Anomalous small angle x-ray scattering determination of ion distribution around a polyelectrolyte biopolymer in salt solution

Abstract

The distribution of counterions in solutions of high molecular mass hyaluronic acid, in near-physiological conditions where mono- and divalent ions are simultaneously present, is studied by small angle neutron scattering and anomalous small angle x-ray scattering. The solutions contain either sodium or rubidium chloride together with varying concentrations of calcium or strontium chloride. The effects of monovalent-divalent ion exchange dominate the amplitude and the form of the counterion cloud. In the absence of divalent ions, the shape of the anomalous scattering signal from the monovalent ions is consistent with the distribution calculated from the Poisson-Boltzmann equation, as found by other workers. In mixtures of monovalent and divalent ions, however, as the divalent ion concentration increases, both the diameter and the amplitude of the monovalent ion cloud decrease. The divalent counterions always occupy the immediate neighborhood of the charged polyanion. Above a given concentration their anomalous scattering signal saturates. Even in a large excess of divalent ions, ion exchange is incomplete.

Molecular simulation of the swelling of polyelectrolyte gels by monovalent and divalent counterions

“…Our new calculations also indicate that divalent counterions have a higher affinity for the chain backbone and displace the monovalent counterion cloud when added to a mixture, such that the overall charge neutrality of our gel is preserved in the process. The spatial extent of the counterion cloud is found to be more contracted for divalent counterions than for monovalent counterions, a finding that is in broad accord with recent x-ray scattering measurements 18 on hyaluronic acid with monovalent and divalent ions ͑Rb + and Ca 2+ ͒ and with previous simulations and density functional calculations. 24 Although the simulations do seem to capture some of the qualitative trends of real polyelectrolyte solutions, we encounter what appears to be a fundamental quantitative difference with direct fits of experimental data.…”

“…16 Flory and Osterheld 17 specifically note that "it seems unnecessary to postulate formation of complex or cross-linkages" to account for the "salting out" tendencies of multivalent ions on polyelectrolyte solutions. This basic conclusion is supported by mechanical measurements by Horkay et al 18 where no evidence of crosslinking effects in polyelectrolyte gels was found for a wide range of multivalent salts.…”

Ions in hyaluronic acid solutions

“…Recent anomalous SAXS measurements on HA solutions, for example, have shown that divalent counterions form a tight sheathlike layer ͑spatial extent of the counterion cloud is much smaller than for the monovalent analog͒ around the polyanion. 19 The calciumsodium exchange may also modify the structure of the surrounding water layer. The validity of Eq.…”

“…15,16 Scattering measurements have been used to investigate the effect of different counterions on the molecular conformation of HA both in the solid state and in solution. [17][18][19] Anomalous SAXS has revealed changes in the counterion distribution around the HA filaments due to monovalentdivalent ion exchange. 19 The abundance of HA and its multiple roles in biological tissues is unique among biopolymers.…”

“…[17][18][19] Anomalous SAXS has revealed changes in the counterion distribution around the HA filaments due to monovalentdivalent ion exchange. 19 The abundance of HA and its multiple roles in biological tissues is unique among biopolymers. Other types of biopolymer, e.g., proteins, lipids, or nucleic acids, are typically localized, and exert specific biological functions.…”