Adsorption of DNA onto Charged Spheres
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Adsorption of DNA onto Charged Spheres Alison J. Hodrien, Alison M. Voice, Thomas A. Waigh School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK. ABSTRACT The complexation behaviour of linear DNA (negatively charged) with surface functionalised sub-micron latex spheres is studied by dynamic light scattering (DLS), small-angle neutron scattering (SANS), small-angle x-ray scattering (SAXS), and micro rheology. The complexes are measured in solution as a function of component concentration (CDNA, CSPH) and added salt concentration (Cs). In the absence of salt, measured radius increases with CDNA (CSPH held constant) up to a plateau value. The presence of salt causes a decrease in the measured complex radius, which may be due either to increased flexibility of the chains allowing them to form a more compact layer on the sphere surface, or dissociation of the two components due to screened electrostatics. More detailed experiments to determine this are underway. INTRODUCTION Motivation for this work stems from the growing body of theoretical and simulation literature on the subject, which currently lacks an equivalent body of experimental work for the purposes of comparison. Generally these works describe a range of structures (often in the form of complexation phase diagrams), from the chain tightly wrapping the sphere to the formation of large loops (or leaves) to give a rosette like complex [1-3]. Parameters in the system under study have been limited to: sphere radius, chain length, sphere charge and salt concentration (affecting both electrostatic screening and chain flexibility). Although much of the literature only examines systems containing one chain per sphere, we expect that general trends (e.g. layer thickness increasing as chain stiffness increases) will remain as predicted in the case of multiple chains. As a secondary motivation, the phenomenon of complexation between oppositely charged structures is already exploited by man and nature (e.g. industrially for colloidal stabilisation, and for DNA packaging within cells), and a better understanding of the physics behind the systems can only serve to provide further applications. CURRENT THEORY In order to simplify the system, most theoretical models consider the interaction of a charged sphere with a single oppositely charged flexible chain. More recent works approach the behaviour of a semi-flexible chain, and specific interactions such as that between DNA and histones. Different studies vary greatly in their methods and approximations, and also in the parameters that they choose to study, for example – the macroion charge, linear charge density of the polyelectrolyte, ionic strength of the solvent, sphere radius, chain length, chain flexibility and solvent pH. Obviously the ideal systems described in the literature are far removed from the experimental reality of multiple adsorptions (a sample containing complexes consisting of a single chain adsorbed onto a macroion would not only be difficult to prepare, but also to measure).
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