High-affinity single and double helical pseudofoldaxanes with cationic guests.

Two aromatic oligoamides, the 8-residue H8 and 16-residue H16, that adopt stable, cavity-containing helical conformations were examined for their complexation of a rodlike dicationic guest, octyl viologen (OV) and -bis(trimethylammonium)benzene (TB). Studies based on 1D and 2D H NMR, isothermal titration calorimetry (ITC), and X-ray crystallography demonstrated that H8 and H16 wraps around two OV ions as a double helix and a single helix, respectively, resulting in 2 : 2 and 1 : 2 complexes. Compared to H8, the longer H16 binds the OV ions with much higher binding affinity and with extraordinary negative cooperativity. In contrast to its 1 : 2 binding with OV, the binding of helix H16 with the bulkier guest TB shows a 1 : 1 ratio. Host H16 also selectively binds OV in the presence of TB. This novel host-guest system features pairwise placement of the otherwise strongly repulsive OV ions in the same cavity, strong negative cooperativity, and mutual adaptability of hosts and guests. The resultant complexes are highly stable [2]-, [3]-, and [4]pseudo-foldaxanes with few known precedents.