Quantum chemical study of the host-guest inclusion complexes of the local anaesthetic drugs, procaine hydrochloride and butacaine hydrochloride, with α- and β-cyclodextrins

Quantum mechanical (QM) calculations were carried out in order to study the host-guest inclusion complexes of procaine hydrochloride (Pro-H) and butacaine hydrochloride (But-H) with α- and β-cyclodextrins (α- and β-CDs) by PM3 and AM1 methods. The systems were studied by a 1:1 (α-CD/Pro-H, α-CD/But-H, β-CD/Pro-H, and β-CD/But-H) stoichiometric ratio. In this work we calculated the energy of complex formation in vacuo, and this investigation was carried out on the basis of the host-guest approach. The stabilization energy results for the 1:1 host-guest inclusion complexes indicate that the β-CD/Pro-H complex is more stable than the α-CD/Pro-H complex. Furthermore, stabilization energy for the 1:1 inclusion complex of α-CD with But-H is lower than that for the 1:1 inclusion complex of β-CD with But-H. The calculation results show that all complexation processes for the four complexes are exothermic. Enthalpy changes for the α-CD/But-H and β-CD/Pro-H host-guest inclusion complexes are more negative than those for the other ones. ΔGo values for both the β-CD/Pro-H and α-CD/But-H complexes are negative.