Switching of Fluorescent Zn/Cd Selectivity in N,N,N',N'-Tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine by One Asymmetric Carbon Atom Inversion.

A quinoline-based hexadentate ligand, (S,S)-N,N,N',N'-tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine ((S,S)-6-MeOTQPh2EN), exhibits fluorescence enhancement at 498 nm upon addition of 1 equiv of Zn2+ (IZn/I0 = 12, φZn = 0.047) in aqueous DMF solution (DMF/H2O = 2:1). Addition of 1 equiv of Cd2+ affords a much smaller fluorescence increase at the same wavelength (ICd/I0 = 2.5, ICd/IZn = 21%). The trivalent metal ions such as Al3+, Cr3+, and Fe3+ also exhibit fluorescence enhancement at 395 nm (IAl/I0 = 22, ICr/I0 = 6 and IFe3+/I0 = 13). In contrast, meso-6-MeOTQPh2EN exhibits a Cd2+-selective fluorescence increase at 405 nm in the presence of 1 equiv of metal ion (ICd/I0 = 11.5, φCd = 0.022), while Zn2+ induces a smaller fluorescent response under the same experimental conditions (IZn/I0 = 3.3, IZn/ICd = 29%). In this case, the fluorescence intensities of meso-6-MeOTQPh2EN in the presence of a large amount of Zn2+ and Cd2+ become similar. This diastereomer-dependent, fluorescent metal ion specificity is derived from the Zn2+-specific intramolecular excimer formation in (S,S)-6-MeOTQPh2EN-Zn2+ complex and higher binding affinity of meso-6-MeOTQPh2EN with Cd2+ in comparison to Zn2+. The more conformationally restricted diastereomeric pair, namely, cis- and trans-TQDACHs (cis- and trans-N,N,N',N'-tetrakis(2-quinolylmethyl)-1,2-diaminocyclohexanes), both exhibit Zn2+-specific fluorescence enhancement because of the high metal binding affinity and intramolecular excimer forming property derived from the rigid DACH backbone.

Methoxy-substituted tetrakisquinoline analogs of EGTA and BAPTA for fluorescence detection of Cd2+ .

EGTA (ethylene glycol bis(2-aminoethyl ether)-N,N,N',N'-tetraacetic acid) and BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) are well-known Ca2+ chelators that have four carboxylates, two nitrogen atoms and two ether oxygen atoms. In the present study, we prepared EGTQ (N,N,N',N'-tetrakis(2-quinolylmethyl)-1,2-bis(2-aminoethoxy)ethane) and BAPTQ (N,N,N',N'-tetrakis(2-quinolylmethyl)-1,2-bis(2-aminophenoxy)ethane) as quinoline alternatives of EGTA and BAPTA, respectively. In methanol-HEPES buffer solution (9 : 1, 50 mM HEPES, 0.1 M KCl, pH = 7.5), EGTQ exhibits fluorescence enhancement induced by Zn2+ and Cd2+ with poor selectivity, but BAPTQ did not exhibit a fluorescence response to either metal ion. Introduction of three methoxy substituents at the 5,6,7-positions of each quinoline moiety in BAPTQ specifically enhanced the fluorescence intensity of the Cd2+ complex, establishing the Cd2+-specific probe TriMeOBAPTQ (N,N,N',N'-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-bis(2-aminophenoxy)ethane). In contrast, TriMeOEGTQ (N,N,N',N'-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-bis(2-aminoethoxy)ethane) maintains a poor Cd2+/Zn2+ selectivity in its fluorescence response. Although the crystal structures of Cd2+/Zn2+ complexes with EGTQ and BAPTQ derivatives reveal the formation of multiple components including mononuclear and dinuclear complexes, the dinuclear Cd2+ and Zn2+ complexes with a linearly extended structure are regarded as possible fluorescent species in the solution. The conformational restriction of BAPTQ due to the orthophenylene moieties in the molecular skeleton is responsible for the formation of the weakly fluorescent, OH-bridged dizinc complex, which is critical to the strict Cd2+-specificity in the fluorescence response of TriMeOBAPTQ.