Submitted to: Journal of Molecular Structure (Theochem)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 13, 2009
Publication Date: March 24, 2009
Citation: Bosma, W.B., Schnupf, U., Willett, J.L., Momany, F.A. 2009. Density Functional Study of the Infrared Spectrum of Glucose and Glucose Monohydrates in the OH Stretch Region. Journal of Molecular Structure (Theochem). 905(1):59-69. Interpretive Summary: A thorough understanding of the interactions between carbohydrate molecules and water is essential for the design of new carbohydrate-based commercial products. This understanding is often hindered by the available experimental data and the large number of ways that the carbohydrate molecule can link with the water molecules in its solvent environment. Even glucose, the monosaccharide building block of starch and cellulose, is a complicated system in terms of the number of possible molecular conformations and arrangements of surrounding water molecules. In this paper, advanced theoretical methods were employed to study the relative energies of the glucose molecule interacting with one water molecule. These studies offer considerable insight into the differences between isolated glucose molecules and glucose in solution. In addition, a type of experiment that is used to characterize carbohydrate structures (infrared spectroscopy) was modeled on these systems, to identify the experimental “fingerprints” of individual glucose-water complexes. Comparison was made to some recent experiments that study systems very similar to these in the gas phase. The theoretical calculations were used to provide a detailed picture of the structure of the complexes that were studied in the experiment. 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: Density functional theory (DFT) has been used to calculate the structures and infrared spectra of glucose and glucose monohydrates. Both the alpha and beta anomers were studied, with all possible combinations of hydroxymethyl rotamer (gg, gt, or tg) and hydroxyl orientation (clockwise or counter-clockwise). A total of 69 glucose monohydrates were studied, representing most of the possible single-donor, single-acceptor complexes. Two monohydrates stand out as being particularly stable relative to the others; these correspond to complexes that require little distortion of the glucose structure in order to accommodate the water molecule. While the alpha anomer is still more stable than the beta anomer at the monohydrate level, the current calculations offer insights into the preference for the beta anomer in solution. The calculated infrared spectrum was studied in the OH stretch region (3300-3800 cm-1). One of the peaks in the spectra produced by the tg rotamers was found to be redshifted by approximately 30 cm-1 relative to its location in the gt and gg rotamers. A second signature redshift (also approximately 30 cm-1) was found to characterize the alpha glucose anomers. In the monohydrates, the water OH stretching motions couple to the glucose vibrations at the hydrogen-bond donor site, giving redshifted peaks characteristic of the glucose-water hydrogen bonds. The extent to which these peaks are conformation-dependent depends strongly on the location of the water molecule. Comparison is made to recent experiments on the OH stretch region of gas-phase monohydrates of glucose derivatives.