A New Zinc(II) Fluorophore 2-(9-Anthrylmethylamino)ethyl-Appended 1,4,7,10-Tetraazacyclododecane

A new 2-(9-anthrylmethylamino)ethyl-appended cyclen, L3 (1-(2-(9-anthrylmethylamino)ethyl)-1,4,7,10-tetraazacyclododecane) (cyclen = 1,4,7,10-tetraazacyclododecane), was synthesized and characterized for a new Zn2+ chelation-enhanced fluorophore, in comparison with previously reported 9-anthrylmethylcyclen L1 (1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane) and dansylamide cyclen L2. L3 showed protonation constants log Kai of 10.57 ± 0.02, 9.10 ± 0.02, 7.15 ± 0.02, <2, and <2. The log Ka3 value of 7.15 was assigned to the pendant 2-(9-anthrylmethylamino)ethyl on the basis of the pH-dependent 1H NMR and fluorescence spectroscopic measurements. The potentiometric pH titration study indicated extremely stable 1:1 Zn2+−L3 complexation with a stability constant log Ks(ZnL3) (where Ks(ZnL3) = [ZnL3]/[Zn2+][L3] (M-1)) of 17.6 at 25 °C with I = 0.1 (NaNO3), which is translated into the much smaller apparent dissociation constant Kd (=[Zn2+]free[L3]free/[ZnL3]) of 2 × 10-11 M with respect to 5 × 10-8 M for L1 ...