The role of intra- and intermolecular hydrogen bonds in the formation of β-cyclodextrin head-to-head and head-to-tail dimers. The results of ab initio and semiempirical quantum-chemical calculations

Abstract

The conformation of a free β-cyclodextrin molecule optimized by the MNDO/PM3 quantum-chemical calculations has C₇ symmetry. The “right” orientation of the interglucose hydrogen bonds in β-cyclodextrin, in which the 2-OH groups act as the proton donors and the O atoms of the nearby 3"-OH groups function as the proton acceptors, is advantageous for thermodynamic reasons. The ring of seven H bonds thus formed stabilizes the symmetrical form of β-cyclodextrin. The β-cyclodextrin head-to-head dimer has D ₇ symmetry and consists of molecules whose 2-OH groups partcipate as proton donors in the formation of fourteen complementary intermolecular hydrogen bonds. The energy of H bonds in the β-cyclodextrin monomer and dimer was estimated to be 1.0--1.4 kcal mol^–1. Of the two possible β-cyclodextrin dimers, the “head-to-tail” dimer is more thermodynamically stable. The thermodynamic preference of the right orientation of the inter-glucose H bonds in β-cyclodextrin was confirmed by the MP2/6-31G(d,p)//6-31G(d,p) ab initio calculations for maltose (α-glucodioside). The maltose molecule with inter-glucose H bonds of the type 2-OH→O(3")-H is more stable than the structure with the H-(2)O←H-O(3") orientation of H bonds with a difference of ∼2.7 kcal mol^–1. According to the MNDO/PM3 method, the maltose structure with the right H bond orientation is more stable by ∼3.1 kcal mol^–1.