Adsorption of DNA onto positively charged amidine colloidal spheres and the resultant bridging interaction

The complexation behaviour of duplex linear DNA (negatively charged) with amidine functionalised sub-micron latex spheres (positively charged) was studied using dynamic light scattering (DLS) and a PALS interferrometric zeta potential sizer. Four types of DNA-sphere complex were investigated as a function of component concentration by combining amidine functionalised polystyrene microspheres with radii of 10.5 nm and 60 nm, and herring DNA of lengths of 35 nm and 85 nm. At low DNA concentrations (cDNA), the undercharged complexes showed a small increase in measured hydrodynamic radius (Rh) and a decrease in zeta potential with increasing cDNA. Within a critical DNA concentration range Rh was seen to peak sharply, and the zeta potentials were ∼0 mV, corresponding to the formation of unstable neutral complexes. Immediately above this concentration region the measured Rh values became comparable with those at low cDNA, and the zeta potential became negative, indicating the formation of stable overcharged complexes. The small and large spheres formed multi-sphere and single sphere overcharged aggregates respectively, which is thought to be determined by the relative magnitude of the chain persistence length (∼50 nm) and the sphere radius, switching on or off the DNA bridging interaction.