Fractional Stokes-Einstein relationship in biological colloids: Role of mixed stick-slip boundary conditions

The bovine serum albumin mutual diffusion coefficient D has been measured by photon correlation spectroscopy as a function of protein concentration. The solution viscosity eta(sol) has been obtained by following the diffusion of spherical probes added, at a small volume fraction, to the protein solution. it is found that D follows a fractional Stokes-Einstein relation D similar to 1/eta(sol)(u) with alpha similar or equal to 0.5 up to a protein concentration C similar or equal to 15 g/dL. The observed behavior has been traced back to interparticle interactions taking into account the possibility of mixed stick-slip boundary condition at the protein-water interface.