Metallocyclodextrins of 6A-(3-aminopropylamino)-6A-deoxy-β-cyclodextrin: their formation and enantioselective complexation of (R)- and (S)-tryptophan anions in aqueous solution

Abstract

From a pH titration study, the complexation of divalent metal ions (M²⁺) by 6^A-(3-aminopropylamino)-6^A-deoxy-β-cyclodextrin (βCDpn) to form the metallocyclodextrins, [M(βCDpn)]²⁺, is characterized by log(K₂/dm³ mol^–1)= 4.22 ± 0.02, 5.2 ± 0.1, 7.35 ± 0.04 and 4.96 ± 0.08 when M²⁺= Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, respectively, in aqueous solution at I= 0.10 (NaClO₄) and 298.2 K. The complexation of the tryptophan anion (Trp^–) by [M(βCDpn)]²⁺ is enantioselective for (S)-Trp^– as indicated by log(K_6R/dm³ mol^–1) and log(K_6S/dm³ mol^–1)= 4.04 ± 0.03 and 4.32 ± 0.05, 4.1 ± 0.2 and 5.1 ± 0.2, and 7.85 ± 0.07 and 8.09 ± 0.05, where the first and second magnitudes refer to the stability constants for [M(βCDpn)(R)-Trp]⁺, and [M(βCDpn)(S)-Trp]⁺, respectively, when M²⁺= Co²⁺, Ni²⁺ and Cu²⁺, respectively. The corresponding magnitudes for M²⁺= Zn²⁺ are both 5.3 ± 0.1, indicating no enantioselectivity. The role of M²⁺ and other factors affecting complexation and enantioselectivity are discussed.