Cucurbit[7]uril host-guest complexes of cholines and phosphonium cholines in aqueous solution

The neutral host cucurbit[7]uril forms very stable complexes with a series of cationic cholines (R3NCH2CH2OR'(+)) and their phosphonium analogues (R3PCH2CH2OR'(+)) (R-3 = Me-3, Et-3, or Me(2)Bz, or R3N = quinuclidinium, and R' = H, COCH3, CO(CH2)(2)CH3, or PO3H), and (+/-)-carnitine, in aqueous solution. The complexation behaviour has been investigated using H-1 and P-31 NMR spectroscopies, and ESI mass spectrometry. The complexation-induced chemical shift changes of the guests clearly indicate the effects of replacing the N(CH3)(3)(+) end group by P(CH3)(3)(+), and changing the nature of R on the position of the guest with respect to the CB[7] cavity and its polar portal-lining carbonyl groups. This study demonstrates that molecular recognition of cholines in aqueous solution is achievable with a neutral host without the need for aromatic walls for cation-pi interactions.