Majority-Rules Effect and Allostery in Molecular Recognition of Calix[4]arene-Based Triple-Stranded Metallohelicates

The triple-stranded metallohelicates 1 a,/1 b-3 a/3 b possessing internal guest binding cavities surrounded by calix[4]arene units were synthesized through coordination-driven self-assembly. UV/Vis titration experiments verified that the metallohelicates encapsulated N-methyl pyridinium cations bearing amino acid groups to form host-guest complexes. The guest chirality was transferred to the helicity of the helicates through the steric contact between the stereogenic center of the amino acid group and the metal cores. The (M) helicity was induced when guests the (R)-4--(R)-6 were accommodated within the cavities. The multiple guest complexation within the self-assembled helicates 2 a and 3 a displayed large positive cooperative effects, indicating that the first guest complexation preorganizes the rest of the cavities to facilitate a subsequent guest binding. This cooperativity results in the majority-rules effect in the chiral guest binding for 2 a and 3 a.