Coordination and Fluorescence of the Intracellular Zn2+ Probe [2-methyl-8-(4-Toluenesulfonamido)-6-quinolyloxy]acetic Acid (Zinquin A) in Ternary Zn2+ Complexes

A potentiometric study of the coordination of the fluorophore, 2-methyl-8-(4-toluenesulfonamido)-6-quinolyloxyacetic acid, 1 LH2 (the intracellular Zn2+ probe, Zinquin A) in its deprotonated form, 1 L 2-, in Zn2+ ternary complexes, [Zn n L 1 L] n (where n is the charge of n L) at 298.2 K in 50% aqueous ethanol (v/v) and I = 0.10 (NaClO4), shows that the formation of [Zn n L 1 L] n from [Zn n L](2+ n )+ is characterized by log(K 5/dm3 mol-1) = 8.23 ± 0.05, 4.36 ± 0.18, 8.45 ± 0.10, 10.00 ± 0.06, 11.53 ± 0.06 and 5.92 ± 0.15, respectively, where n L = 2 L − 6 L and 7 L 3- are 1,4,7,10-tetraazacyclododecane, 1,4,8,11-tetraazacyclotetradecane, 1,4,7-triazacyclononane, 1,5,9-triazacyclododecane, tris(2-aminoethyl)amine and nitrilotriacetate, respectively, and K 5 is the stepwise complexation constant. Dissociation of a hydroxo proton from triethanolamine, 8 L, occurs in the formation of [Zn 8 LH- 1]+ that subsequently forms [Zn 8 LH- 1 1 L]- for which log(K 5/dm3 mol-1) = 9.87 ± 0.08. The variation of K 5 and the 5-fold variation of quantum yield of 1 L 2- as its coordination environment changes in Zn2+ ternary complexes are discussed with reference to the use of 1 L 2- in the detection of intracellular Zn2+.