Biomacromolecules are mostly polyelectrolytes (PE), dissociating in aqueous envirnoment into polyions and small counterions. Their long-range electrostatic interaction leads to behavior qualitatively different than for neutral polymers1. Conformations depend on the PE and added salt concentration, PE rigidity and valence of counterions2.
We studied salt-free binary mixtures of DNA and hyaluronic acid (HA) by small angle x-ray scattering (SAXS) and observed a PE correlation peak. Indeed, this peak is regularly observed for DNA but for HA this feature is lacking3. The peak at q* wave vector directly relates to DNA mesh size ξ=2π/q*~c-1/2 and scales with c (monomer concentration)4. We concluded that q* observed for a mixture is a measure of concentration of DNA subphase, cDNA, since by polarizing microscopy we found that DNA+HA mixtures were phase separated (contrary to some expectations for PEs5). From this we inferred cHA of HA subphase. As DNA and HA subphases are in the osmotic pressure equilibrium, HA osmotic pressure ΠHA(cHA)=ΠDNA(cDNA) is inferred from cDNA, since DNA equation of state is known.
We concluded that ΠHA(c) (in our range c=0.01-1M) scales as for DNA and other PEs, Π~c9/8 6,7. More importantly, for equal concentrations c=cHA=cDNA, ΠHA(c)≈½ΠDNA(c). In comparison, the osmotic pressure of another PE polystyrene sulphonate (PSS), ΠPSS(c)≈ΠDNA(c). These relations are somewhat surprising since HA is a weak PE (no Manning condensation) where all the counterions are free to contribute to the osmotic pressure. On the contrary, both DNA and PSS are strong PEs, where Manning condensed counterions do not contribute to the osmotic pressure (only 1/4 to 1/3 of the counterions are free).