A preparative and spectroscopic study of fluorophores for zinc(II) detection

Zinc(II) specific fluorophores are of substantial importance in the study o f intracellular Zn2+. Two such widely used fluorophores are 2-methyl-8-(4-t oluenesulfonamido)-6-quinolyloxyacetic acid, Zinquin (1), and its ethyl est er, Zinquin ester (2), which fluoresce strongly when bound by Zn2+. To gain insight into the factors affecting the fluorescence of such fluorophores t he closely related 4-methyl-N-(6-methoxy-2-methyl-8-quinolyl)-benzenesulfon amide (3), and nine analogues (4-12), substituted at sulfur by nine differe nt substituents have been prepared and their fluorescing characteristics an d those of their Zn2+ complexes have been examined. The nine substituents a re: 2,2,2-trifluoroethyl (4), 4-methoxyphenyl (5), 4-acetamidophenyl (6), 4 -bromophenyl (7), 4-nitrophenyl (8), 3-trifluoromethylphenyl (9), naphth-1- yl (10), naphth-2-yl (11) and 5-dimethylaminonaphth-1-yl (12). Under neutra l conditions in 75/25% v/v ethanol/water solutions, (3)-(7) and (9)-(11) fl uoresce weakly in the free state, (8) does not fluoresce and (12) fluoresce s strongly. Ultraviolet (UV)- visible spectroscopy shows that (3)-(12) comp lex to Zn2+, but unlike the remainder, the complexes of (8) and (12) do not fluoresce, with those possessing electron-withdrawing substituents, (4) an d (9), being the most fluorescent. On this basis ethyl-2-(2-methyl- quinoly loxy-8-(2,2,2-trifluoroethylsulfonamido)) acetate (19) was prepared and its Zn2+ complex was found to be substantially more fluorescent than that of ( 2).