engleski

Unraveling the Structure-Affinity Relationship Between Cucurbit[n]urils (n = 7, 8) and Cationic Diamondoids

We report measurement of the binding constants (Ka) for cucurbit[n]uril (n = 7, 8) toward four series of guests based on 2, 6-disubstituted adamantanes, 4, 9-disubstituted diamantanes, 1, 6-disubstituted diamantanes, and 1-substituted adamantane ammonium ions by direct and competitive 1H NMR spectroscopy. Compared to B[7]•Diam(NMe3)2, the adamantane diammonium ion complexes (e.g. CB[7]•2, 6-Ad(NH3)2 and CB[7]•2, 6-Ad(NMe3)2 are less effective at realizing the potential 1000-fold enhancement in affinity due to ion dipole interactions at the second ureidyl C=O portal. Comparative crystallographic investigation of CB[7]•Diam(NMe3)2, B[7]•DiamNMe3, and CB[7]•1-AdNMe3 reveals that the preferred geometry positions the +NMe3-groups ≈ 0.32 Å above the C=O portal ; the observed 0.80Å spacing observed for CB[7]•Diam(NMe3)2 reflects the simultaneous geometrical constraints of CH2•••O=C close contacts at both portals. Remarkably, the CB[8]•IsoDiam(NHMe2)2 complex displays femtomolar binding affinity placing it firmly alongside the CB[7]•Diam(NMe3)2 complex. Primary or quaternary ammonium ion looping strategies lead to larger increases in binding affinity for CB[8] than for CB[7] which we attribute to the larger size of the carbonyl portals of CB[8] which suggests routes to develop CB[8] as the tightest binding host in the CB[n] family. We report that alkyl group fluorination (e.g. CB[7]•1-AdNH2Et versus CB[7]•1-AdNH2CH2CF3) does not result in the expected increase in Ka value. Finally, we have discussed the role of solvation in non-empirical quantum mechanical computational methodology which is used to estimate the relative changes in Gibbs binding free energies.

diamondoids ; ammonium salts ; cucurbit[n]urils ; high affinity host-guest pairs ; X-ray diffraction

Znanstveni projekt HAZU - Diamantoidni amini – novi ligandi u supramolekulskim sustavima ; voditelj: Marina Šekutor