A new zinc(II) fluorophore 2-(9-anthrylmethylamino)ethyl-appended 1,4,7,10-tetraazacyclododecane

A new 2-(9-anthrylmethylamino)ethyl-appended cyclen, L-3 (1-(2-(9-anthrylmethylamino)ethyl)-1,4,7,10-tetraazacyclocloclecane) (cyclen = 1,4,7,1 0-tetraazacyclododecane), was synthesized and characterized for a new Zn2+ chelation-enhanced fluorophore, in comparison with previously reported 9-anthrylmethylcyclen L-1 (1-(9-anthrylmethyl)-1,4,7,1 0-tetraazacyclododecane) and dansylamide cyclen L-2. L-3 showed protonation constants log K-ai of 10.57+/-0.02, 9.10 +/- 0.02, 7.15 +/- 0.02, <2, and <2. The log K-a3 value of 7.15 was assigned to the pendant 2-(9-anthrylmethylamino)-ethyl on the basis of the pH-dependent H-1 NMR and fluorescence spectroscopic measurements. The potentiometric pH titration study indicated extremely stable 1:1 Zn2+-L-3 complexation with a stability constant log K-s(ZnL3) (where K-s(ZnL3) = [ZnL3]/[Zn2+][L-3] (M-1)) of 17.6 at 25 degreesC with I = 0.1 (NaNO3), which is translated into the much smaller apparent dissociation constant K-d (=[Zn2+](free)[L-3](free))/[ZnL3]) of 2 x 10(-11) M with respect to 5 x 10(-8) M for L-1 at pH 7.4. The quantum yield (Phi = 0.14) in the fluorescent emission of L-3 increased to Phi = 0.44 upon complexation with zinc(II) ion at pH 7.4 (excitation at 368 nm). The fluorescence of 5 muM L-3 at pH 7.4 linearly increased with a 0.1-5 muM concentration of zinc(II). By comparison, the fluorescent emission of the free ligand L-1 decreased upon binding to Zn2+ (from Phi = 0.27 to Phi = 0.19) at pH 7.4 (excitation at 368 nm). The Zn2+ complexation with L-3 occurred more rapidly (the second-order rate constant k(2) is 4.6 x 10(2) M-1 s(-1)) at pH 7.4 than that with L-1 (k(2) = 5.6 x 10 M-1 s(-1)) and L-2 (k(2) = 1.4 x 10(2) M-1 s(-1)). With an additionallyinserted ethylamine in the pendant group, the macrocyclic ligand L-3 is a more effective and practical zinc(II) fluorophore than L-1.