The Shape of Sucrose Molecules
French, A. D.; Johnson, G. P.; Kelterer, A.-M.; Cramer, C. J.
Proc. Sugar Process. Res. Conf., Sugar Processing Research Inst., New Orleans, LA: 2004, 417.
The shape properties of sucrose have many important ramifications. They are responsible for its sweet taste, for its crystallization behavior and for many of the intermolecular interactions that are unique to sucrose. The shapes of sucrose are conveniently described in terms of the extent of rotation of the glucose and fructose monomers about their bonds to the mutually held oxygen atom. The most definitive method for studying shape is diffraction crystallography. It precisely locates each of the atoms but it requires a crystal and in most cases gives only a single structure when the desired result is probabilities for the range of plausible structures. The problem of a limited number of structures has been overcome by finding crystals of very similar molecules and complexes, giving a wide range of observed structures. Another precise method is to calculate the energy of the different shapes to learn the shape with lowest energy. In principle, that structure is the most likely one, and structures with progressively higher energy are progressively less probable. Although this method can give a very precise answer, it may not be accurate. There are many ways to calculate the energy and they give different answers. Our poster presents a method that shows all of the crystallographically observed structures to have fairly low energy. This suggests that other shapes are unlikely to be observed in future experiments. The lowest energy occurs when hydrogen bonds connect the glucose and fructose rings, but many of the observed structures do not have such hydrogen bonds. This suggests that hydrogen bonds do not determine the structure but that they can form if the molecule otherwise has the correct shape.