Binding and Selectivity of Essential Amino Acid Guests to the Inverted Cucurbit[7]uril Host

Interactions between inverted cucurbit[7]uril (iQ[7]) and essential amino acids have been studied at pH = 7.0 by ¹H NMR spectroscopy, electronic absorption spectroscopy, isothermal titration calorimetry, and mass spectrometry. The interactions can be divided into three binding types at pH = 7.0. Experimental results from the present study showed that the host displays a strong binding to the aromatic amino acids, Trp and Phe, and the guests of Lys, Arg, and His lie outside the cavity portal of the host. Meanwhile, the alkyl moieties of the guests Met, Leu, and Ile were accommodated within the cavity of iQ[7], but there was no significant interaction between iQ[7] and Thr or Val. The complexation behavior of iQ[7] with essential amino acids was explored at pH = 3, and the binding of Lys, Arg, and His revealed an unexpected behavior, with their side chains located in the cavity of iQ[7], whereas those of the aromatic Trp and Phe were deeper within the iQ[7] cavity. The alkyl side chains of the guests Met, Leu, Ile, Thr, and Val were also located inside the iQ[7] cavity and formed the host-guest complexes.