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Intramolecular Hydrogen Bonding and Cooperative Interactions in Carbohydrates via the Molecular Tailoring Approach

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journal contribution
posted on 17.01.2008, 00:00 by Milind M. Deshmukh, Libero J. Bartolotti, Shridhar R. Gadre
In spite of many theoretical and experimental attempts for understanding intramolecular hydrogen bonding (H-bonding) in carbohydrates, a direct quantification of individual intramolecular H-bond energies and the cooperativity among the H-bonded networks has not been reported in the literature. The present work attempts, for the first time, a direct estimation of individual intramolecular O−H···O interaction energies in sugar molecules using the recently developed molecular tailoring approach (MTA). The estimated H-bond energies are in the range of 1.2−4.1 kcal mol-1. It is seen that the OH···O equatorial−equatorial interaction energies lie between 1.8 and 2.5 kcal mol-1, with axial−equatorial ones being stronger (2.0−3.5 kcal mol-1). The strongest bonds are nonvicinal axial−axial H-bonds (3.0−4.1 kcal mol-1). This trend in H-bond energies is in agreement with the earlier reports based on the water−water H-bond angle, solvent-accessible surface area (SASA), and 1H NMR analysis. The contribution to the H-bond energy from the cooperativity is also estimated using MTA. This contribution is seen to be typically between 0.1 and 0.6 kcal mol-1 when H-bonds are a part of a relatively weak equatorial−equatorial H-bond network and is much higher (0.5−1.1 kcal mol-1) when H-bonds participate in an axial−axial H-bond network.