A study of factors influencing hydration of sodium hyaluronate from compressibility and high-precision densimetric measurements.

A study of the factors influencing the hydration of the biopolymer hyaluronic acid was made by compressibility and density measurements. The factors investigated were the hydration changes on glycosidic bond formation, and also the influence of counterion type, solution ionic strength and temperature. The results indicate that, with this biopolymer, the hydration of the glucuronate residue is significantly more than that of the N-acetylglucosamine residue, and further that the biopolymer is less hydrated than the sum of its component monosaccharide residues. Change of the counterion salt form of this polyelectrolyte from univalent to bivalent counterion type (Na+ to Ca2+) leads to a small though significant increase in the total hydration sheath surrounding the polymer. An increase in the background ionic strength of the solvent leads to a quantifiable lowering of the hydration of the polymer at physiological ionic strength compared with its value in salt-free aqueous solution. A decrease in hydration with increase in temperature in the range 20-50 degrees C is the opposite of previous reports, and was observed when the polymer was dissolved both in pure water and in 0.15 M-NaCl.

An investigation of calcium ion binding to heparins.

Using the line charge model of Manning and, where appropriate, a form of the empirical corrections to his limiting laws proposed by Wells, we have investigated the ion condensation phenomenon for four pure calcium heparin preparations and similar Ca-heparin/CaCl2 mixtures in aqueous solution. This has been expressed in terms of the single ion activity coefficient of the total calcium counter ion present. The results found for the single counter ion activity coefficient of pure Ca-heparin agree moderately well with the limiting law of Manning for a pure divalent counter ion polyanion salt in dilute solution. The results for Ca-heparin/CaCl2 mixtures when corrected for small ion-small ion interactions give a surprisingly good fit to the theoretical counter ion activity coefficient for a divalent counter ion/divalent cation simple electrolyte system, for values of X (X = the polyanion/co-ion ratio) up to 4, with slight deviations for values of X > 4. In the mixtures with the largest excess of polyelectrolyte, the single ion activity coefficients of the Ca2+ ion are in excess of the value predicted by Manning's theory. Ion exchange of sodium heparins to give the Ca-heparin samples used in the work did not appear to cause regular changes in the anticoagulant activity.