A new fluorescent probe for zinc(II):: An 8-hydroxy-5-N,N-dimethylaminosulfonylquinoline-pendant 1,4,7,10-tetraazacyclododecane

A new fluorescent probe for Zn2+, namely, 8-hydroxy-5-N,N-dimethylaminosulfonylquinolin-2-ylmethyl- pendant cyclen (0), was designed and synthesized (cyclen=1,4,7,10-tetraazacyclododecane). By potentiometric pH, H-1 NMR, and UV spectroscopic titrations, the deprotonation constants pK(a1)-pK(a6) of L-8 center dot 4HCl were determined to be < 2, < 2, < 2 (for amino groups of the cyclen and quinoline moieties), 7.19 +/- 0.05 (for 8-OH of the quinoline moiety), 10.10 +/- 0.05, and 11.49 +/- 0.05, respectively, at 25 degrees C with I=0.1 (NaNO3). The results of H-1 NMR, potentiometric pH, and UV titrations, as well as single-crystal X-ray diffraction analysis, showed that L-8 and Zn2+ form a 1:1 complex [Zn(H-1L8)], in which the 8-OH group of the quinoline ring of L-8 is deprotonated and coordinates to Zn2+, in aqueous solution at neutral pH. On addition of one equivalent of Zn2+ and Cd2+ the fluorescence emis sion of L-8 (5 mu m) at 512 nm in aqueous solution at pH 7.4 [10 mm HEPES with I=0.1 (NaNO3)] and 25 degrees C increased by factors of 17 and 43, respectively. We found that the cyclen moiety has the unique property of quenching the fluorescence emission of the quinolinol moiety when not complexed with metal cations, but enhancing emission when complexed with Zn2+ or Cd2+. In addition, the Zn2+-L-8 complex [Zn(H-1L8)] is much more thermodynamically and kinetically stable (K-d[Zn(H-1L8)] = [Zn2+](free)[L-8](free)/[Zn(H-1L8)] = 8 f(M) at pH 7.4) than the Zn2+ complexes of our previous Zn2+ fluorophores ([Zn(H-1L2)] and [Zn(L-3)]). Furthermore, formation of [Zn(H-1L8)] is much faster than those of [Zn(H-1L2)] and [Zn(L-3)]. The staining of early-stage apoptotic cells with L-8 is also described.