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 Zn(2+) fluorophore (cyclen = 1,4,7,10-tetraazacyclododecane) and its caged derivative, 8-(benzenesulfonyloxy)-5-(N,N-dimethylaminosulfonyl)quinolin-2-ylmethyl-pendant cyclen (BS-caged-L(5)), which can be reactivated by hydrolysis of benzenesulfonyl group upon complexation with Zn(2+) at neutral pH to give a 1:1 Zn(2+)-L(5) complex (Zn(H(-1)L(5))). 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, (1)H NMR, and UV-vis spectroscopic titrations, the deprotonation constants pK(a1)-pK(a6) of H(5)L(6) 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 (NaNO(3)). The results of (1)H NMR, potentiometric pH, UV-vis, and fluorescent titrations showed that L(6) rapidly forms a 1:1 complex with Zn(2+) (Zn(H(-1)L(6))), the dissociation constant of which is 50 fM at pH 7.4. The fluorescent emission of Zn(H(-1)L(6)) 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(-1)L(6)) (Phi(F) = 0.41) is much greater than that of Zn(H(-1)L(5)) (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(2+) in sample solutions and living 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.