Figure 3.

Characterization of the intermolecular interactions in solution at pH = 7.8. (a) The dependence of the second osmotic virial coefficient B₂ on the ionic strength I, varied through the concentration of HEPES buffer or by adding KCl, NaCl, or (NH₄)₂SO₄. The values of B₂ computed using the model represented in Fig. S3 are also shown. B₂(hs) = 4 V_MN_AM_w⁻² = 2.35 × 10⁻⁴ mol m³ kg⁻² for hard spheres (V_M = 2.0 × 10⁻²⁶ m³, molecular volume; N_A, Avogadro’s number; M_w = 14.5 kg mol⁻¹, lysozyme molecular weight) is shown for comparison. (b) The dependence of monomer diffusivity in dilute (D₁^dilute, measured at 9 mg mL⁻¹, open symbols) and concentrated (D₁^conc, measured at 100 mg mL⁻¹, solid symbols) solutions on the ionic strength, varied by the addition of the same electrolytes as in (a). The Stokes-Einstein diffusivity (or self-diffusivity) D₀ = 1.20 × 10⁻¹⁰ m² s⁻¹ of a sphere of radius 1.7 nm in a solution with viscosity 1.06 mPa s is shown. Solid and dashed lines are guides for the eye. (c) Potential of mean force (PMF) between a pair of molecules as a function of the distance between their centers of mass calculated using a numerical model illustrated in Fig. S3 at ionic strength I = 13.3 mM. (d) The dependences of the energy at contact, repulsive hump, and detachment barrier, defined in (c) on the ionic strength. To see this figure in color, go online.