Dynamic light scattering and turbidimetry, carried out on solutions of hyaluronic acid (HA) and bovine or human serum albumin (SA) at fixed ionic strength (I), revealed a critical pH corresponding to the onset of HA-SA soluble complex formation. Subsequent reduction of pH below pH c , corresponding to an increase in protein net positive charge, results in phase separation of the complex. The sensitivity of pH c to I indicated the primacy of electrostatic interactions in this process. Since pH c was always above the pK a of HA, these effects could be attributed to the influence of protein charge. The electrostatic potential around HSA was modeled using DelPhi (MSI) under pH, I conditions corresponding to incipient binding, phase separation, and noninteraction. At all incipient binding conditions (i.e., pH c , at varying I), an identical region of positive potential 5 Å from the protein van der Waals surface appeared. This unique domain intensified with a decrease in pH or I (corresponding to stronger binding), and diminished with an increase in pH or I (i.e., at noninteracting conditions). The size and low curvature of this domain could readily accommodate a 12 nm (decamer) sequence of HA. Simple electrostatic considerations indicate an electrostatic binding energy for the formation of this complex of ca. 1 kT, consistent with the condition of incipient complex formation. We suggest that such weak electrostatic binding may characterize nonspecific interactions for other proteingylcosaminoglycan pairs.

The effect of a polyelectrolyte's chain stiffness on its interaction with an oppositely charged colloid particle was studied by measuring the relative affinity of two polyelectrolytes for (1) mixed cationic/nonionic micelles (DTAB/TX100), and (2) the protein serum albumin. The binding affinity as manifested, respectively, in the critical ionic surfactant mole fraction required for polyelectrolyte-micelle complex formation, and in the critical pH for polyelectrolyte-protein association, was determined by turbidimetric titrations over a range of ionic strengths. Binding was generally weaker for the stiffer chain, hyaluronic acid (HA), relative to the more flexible chain, a copolymer of acrylamidomethylpropanesulfonate (AMPS) and acrylamide (AAm), chosen to have the same linear charge density as HA at neutral pH. In the case of serum albumin, comparisons were also made to AMPS-AAm copolymers of higher charge densities, and to heparin, a highly charged and flexible biopolyelectrolyte. The results are discussed in terms of the ionic strength dependence of the relevant persistence lengths.

The translational diffusion constants, DT , of the bis(maleonitriledithiolato)nickel anion and dianion, Ni(mnt)2 - and Ni(mnt)2 2-, respectively, have been measured in acetone and ethyl alcohol solutions drawn through a microcapillary by reduced pressure at 25 °C. The translational radius, r t, obtained from DT is larger for Ni(mnt)2 2- than for Ni(mnt)2 - in both solvents. The larger r t for Ni(mnt)2 2- is attributed to ion pairing with (n-Bu)4N+, while Ni(mnt)2 - does not appear to be ion paired. r t for Ni(mnt)2 - has been used to determine the rotational radius, r o, of this paramagnetic ion; r o has been used with ESR results to determine κ⊥, the solvent interaction parameter for Ni(mnt)2 - in several solvents. ESR studies of Ni(mnt)2 - in dimethyl phthalate (DMPT) and tris(2-ethylhexyl) phosphate (TEHP) have also been made. The analysis of the widths shows the reorientation of Ni(mnt)2 - is fastest about its long in-plane axis by a factor of ≈3 in both solvents. The temperature dependence of the widths is discussed using the modified Stokes−Einstein−Debye (SED) model and the Vogel−Tammann−Fulcher (VTF) equation. The SED results indicate that Ni(mnt)2 - has relatively weak interactions with both DMPT and TEHP. The ESR VTF parameters are consistent with those from viscosities and produce calculated values of the glass transition temperatures, T g, that are also in general agreement with experiment.