Binary and ternary metallo-β-cyclodextrins of 6^A-[bis(carboxylatomethyl)amino]-6 ^A-deoxy-β-cyclodextrin

The binary metallo-β-cyclodextrins formed by 6^A-[bis(carboxylatomethyl)amino]-6 ^A-deoxy-β-cyclodextrin ([M(βCDida)], where M²⁺ = Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) are characterized by log(K/dm³ mol^-1) = 7.29±0.01, 9.74± 0.02, 8.89±0.03 and 6.08±0.02, respectively, in aqueous solution at 298.2 K and I = 0.10 mol dm^-3 (NaClO₄), where K is the stability constant. The ternary metallo-β-cyclodextrins formed by [M(βCDida)] and (R)- and (S)-tryptophanate, [M(βCDida)(R)-Trp]^- and [M(βCDida)(S)-Trp]^-, are characterized by log(K/dm³ mol^-1) = 5.49±0.04 and 5.16±0.07, 6.33±0.04 and 6.34±0.02, 6.91±0.02 and 6.99±0.01, and 5.34±0.02 and 5.36±0.04, respectively, when M²⁺ = Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺. For [M(βCDida)(R)-Trp]^- and [M(βCDida)(S)-Trp]^-, pK_a values of 8.07±0.04 and 8.4±0.1, 7.45±0.02 and 7.88±0.04, 8.46±0.09 and 8.40±0.01, and 8.92±0.05 and 8.7±0.2 are found when M²⁺ = Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, respectively, and are attributed to the protolysis of an aqua ligand occupying a sixth coordination site on M²⁺.