We report here NMR and Isothermal Titration Calorimetry studies of the binding of ionisable guests (carboxylate acids) to a deep-cavity cavitand. These studies reveal that the shortest guests favoured 1:1 complex formation, but the longer the alkyl chain the more the 2:1 host-guest capsule is favoured. For intermediate-sized guests, the equilibrium between these two states is controlled by pH; at low values the capsule containing the carboxylic acid guest is favoured, whereas as the pH is raised deprotonation of the guest favours the 1:1 complex. Interestingly, for one host–guest pair the energy required to decap the 2:1 capsular complex and form the 1:1 complex is sufficient to shift the pKa of the guest by ~3–4 orders of magnitude (4.1–5.4 kcal mol−1). The two largest guests examined form stable 2:1 capsules, with in both cases the guest adopting a relatively high energy J-shaped motif. Furthermore, these 2:1 complexes are sufficiently stable that at high pH guest deprotonation occurs without decapping of the capsule.

中文翻译：

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水溶性空腔及其二聚体胶囊中脂肪酸包封的 ITC 和 NMR 分析

我们在这里报告了可电离客体（羧酸）与深腔空腔结合的核磁共振和等温滴定量热研究。这些研究表明，最短的客体倾向于 1:1 的复合物形成，但烷基链越长，2:1 的主客体胶囊就越受青睐。对于中等大小的客体，这两种状态之间的平衡受 pH 值控制；在低值下，含有羧酸客体的胶囊是有利的，而随着 pH 值升高，客体的去质子化有利于 1:1 复合物。有趣的是，对于一对主客体，打开 2:1 囊状复合物并形成 1:1 复合物所需的能量足以将客体的 pKa 移动约 3-4 个数量级（4.1-5.4 kcal mol−）。 1）。检查的两个最大的客人形成稳定的 2:1 胶囊，在这两种情况下，客人都采用了能量相对较高的 J 形图案。此外，这些 2:1 复合物足够稳定，以至于在高 pH 值下客体去质子化发生而不会使胶囊开盖。