Design and Synthesis of a Fluorescent Probe for Zn2+, 5,7-Bis(N,N-dimethylaminosulfonyl)-8-hydroxyquinoline-Pendant 1,4,7,10-Tetraazacyclododecane and Zn2+-Dependent Hydrolytic and Zn2+-Independent Photochemical Reactivation of Its Benzenesulfonyl-Caged Derivative

We previously reported on a 8-quinolinol-pendant cyclen (L-5) as a Zn2+ fluorophore (cyclen = 1,4,7,10-tetraazacyclododecane) and its caged derivative, 8-(benzenesulfonyloxy)-5-(N,N-dimethylaminosultonyl)quinolin-2-ylmethyl-pendant cyclen (BS-caged-L-5), which can be reactivated by hydrolysis of benzenesulfonyl group upon complexation with Zn2+ at neutral pH to give a 1:1 Zn2+-L-5 complex (Zn(H-1L5)). We report herein on the synthesis of 5,7-bis(N,N-dimethylaminosulfonyl)-8-hydroxyquinolin-2-ylmethyl-pendant cyclen (L-6) and its caged derivative (BS-caged-L-6) for more sensitive and more efficient cell-membrane permeability than those of L-5 and BS-caged-L-5. By potentiometric pH, H-1 NMR, and UV-vis spectroscopic titrations, the deprotonation constants pK(a1)-pK(a6) of H5L6 were determined to be <2, <2, <2, 2.5 +/- 0.1 (for the 8-OH group of the quinoline moiety), 9.7 +/- 0,1, and 10.8 +/- 0.1 at 25 degrees C with I = 0.1 (NaNO3). The results of H-1 NMR, potentiometric pH, UV-vis, and fluorescent titrations showed that L-6 rapidly forms a Zn2+ complex with Zn2+ (Zn(H-1L6)), the dissociation constant of which is 50 fM at pH 7.4. The fluorescent emission of Zn(H-1L6) at 478 nm is 32 times as large as that of L-6 (excitation at 370 nm), and the fluorescent quantum yield of Zn(H-1L6) (Phi(F) = 0.41) is much greater than that of Zn(H-1L5) (Phi(F) = 0.044). The BS-caged-L-6 was reactivated by hydrolysis of the benzenesulfonyl moiety more rapidly (completes in 30 min at pH 7.4 at 37 degrees C) than BS-caged-L-5, presumably enabling the practical detection of Zn in sample solutions and living 6 cells, The photochemical deprotection of BS-caged-L-6 and the cell membrane permeability of L-6 and BS-caged-L-6 are also described,