Ferrocene-Conjugated l-Tryptophan Copper(II) Complexes of Phenanthroline Bases Showing DNA Photocleavage Activity and Cytotoxicity

Ferrocene-conjugated l-tryptophan (l-Trp) reduced Schiff base (Fc-TrpH) copper(II) complexes [Cu(Fc-Trp)(L)](ClO4) of phenanthroline bases (L), viz. 2,2′-bipyridine (bpy in 1), 1,10-phenanthroline (phen in 2), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq in 3), and dipyrido[3,2-a:2′,3′-c]phenazine (dppz in 4), were prepared and characterized and their photocytotoxicity studied. Cationic reduced Schiff base (Ph-TrpH) complexes [Cu(Ph-Trp)(L)(H2O)](ClO4) (L = phen in 5; dppz in 6) having the ferrocenyl moiety replaced by a phenyl group and the Zn(II) analogue (7) of complex 4 were prepared and used as control species. The crystal structures of 1 and 5 with respective square-planar CuN3O and square-pyramidal CuN3O2 coordination geometry show significantly different core structures. Complexes 1–4 exhibit a Cu(II)–Cu(I) redox couple near −0.1 V and the Fc+–Fc couple at ∼0.5 V vs SCE in DMF–0.1 M [Bun4N](ClO4) (Fc = ferrocenyl moiety). The complexes display a copper(II)-based d–d band near 600 nm and a Fc-centered band at ∼450 nm in DMF–Tris-HCl buffer. The complexes are efficient binders to calf thymus DNA. They are synthetic chemical nucleases in the presence of thiol or H2O2, forming hydroxyl radicals. The photoactive complexes are cleavers of pUC19 DNA in visible light, forming hydroxyl radicals. Complexes 2–6 show photocytotoxicity in HeLa cancer cells, giving IC50 values of 4.7, 10.2, 1.3, 4.8, and 4.3 μM, respectively, in visible light with the appearance of apoptotic bodies. The complexes also show photocytotoxicity in MCF-7 cancer cells. Nuclear chromatin cleavage has been observed with acridine orange/ethidium bromide (AO/EB) dual staining with complex 4 in visible light. The complexes induce caspase-independent apoptosis in the HeLa cells.

本研究合成并表征了一系列基于二茂铁缀合L-色氨酸（L-tryptophan, L-Trp）还原席夫碱（reduced Schiff base, Fc-TrpH）的铜(II)配合物[Cu(Fc-Trp)(L)](ClO4)，其中配体L为菲咯啉类碱：2,2′-联吡啶（2,2′-bipyridine, bpy，对应配合物1）、1,10-菲咯啉（1,10-phenanthroline, phen，对应配合物2）、二吡啶并[3,2-d:2′,3′-f]喹喔啉（dipyrido[3,2-d:2′,3′-f]quinoxaline, dpq，对应配合物3）以及二吡啶并[3,2-a:2′,3′-c]吩嗪（dipyrido[3,2-a:2′,3′-c]phenazine, dppz，对应配合物4），并对其光细胞毒性进行了研究。同时合成了以苯基替代二茂铁基的阳离子型还原席夫碱（Ph-TrpH）配合物[Cu(Ph-Trp)(L)(H2O)](ClO4)（其中L为phen时对应配合物5，L为dppz时对应配合物6），以及配合物4的锌(II)类似物（配合物7），将其作为对照体系。配合物1和5的晶体结构分别呈现平面正方形CuN3O和四方锥型CuN3O2配位几何，二者的核心结构存在显著差异。在N,N-二甲基甲酰胺（DMF）-0.1 M [Bun4N](ClO4)体系中（相对于饱和甘汞电极Saturated Calomel Electrode, SCE），配合物1~4均表现出位于-0.1 V附近的Cu(II)/Cu(I)氧化还原对，以及位于~0.5 V的二茂铁阳离子/二茂铁（Fc+/Fc）氧化还原对（Fc为二茂铁基）。在DMF-Tris-HCl缓冲液中，该系列配合物均显示出基于Cu(II)的d-d跃迁吸收带（位于~600 nm附近）以及二茂铁中心的特征吸收带（位于~450 nm附近）。该系列配合物均为有效的小牛胸腺DNA结合剂；在巯基试剂或H2O2存在下，它们可作为合成化学核酸酶，通过产生羟基自由基切割DNA。其中的光活性配合物在可见光照射下可通过产生羟基自由基切割pUC19质粒DNA。配合物2~6在海拉（HeLa）癌细胞中表现出光细胞毒性，在可见光照射下的半数抑制浓度（IC50）分别为4.7、10.2、1.3、4.8和4.3 μM，且可观察到凋亡小体的形成；该系列配合物对MCF-7乳腺癌细胞同样具有光细胞毒性。采用吖啶橙/溴化乙锭（acridine orange/ethidium bromide, AO/EB）双染色法观察到，配合物4在可见光照射下可引发HeLa细胞的核染色质裂解。该系列配合物可在HeLa细胞中诱导不依赖半胱天冬酶（caspase）的细胞凋亡。