Quantumchemical calculations of the structural stability of β-Cyclodextrin/Dopamine and β-Cyclodextrin/Ascorbic Acid systems

A theoretical study of the complex formation and stability between beta-cyclodextrin/dopamine (beta CD/DA) and beta-cyclodextrin/ascorbic acid (beta CD/AA) was performed by means of quantum chemical calculations using Density Functional Theory (DFT). The B3LYP functional and the 6-31G (d) basis set were used for calculations under the so called all electrons scheme. The mechanistic considerations for interactions in both chemical systems were evaluated assuming entrance of either DA or AA into the ring of the guest molecule (beta CD). According to our results, the most preferred orientations of beta CD/DA system was obtained either when the DA catechol group or the amino group entered to the beta CD ring, while for the beta CD/AA system the most likely interaction occurred when AA functional groups, alcohol or lactone, entered to the beta CD ring. Geometry, molecular energy and some electronic properties were obtained from calculations that led to determine the conformer's structural stability. The effect of acidic media was studied by setting pH values lower than 4.1 in which amino group of DA is protonated and carboxylic group of AA keeps its proton bonded. Vibrational frequencies for each reactant molecule as well as for complex system were calculated to ensure that obtained values corresponded to structures with minimal energy. The calculated IR spectra of conformers were analyzed to characterize the interaction between the host-guest complexes by means of its coupled vibrational frequencies. Theoretical UV spectra were calculated to verify the maximum UV absorption corresponding to fully protonated DA (H(3)DA(+)) and AA (H(2)AA) species when located within the beta CD ring and is compared with reported experimental data. The zero point energy values and enthalpy and Gibb's free energy were calculated and discussed.