Molecular mechanics study of the complexes of β-Cyclodextrin with 4-(dimethylamino)benzonitrile and benzonitrile

Molecular Mechanics calculations with the Tripos Force Field were employed to study the complexation of 4-(dimethylamino)benzonitrile (DMABN) and/or benzonitrile (BN) with β-cyclodextrin (βCD). The systems studied have 1 : 1 (DMABN : βCD and BN : βCD), 2 : 2 (DMABN : βCD) and 1 : 1: 2 (DMABN : BN : βCD) stoichiometries. Evidence for the formation of such complexes, binding constants and other thermodynamic parameters were extracted from the analysis of the steady state fluorescence measurements performed in a previous work. The Molecular Mechanics study, based on the energy changes upon guest-host approaching, was performed in vacuo and in the presence of water as a solvent. Results show that the driving forces for 1: 1 complexation are mainly dominated by non-bonded van der Waals host : guest interactions. However, the driving forces for association between 1 : 1 complexes to give 2 : 2 homo-or 1 : 1: 2 heterodimers are dominated by non-bonded electrostatic interactions. Head-to-head electrostatic interactions between βCDs, which are presumably due to the hydrogen bonding formation between secondary hydroxyl groups of CDs, are responsible for most of the stability of the dimers.