Evaluating Intramolecular Hydrogen Bond Strengths in (1−4) Linked Disaccharides from Electron Density Relationships

The binding energy of intermolecular hydrogen bonds can be readily and accurately assessed using model pairs of functional groups; however, the energy of intramolecular hydrogen bonds as found in carbohydrates and folded proteins has not been computationally and experimentally accessible. We present a procedure to evaluate the intramolecular hydrogen bond strength from a set of correlation curves based on the electron density within the hydrogen bond. B3LYP 6-31G(d), 6-31+G(d,p), and 6-311++G(d,p) were used to optimize all combinations of functional group OH, NH 2, and NHCOCH 3 pairs. The intermolecular hydrogen bonds and the electron density in the hydrogen bond were measured using Bader's atoms in molecules method. A linear relationship between the binding energy/hydrogen bond strength (Δ E HB ) and a modified Grabowski complex parameter (Δ el+lap ) for the entire set of functional group pairs was established. Correlation curves (Δ E HB vs Δ el+lap ) for each individual pair of functional groups were then constructed by measuring the electron density (using Bader's atoms in molecules method) and the intermolecular binding energy as the hydrogen bond is compressed and extended. This Δ E HB vs Δ el+lap curve was then used to estimate the intramolecular cross glycosidic hydrogen bond strength of 1−4 linked disaccharides by calculating Δ el+lap for the minima conformation. On the basis of these data, the hydrogen bond strengths between functional groups located across the glycosidic linkage from each other were evaluated and then ranked from the strongest (viz., NHCOCH 3 (D)···NH 2 (A)) to the weakest (viz., NH 2 (D)···NH 2 (A)).