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

Abstract

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.