Inclusion of neutral guests by water-soluble macrocyclic hosts – a comparative thermodynamic investigation with cyclodextrins, calixarenes and cucurbiturils
The driving forces of association between three different families of macrocycles as hosts, namely cyclodextrins (α-, β-, and γ-), p-sulfonatocalix[n]arenes (n = 4–6) as well as cucurbit[n]urils (n = 6–8), and three different bicyclic azoalkane homologues as guests, namely 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH), 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as well as 2,3-diazabicyclo[2.2.3]non-2-ene (DBN), were examined by means of calorimetric titrations, NMR spectroscopy and molecular dynamics simulation, all in aqueous solution. The small, spherical and uncharged guests preferably bind inside the cavities of the medium sized hosts. The inclusion complexation by β-cyclodextrin and p-sulfonatocalixarene shows medium binding affinities (millimolar), while cucurbituril macrocycle shows very strong binding (micromolar). For all types of macrocycles, the complex formation is enthalpically driven (ΔH° < 0), accompanied by slightly unfavourable entropy changes (ΔS° < 0). The results are discussed in terms of the flexibility of the hosts, the hydrophobic character of their cavities and the release of high-energy water upon binding, and generalised by including two additional guests, the ketones cyclopentanone and (+)-camphor.