The Cucurbit[8]uril effect on the properties of Oroxin A

In this study, we investigated host–guest interactions between Oroxin A (OA) and cucurbit[8]uril (Q[8]) using H NMR, MS, UV-Vis and IR spectroscopy. The results showed that OA and Q[8] formed an inclusion compound (OA@Q[8]) with a molar ratio of 1:1 and a binding constant of 1.299 × 10 L·mol. In addition, the effect of Q[8] on the properties of OA was investigated through comparative experiments. The solubility of OA in water increased 22.47-fold when the concentration of Q[8] was 1 × 10 mol·L. Q[8] hardly affected the antioxidant capacity of OA, while the cumulative release of OA in gastric juice increased 2.3-fold after forming the inclusion compound with Q[8].


Introduction
Cucurbit[n]urils (Q[n]s) are a family of macrocyclic cage compounds synthesized by the condensation of glycoluril and formaldehyde in a strong acid solution [1][2][3]. As a consequence of the specific structural features of Q[n]s, which have two hydrophilic portals decorated with partially negatively charged carbonyl groups and a hydrophobic cavity [4], cucurbit[n]urils are able to form host-guest complexes with a range of drugs [5][6][7]. These complexes involve three main intermolecular forces: a hydrophobic effect, hydrogen bonding and ion-dipole interactions at the carbonyl portals [7][8][9]. The high thermal stability [10], ease of synthesis [11], general absence of cytotoxicity or toxicity [12,13] and their good molecular recognition and binding constants [14] have shown that Q[n]s are ideal drug carriers [15,16]. Moreover, Q[n]s can enhance physical stability [17,18] and increase solubility [19,20] of drug molecules after complexation.
Herein, we selected Q [8] as a host molecule and investigated its host-guest interactions with OA, as well as its effect on the properties of OA. Our results provide an approach and theoretical basis for the development and utilization of Oroxin A.
3 Figure 1 The molecular structures of OA and Q [8] Results and discussion

Host-guest interactions
The host-guest interaction can be effectively observed using 1 H NMR spectroscopy, and the mode of action of the cucurbit[n]uril-guest can be inferred from the chemical shift changes of the guest proton resonance peaks. 1 H NMR titration experiments were performed in D2O containing 10% DMSO by volume at room temperature. As shown in Figure 2, upon addition of Q [8], some of the peaks due to protons on the OA aglycone shifted to higher field, while peaks due to the glycosidic proton shifted to lower field. At the same time, the proton peaks on Q [8] were split, indicating that OA interacted with Q [8]. When the host-guest molar ratio was 1:1, all of the OA aglycone proton peaks moved to higher field, indicating entry into the cavity of Q [8]. Proton peaks due to the glycosidic -H and glucose were shifted to lower field, indicating that they were located at the portal of Q [8].  To further determine the host-guest ratio of the inclusion complex formed by Q [8] and OA, their interaction was investigated using UV-visible absorption spectroscopy via a molar ratio method and Job's method.

Phase-solubility
Phase solubility studies were conducted to investigate the solubility of OA in the presence of Q [8]. As can be seen from Figure 6, the solubility of OA in water is very poor (4.62 × 10 −6 mol·L −1 ). The solubility of OA increased linearly in water with addition of Q [8]. When the concentration of Q [8]    In artificial intestinal fluid (Figure 8(B)), the release rate of OA was faster than that of OA@Q [8]. After 12 h, the cumulative release of OA was 12.02%, while there was only 3.31% release of OA@Q [8].
8 Figure 8 The release curves of OA and OA@Q [8]

Conclusions
In summary, the experimental results showed that OA and Q [8]

Supporting Information
Supporting Information File 1: Title: Apparatus, Materials and methods.