Synthesis, structural analysis, solution equilibria and biological activity of rhodium(iii) complexes with a quinquedentate polyaminopolycarboxylate

Abstract

Two rhodium(III) complexes [Rh(ed3a)(OH₂)]·H₂O (1) and Na[Rh(ed3a)Cl]·H₂O (2) with ethylenediamine-N,N,N′-triacetate (ed3a) have been synthesized and characterized by elemental, spectroscopic and structural analyses. The crystal structure of (1) and (2) and the spectroscopic analysis of the two rhodium(III)–ed3a complexes are discussed in detail. The protonation constants of H₃ed3a and the conditional stability constants of its Rh^III complexes have been determined in aqueous solution by pH potentiometry and UV-Vis spectrophotometry. Molecular mechanics (MM) and density functional theory (DFT) have been used to model all possible geometric isomers, determine the global energy minimum and compare the computed with the experimentally observed structures. The cytotoxic activity of the new Rh^III complexes was evaluated by an MTT assay against four human cancer lines (MCF-7, A549, HT-29 and HeLa) and a normal human cell line (MRC-5). A549, HT-29 and HeLa cells were sensitive to all compounds tested, while the breast carcinoma cell line MCF-7 was only sensitive to the reference compounds (doxorubicin and cisplatin). Western blot (WB) analysis of the effects of the tested compounds indicates that both complexes increase the expression of caspase 3 and consequently the involvement of this enzyme in apoptotic processes of the treated cells. WB also demonstrates proteolytic cleavage of poly-(ADP-ribose)polymerase (PARP) in HeLa cells after treatment with both tested substances. Flow cytometry confirmed apoptotic cell death and showed the induction of cell cycle termination as a possible promoter of apoptosis.