Ion Sensitivity of Dna Gel Structure

Introduction Biological systems operate at the cellular and subcellular levels. Understanding the behavior of these systems, material properties including structure, osmotic and mechanical properties must be determined to dimensions below 100 nm. Synthetic polymers have been recently designed to mimic the hierarchical structure and function of biological macromolecules. Knowledge of the structure at molecular and nanometer scale is necessary for engineering useful and novel properties. Examples of applications of engineered structures include materials for tissue engineering, medical implants for diagnosis and therapy, and in-vivo drug-delivery. We developed a multiscale approach to examine the structural hierarchy and equilibrium properties of natural and synthetic polyelectrolyte gels. In these systems different kind of interactions (electrostatic forces, van der Waals forces, hydrophobic interactions, etc.) drive structure formation. Many natural and synthetic polyelectrolytes (e.g., polyacrylic acid, DNA) exhibit a strong sensitivity to ionic strength and, in particular, to counterion valence. The complexity of the behavior of charged macromolecular systems makes necessary to investigate their structure and physical properties on all length scales from the atomic level up to the macroscopic level. Small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) are ideal techniques to study biopolymer molecules and assemblies in their natural environment and to correlate the changes in environmental conditions (e.g., ionic composition, solvent quality) with physical properties. In the present work we report SANS and osmotic swelling pressure measurements made on covalently cross-linked DNA gels swollen in NaCl and CaCl2 solutions. Theory. The neutron scattering intensity of a neutral polymer solution can be described by a Lorentzian function