Polyelectrolyte composite: hyaluronic acid mixture with DNA*

Biomacromolecules are mostly polyelectrolytes (PE), dissociating into polyions and small counterions. Their long-range electrostatic interaction leads to arrangements different than for neutral polymers and generates difficulties in physical understanding. For the last decade we have adressed these by studying the structure and dynamics of two semirigid (bio)PEs, DNA and HA (hyaluronic acid).
Here we present a study of mixtures of DNA and HA, as a system of like-charged semi-rigid chains, differing, however, by the respective persistence lengths of 50 and 1 nm. We studied salt-free mixtures across a broad range of concentration ratios c_HA/c_DNA=0.05-50 (c=5-200 g/L). By polarizing microscopy we established that DNA and HA form clearly separated thread-like domains defined and oriented by solution shear. We applied small angle x-ray scattering (SAXS) and observed a PE correlation peak at q* wave vector, ascribed to DNA subphase and thus reporting on its effective concentration c*_DNA. From c*_DNA it was possible to infer the c*_HA of HA subphase. We found a ratio G= c*_HA/c*_DNA≈0.85 across the studied range. As there is the osmotic pressure, P equilibrium between DNA and HA subphases, the constant G indicates that P~c^9/8 scaling commonly found for DNA and other highly charged PEs is valid also for HA which does not feature counterion condensation – so it’s not due to the concentration dependence of the latter. Furthermore, as for HA all counterions contribute to P, the HA osmotic coefficient f_HA is a measure of f_DNA. We found the latter to be 0.28 and corroborated previous workers who found the concentration of counterions controlling the osmotic pressure to be double the Manning-condensation theory value for DNA.