Interactions with Charged Cyclodextrins and Chiral Recognition

Anionic N-acetylated α-aminoacids (AcTrp-, AcPhe-, AcLeu- and AcVal-) are bound to protonatedheptakis(6-amino-6-deoxy)-β-cyclodextrin(per-NH3+-β-CD) by a cooperative work of inclusion and Coulomb interactions.Such complexation occurs enantioselectively ((S)-selective)and is accompanied bypositive entropy changes. Similar (S)-selectivecomplexation occurs in the oppositelycharged system. Namely, cationic α-aminoacid methyl esters are enantioselectivelybound to dissociatedheptakis(6-carboxymethylthio-6-deoxy)-β-cyclodextrin(per-COO--β-CD). In order to obtain thegeneral mechanism for complexationof a charged host with an oppositely charged guest,we examined the 1H NMR spectra oncomplexation of simple carboxylate anions suchas p-methylbenzoate anion andalkanoate anions with per-NH3+-β-CD.Both Coulomb interactions andinclusion are essential to form stable complexesof these carboxylate anions. In allcases, positive entropy changes promote thecomplexation between the carboxylateanions and per-NH3+-β-CD. Dehydrationfrom both charged host and guestis the origin of entropic gains. The mechanism forcomplexation of a charged host withan oppositely charged guest involving the cooperativework of inclusion and Coulombinteractions and positive entropy change due todehydration upon complexation isgenerally applied for related systems such asenantioselective complexation ofRu(phen)32+ with per-COO--β-CDand of Ru(phen)32+with DNA.