Role of the coulombic interaction in ligand-induced biopolymer aggregation

The interaction mechanisms responsible for the binding between metal complexes and biopolymers in aqueous solution, as well as the consequent aggregation process of biopolymers themselves, involve many factors, from geometrical aspects and hydrophobic contributions, as examples, to the electrostatic potential. In this paper aqueous solutions of a polynucleotide, polyadenylic acid (PolyA), which mimics the helix arrangement of RNA or single-stranded DNA but has a simpler structure, are used as a model system. The role of the electrostatic interactions in the binding process between some platinum(II) complexes and PolyA and in the aggregation among PolyA molecules is investigated, by means of elastic and quasielastic light scattering and electrophoretic mobility. The results show that the presence of large, planar aromatic moiety in the dicationic platinum(II) complexes is essential for the binding with PolyA and suggest that the consequent lowering of the local electrostatic barrier between PolyA molecules can be involved in triggering the aggregation process.