Formation of metallo-6(A)-((2-(bis(2-aminoethyl)amino)ethyl)amino)-6(A)-deoxy-beta-cyclodextrins and their complexation of tryptophan in aqueous solution

A pH titration study shows that 6(A)-((2-(bis(2-aminoethyl)amino)ethyl)amino)-6(A)-deoxy-beta-cyclodextrin)(beta CDtren) forms binary metallocyclodextrins, [M(beta CDtren)](2+), for which log(K/dm(3) mol(-1)) = 11.65 +/- 0.06, 17.29 +/- 0.05, and 12.25 +/- 0.03, respectively, when M(2+) = Ni2+, Cu2+, and Zn2+, where K is the stability constant in aqueous solution at 298.2 K and I = 0.10 mol dm(-3) (NaClO4). The ternary metallocyclodextrins [M(beta CDtren)Trp](+), where Trp(-) is the tryptophan anion, are characterized by log(K/dm(3) mol(-1)) = 8.2 +/- 0.2 and 8.1 +/- 0.2, 9.5 +/- 0.3 and 9.4 +/- 0.2, and 8.1 +/- 0.1 and 8.3 +/- 0.1, respectively, where the first and second values represent the stepwise stability constants for the complexation of (R)- and (S)-Trp(-), respectively, when M(2+) = Ni2+, Cu2+, and Zn2+. From comparisons of stabilities and UV-visible spectra, the binary and ternary metallocyclodextrins appear to be six-coordinate when M(2+) = Ni2+ and Zn2+ and five-coordinate when M(2+) = Cu2+. The factors affecting the stoichiometries and stabilities of the metallocyclodextrins, are discussed and comparisons are made with related systems.