Spectroscopic and molecular simulation studies on the interaction of imazaquin herbicide with cucurbiturils (n = 6–8)

Imazaquin (IMQ) inclusion complexes with cucurbit[n]uril, CBn, (n = 6–8), have been investigated in both aqueous media and solid states. The aqueous solutions were examined and characterized by fluorescence spectroscopy, ¹HNMR, electrospray-ionization mass spectrometry (ESI-MS), and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The solid samples were analyzed by Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (PXRD). The predominant IMQ@CBn species was found to be 1:1 complexes, with stability constants 1134±140, 493±123, and 890±13 M⁻¹ for CB6, CB7, and CB8 complexes, respectively. A ternary complex of 2:1 guest to host stoichiometry was found to form between IMQ and CB8 with a stability constant of (1.88 × ±0.05)10⁷ M⁻¹. The formation of these complexes was further confirmed by the mass spectrometry results. The stabilities and mode of binding of these complexes in solutions were also studied with the molecular dynamic (MD) simulation methods. IMQ was found to form complexes with different hosts through the insertion of the quinolone moiety into the smaller host's nanocavities, whereas the larger CB8 cavity favors the accommodation of the imidazolinone group. Hydrogen bonding, van der Waals and hydrophobic interactions as well as CH—O interactions, are among the most important factors that stabilized these complexes.