Molecular electrostatic potentials and hydrogen bonding in alpha-, beta-, and gamma-cyclodextrins.
Cyclodextrins (CDs) are cyclic oligomers of glucose having the toroid of sugars elaborating a central cavity of varying size depending on the number of glucoses. The central hydrophobic cavity of CD shows a binding affinity toward different guest molecules, which include small substituted benzenes to long chain surfactant molecules leading to a variety of inclusion complexes when the size and shape complementarity of host and guest are compatible. Further, interaction of guest molecules with the outer surface of alpha-CD has also been observed. Primarily it is the electrostatic interactions that essentially constitute a driving force for the formation of inclusion complexes. To gain insights for these interactions, the electronic structure and the molecular electrostatic potentials in alpha-, beta-, and gamma-CDs are derived using the hybrid density functional theory employing the three-parameter exchange correlation functional due to Becke, Lee, Yang, and Parr (B3LYP). The present work demonstrates how the topography of the molecular electrostatic potential (MESP) provides a measure of the cavity dimensions and understanding of the hydrogen-bonded interactions involving primary and secondary hydroxyl groups. In alpha-CD, hydrogen-bonded interactions between primary -OH groups engender a "cone-like" structure, while in beta- or gamma-CD the interactions from the primary -OH with ether oxygen in glucose ring facilitates a "barrel-like" structure. Further, the strength of hydrogen-bonded interactions of primary -OH groups follows the rank order alpha-CD > beta-CD > gamma-CD, while the secondary hydrogen-bonded interactions exhibit a reverse trend. Thus weak hydrogen-bonded interactions prevalent in gamma-CD manifest in shallow MESP minima near hydroxyl oxygens compared to those in alpha- or beta-CD. Furthermore, electrostatic potential topography reveals that the guest molecule tends to penetrate inside the cavity forming the inclusion complex in beta- or gamma-CD.
Pinjari
Rahul V
RV
Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India.
Joshi
Kaustubh A
KA
Gejji
Shridhar P
SP
D003198:Computer Simulation; D005947:Glucose / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D055672:Static Electricity; D047391:alpha-Cyclodextrins / Q000737:chemistry*; D047392:beta-Cyclodextrins / Q000737:chemistry*; D047408:gamma-Cyclodextrins / Q000737:chemistry*
D047391:alpha-Cyclodextrins; D047392:beta-Cyclodextrins; D047408:gamma-Cyclodextrins; D005947:Glucose