The concentration dependence of the hemoglobin mutual diffusion coefficient

Laser correlation spectroscopy was used to measure the mutual diffusion coefficient, D, of human cyanomethemoglobin (Fe+++:CN) at varying protein concentrations. These measurements were made at 20 degrees C in a 0.1 M phosphate buffer solution at pH 7.0. For low protein concentrations we find D = (6.43 +/- 0.26) X 10(-7) cm2/s and that there is a near linear decrease from this value at higher concentrations. The linear relation between the diffusion coefficient and protein concentration allows us to deduce the value of the linear frictional volume fraction coefficient, Kf=7.75, and to extrapolate to hemoglobin concentrations equivalent to that in the red blood cell where we estimate D = 4.25 X 10(-7) cm2/s. Various theoretical predictions of the dependence of the mutual diffusion coefficient on concentration are tested; we find that the generalized Stokes-Einstein relation can be made to fit our high concentration data if we assume a hard-sphere model and if we include a term involving a hydrodynamic interaction integral.