rasmussen has undergone several developmental refinements leading to numerous parameter sets. The emphasis of the PEF development has been to employ as few parameters and as simple energy expressions as possible. As such, torsion terms were kept to a minimum and no explicit term for the exo-anomeric effect was introduced. The consistent force field (CFF) developed by Lifson and Warshel was chosen as the original framework for the PEF development. The CFF-PEF functional form is similar to that of most macromolecular force fields (equation 9). ... 

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