Deep Inside Cucurbiturils: Physical Properties and Volumes of their Inner Cavity Determine the Hydrophobic Driving Force for Host-Guest Complexation

Cucurbit[n]urils (CBn) bind guest molecules through a combination of electrostatic interactions with the carbonyl rims and hydrophobic interactions with the inner cavity. Investigations with solvatochromic probes in CB7 reveal that the polarity of the cavity resembles that of alcohols (e. g., n-octanol), while its polarizability (P=0.12) and apparent refractive index (n(D)=1.10 +/- 0.12) are extremely low, close to the gas phase. The calculated molecular quadrupole moments of CBs are extremely large (Theta(zz)=-120 to -340 Buckingham). A survey of reported binding constants of neutral guests and hydrophobic residues that form 1 : 1 inclusion complexes with CB6, reveals a preferential inclusion of C3-C5 residues in its cavity. The largest guests which show non-negligible binding contain 7 heavy atoms (excluding hydrogen). For CB7, the strongest binding is observed for guests with adamantyl (10 heavy atoms) and ferrocenyl groups (11 heavy atoms), while the largest guests known to be complexed are carborane and the adduct of two pyridine derivatives (12 heavy atoms). The evaluation of different volumes shows that the most meaningful cavity, namely that responsible for binding of hydrophobic residues, is confined by the planes through the oxygen carbonyls. The volume of this inner cavity follows the formula V/angstrom(3)=68+62(n-5) + 12.5(n-5)(2), affording representative cavity volumes of 68 angstrom(3) for CB5, 142 angstrom(3) for CB6, 242 angstrom(3) for CB7, and 367 angstrom(3) for CB8. The volume of the 2 bond dipole regions is comparably smaller, amounting, for example, to 2 x 35 angstrom(3) for CB6. The analysis of packing coefficients for representative sets of known guests with clearly defined hydrophobic binding motifs reveals average values of 47% for CB5, 58% for CB6, 52% for CB7, and 53% for CB8, which are well in line with the preferred packing ("55% solution", see S. Mecozzi, J. Rebek, Chem. Eur. J. 1998, 4, 1016-1022) in related supramolecular host-guest assemblies. The driving force for binding of hydrophobic guests and residues by CBs is interpreted in terms of the unimportance of dispersion interactions (owing to the low polarizability of their cavity) and the dominance of classical and nonclassical hydrophobic effects related to the removal of very-high-energy water molecules (2 for CB5, 4 for CB6, 8 for CB7, and 12 for CB8) from the cavity.