Complexation of phenylalanine and histidine byβ-cyclodextrin, 6A-(3-aminopropylamino)-6A-deoxy-β-cyclodextrin and its metallocyclodextrins in aqueous solution

Abstract

A pH titration study shows that for the complexation of phenylalanine anion [(R)- and (S)-Phe^–] by β-cyclodextrin (βCD), log(K_1R/dm³ mol^–1)= 2.91 ± 0.08 and log(K_1S/dm³ mol^–1)= 2.83 ± 0.06, and that by 6^A-(3-aminopropylamino)-6^A-deoxy-β-cyclodextrin (βCDpn) is characterized by log(K_2R/dm³ mol^–1)= 2.51 ± 0.07 and log(K_2S/dm³ mol^–1)= 2.74 ± 0.05. No complexation of histidine (HisH) by βCD was detected, but for the complexation of histidine anion (His^–) by βCDpnH⁺ log(K_3R/dm³ mol^–1)= 2.50 ± 0.02 and log(K_3S/dm³ mol^–1)= 2.37 ± 0.09; and for the complexation of HisH by βCDpnH⁺ log(K_4R/dm³ mol^–1)= 2.31 ± 0.05 and log(K_4S/dm³ mol^–1)= 2.18 ± 0.05. For the complexation of Phe^– by the metallocyclodextrin, [M(βCDpn)]²⁺, log(K_11R/dm³ mol^–1) and log(K_11S/dm³ mol^–1)= 3.6 ± 0.2 and 3.69 ± 0.06, <3.6 and 4.4 ± 0.1, 7.2 ± 0.1 and 6.9 ± 0.1, 4.7 ± 0.1 and 4.7 ± 0.2, when M²⁺= Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, respectively. For the complexation of His^– by [Cu(βCDpn)]²⁺, log(K_11R/dm³ mol^–1)= 8.38 ± 0.04 and log(K_11S/dm³ mol^–1)= 8.42 ± 0.02, and for the complexation of a second His^– log(K_12R/dm³ mol^–1)= 7.75 ± 0.05 and log(K_12S/dm³ mol^–1)= 7.6 ± 0.1. The roles of the cyclodextrins, divalent metal ions and amino acids affecting complexation are discussed.