Design and non-covalent DNA binding of platinum (II) metallacalix[4]arenes

A set of cyclic tetranuclear complexes of the metallacalix[4]arene type with formula [{Pt(en)(L)}₄]⁴⁺ (en = ethylenediamine; 2: LH = 5-chloro-2-hydroxypyrimidine (5-Cl-Hpymo); 3: LH=5-bromo-2-hydroxypyrimidine (5-Br-Hpymo); 4: LH = 5-iodo-2-hydroxypyrimidine (5-I-Hpymo)) have been obtained from the reaction between cis-protected square-planar [Pt(en)(H ₂O)₂]²⁺ metal entities and LH in aqueous media. Additionally, the binding properties of 2, 3, 4 and their congener [{Pt(en)(L)}₄]⁴⁺ (1: LH = 2-hydroxypyrimidine (Hpymo)) with calf thymus-DNA (ct-DNA) have been studied by using different techniques including circular and linear dichroism (CD and LD, respectively) and UV-visible absorbance spectroscopies, gel electrophoresis, fluorescence competitive-binding studies and atomic force microscopy (AFM). The results are consistent with significant non-covalent interactions taking place between the polynuclear cyclic species and ct-DNA. Moreover, gel electrophoresis, linear dichroism titrations and AFM images of ct-DNA with metallacalixarenes show ct-DNA coiling at low metallacalixarene concentrations and upon subsequent increments in metallacalixarene concentration ct-DNA can be seen to uncoil with concomitant formation of long and inflexible ct-DNA structures.