Submitted to: Carbohydrate Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/16/2003
Publication Date: 1/30/2004
Citation: Momany, F.A., Appell, M., Strati, G., Willett, J.L. 2004. B3LYP/6-311++G** study of monomhydrates of x - and B-D glucopyranose; hydrogen bonding, stress energies, and effect of hydration of internal coordinates. Carbohydrate Research. n. 339. p. 553-567. Interpretive Summary: Starch and cellulose are two of the most important biological molecules known, in addition to being the largest renewable bio-materials on the planet. Unfortunately, the 3-dimensional structure of starch granules has to date defied description at the atomic and molecular level, while cellulose has become somewhat better understood. In particular, we require an understanding of the electronic structural features of amylose, amylopectin, maltose, and their building blocks, i.e. glucose, since they are the basic components of starch and cellulose polymers. In this paper we present high level electronic structure results on mono-hydrates of a/B-D-glucose. These studies are carried out using powerful computer methods employed in our laboratory. With these computational tools, we can relate previous information from structural observations obtained by other researchers to details on the basic interactions between glucose and water molecules. This work has allowed us to better understand the flexibility and structural organization of components of solvated starch molecules, as well as more accurately define the role of the D-glucose residues in making up the amylose and amylopectin in a solvated starch granule. This work will lead to more efficient design methods for chemical modifications of starch that will result in biodegradable polymers with physical and structural properties useful for numerous commercial applications.
Technical Abstract: Twenty-six monohydrates of a- and B-glucopyranose were studied using gradient methods at the B3LYP/6-311++G** level of theory. Geometry optimization was carried our with the water molecules at different configurations around the glucose molecule. A new nomenclature for hydrated carbohydrates was developed to describe the water configurations. Zero point vibrational energy, enthalpy, entropy, and relative free energy were obtained using the harmonic approximation. Hydrogen bond energies for the monohydrates ranged from -5 to -12 kcal/mol and the average relative free energy is -5 kcal/mol. The 1-hydroxy position is the most energetically favored site for hydration and the region between the two and three positions is the next most favored site. A water molecule approaching a-D-glucose between the 1- and 2-hydroxy positions, pulls the 2-hydroxyl hydrogen atom away from the 1-hydroxy oxygen atom increasing the hydrogen bond length and also increasing the a-D-glucose energy. The increase in energy that occurs with a similar interaction on the B-anomer is much less effective since the hydrogen bond is much longer. Using the calculated free energies of all 26 configurations, the anomer population (a/B) increases in B-anomer population relative to the in vacuo case by -10% at the expense of the a-anomer, giving an (a/B) ratio of -50/50. This result arises from entroopy contributions favoring the B-anomer more than the a-anomer. From analysis of donor and acceptor hydrogen bond lengths, excellent correlation is found between the DFT calculated distances and those taken from carbohydrate structures in the Cambridge Crystallographic Data Bank.