Specific molecular recognition by chiral cage-type cyclophanes having leucine, valine, and alanine residue

Chiral cage-type cyclophanes were constructed with two rigid macrocyclic skeletons and four bridging components bearing chiral leucine, valine, and alanine residues, individually. These host molecules strongly bind anionic and hydrophobic guests, such as 8-anilinonaphthalene-1-sulfonate and 6-p-toluidinonaphthalene-2-sulfonate. Thermodynamic parameters were evaluated from temperature-dependent complexation constants determined by fluorescence spectroscopy, and gave negative ΔH and positive ΔS values; especially large values for the cage-type cyclophanes having leucine residues. The positive ΔS values come primarily from effective desolvation of the guest molecules when incorporated into the hydrophobic host cavities, as evidenced by fluorescence parameters. The four bridging segments of the cage-type hosts having chiral amino acid residues seem to undergo chiral twist in the same directions in the light of circular dichroism (CD) spectroscopy. Such helical conformations of the cyclophanes must be caused by chiral nature of the amino acid residues, and the extent of twist in helical conformations is as follows; leucine > valine > alanine. In addition, the twisted direction of bridging segments in the cage-type hosts having L-amino acid residues is opposite to that evaluated for those having D-amino acid residues, so that the former and latter cyclophanes furnish M- and P-helical cavities, respectively. The chirality-based molecular recognition of the cage-type hosts toward an enantiomeric guest, bilirubin-IXα, was investigated by CD spectroscopy in aqueous media.