Interactions with Charged Cyclodextrins and Chiral Recognition

Anionic N-acetylated -aminoacids (AcTrp^-, AcPhe^-, AcLeu^- and AcVal^-) are bound to protonatedheptakis(6-amino-6-deoxy)--cyclodextrin(per-NH₃ ⁺--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 -aminoacid methyl esters are enantioselectivelybound to dissociatedheptakis(6-carboxymethylthio-6-deoxy)--cyclodextrin(per-COO^---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 ⁺--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 ⁺--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)₃ ²⁺ with per-COO^---CDand of Ru(phen)₃ ²⁺with DNA.