Theoretical and electrochemical studies of host-guest inclusion complexes formed between L-Tryptophan with α- and β-cyclodextrins

Abstract

L-Tryptophan (L-Trp) is an essential amino acid supplement that plays a role in regulating the physiological functions of the body. Inclusion complexes of cyclodextrins (CDs) with organic biomolecules are widely used for biomedicine, pharmaceutical applications, and drug delivery. So, the formation of inclusion complexes of amino acid L-Trp with α- and β-CDs was theoretically investigated using AutoDock Vina 1.1.2, two-hybrid ONIOM method [ONIOM2 (B3LYP/321-G and PM6)], molecular dynamic simulation (MD), and electrochemically using cyclic voltammetry (CV). The free energy of binding values for α- and β-CDs/L-Trp inclusion complexes obtained using AutoDock Vina 1.1.2 and ONIOM2 are -4.6 & -5.5 kcal mol−1 and -684.34 & -685.09 kcal mol−1, respectively. From the binding energy values, the inclusion complexes of β-CD/L-Trp are more stable than α-CD/L-Trp. The MD simulations for 2 ns were further performed to calculate the hydrogen (H) bonding interaction process of the α- and β-CDs molecules with the L-Trp. Also, the electrochemical responses to the variations of α- and β-CDs concentrations with fixed L-Trp and vice versa were measured using CV. The formation of stable inclusions of L-Trp in α- and β-CDs causes significant changes in L-Trp redox properties. The binding constant values of α-CD/L-Trp (370 M-1) and β-CD/L-Trp (434 M-1) inclusion complexes at 298 K and thermodynamic parameters (∆G) were calculated from the above CV data using a Benesi–Hildebrand linear plot. The α- and β-CDs/L-Trp inclusion complexes binding energies from theoretical studies are in good agreement with the results obtained through experimental methods. These results clearly revealed the formation of inclusion complexes in which the guest molecule L-Trp, was tightly entrapped inside the cavity of the host β-CD than α-CD molecules.