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, (LH2)-L-1 (the intracellular Zn2+ probe, Zinquin A) in its deprotonated form, L-1(2-), in Zn2+ ternary complexes, [(ZnLL)-L-n-L-1](n) (where n is the charge of L-n) at 298.2 K in 50% aqueous ethanol (v/v) and I = 0.10 (NaClO4), shows that the formation of [(ZnLL)-L-n-L-1](n) from [(ZnL)-L-n]((2+n)+) is characterized by log(K-5/dm(3) 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 L-n = L-2 - L-6 and L-7(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, L-8, occurs in the formation of [(ZnLH-1)-L-8](+) that subsequently forms [(Zn8LH-1L)-L-1](-) for which log(K-5/dm(3) mol(-1)) = 9.87 +/- 0.08. The variation of K-5 and the 5-fold variation of quantum yield of L-1(2-) as its coordination environment changes in Zn2+ ternary complexes are discussed with reference to the use of L-1(2-) in the detection of intracellular Zn2+.