N,N'-ビス(5,6,7-トリメトキシ-2-キノリルメチル)-N,N'-ジアルキルエチレンジアミンをCd2+特異的蛍光センサーとして利用した嵩高さ/ダイナミシティおよびpHの最適化

Two methyl substituents of TriMeOBQDMEN (L0, N,N'-bis(5,6,7-trimethoxy-2-quinolylmethyl)-N,N'-dimethylethylenediamine) have been changed to 3-pentyl, cyclopentyl, and cyclohexyl groups to achieve better Cd²⁺ selectivity on fluorescence enhancement upon metal ion binding. Bulky alkyl substituents result in weak metal binding affinity of the ligand, which decreases the fluorescence intensity in the presence of excess amount of metal salt, even for Cd(ClO₄)₂ in DMF-H₂O (1:1). During the titration, the protons generated in the presence of excess metal perchlorate inhibit the complexation by protonation of the ligand. Addition of chloride ion minimizes the fluorescence drop in the presence of excess cadmium salt via formation of dinuclear bis(μ-chloro) cadmium complex. Intrinsic fluorescent Cd/Zn selectivity of the ligand can be assessed in DMF-HEPES buffer (1:1, 50 mM HEPES, 0.1 M KCl, pH = 7.5). The use of DMF-borate buffer (1:1, 50 mM M boric acid, 50 mM KCl, pH = 10) effectively discriminates Cd²⁺ from Zn²⁺ (I_Zn/I_Cd = 0.7 % in the presence of 3 equiv. of metal ions) by utilizing the TriMeOBQDPEN (L2, N,N'-bis(5,6,7-trimethoxy-2-quinolylmethyl)-N,N'-di(3-pentyl)-ethylenediamine) with moderate metal binding affinity. Lowering pH also increases the Cd²⁺ selectivity (I_Zn/I_Cd = 9 % in the presence of 5 equiv. of metal ions) of TriMeOBQDCPEN (L3, N,N'-bis(5,6,7-trimethoxy-2-quinolylmethyl)-N,N'-di(cyclopentyl)ethylenediamine) in DMF-glutarate buffer (1:1, 10 mM 3,3-dimethylglutaric acid, 0.1 M NaCl, pH = 6).