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

A pH titration study shows that for the complexation of phenylalanine anion [(R)- and (S)-Phe–] by β-cyclodextrin (βCD), log(K1R/dm3 mol–1)= 2.91 ± 0.08 and log(K1S/dm3 mol–1)= 2.83 ± 0.06, and that by 6A-(3-aminopropylamino)-6A-deoxy-β-cyclodextrin (βCDpn) is characterized by log(K2R/dm3 mol–1)= 2.51 ± 0.07 and log(K2S/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(K3R/dm3 mol–1)= 2.50 ± 0.02 and log(K3S/dm3 mol–1)= 2.37 ± 0.09; and for the complexation of HisH by βCDpnH+ log(K4R/dm3 mol–1)= 2.31 ± 0.05 and log(K4S/dm3 mol–1)= 2.18 ± 0.05. For the complexation of Phe– by the metallocyclodextrin, [M(βCDpn)]2+, log(K11R/dm3 mol–1) and log(K11S/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(K11R/dm3 mol–1)= 8.38 ± 0.04 and log(K11S/dm3 mol–1)= 8.42 ± 0.02, and for the complexation of a second His– log(K12R/dm3 mol–1)= 7.75 ± 0.05 and log(K12S/dm3 mol–1)= 7.6 ± 0.1. The roles of the cyclodextrins, divalent metal ions and amino acids affecting complexation are discussed.