Concentration-dependent structure, diffusion and viscosity in a model macromolecular crowder*

A living biological cell has multiple macromolecules that occupy 30-40% of the total volume, or more. This phenomenon is known as macromolecular crowding.  Crowding conditions can be mimicked in vitro using colloidal suspensions of artificial crowders, a common choice being spherical polysucrose (Ficoll) nanoparticles. Despite many studies where Ficoll is considered as providing a hard-sphere-like excluded volume interaction, the structure of Ficoll has not yet been elucidated.

We carried out nuclear magnetic resonance spectroscopy studies on diffusion and relaxation of Ficoll 400 (molecular mass 400 kD) as well as water as a function of Ficoll 400 concentration. These studies provide strong evidence that Ficoll 400 is highly porous. The form factor obtained from small angle neutron scattering experiments reveal that the Ficoll 400 particle is closer to a diffusive blob or indeed a Gaussian polymer. The osmotic compressibility, extracted from the structure factor, is much smaller than Carnahan-Starling model for hard spheres, implying a system with soft inter-particle repulsion. Finally, measurements of suspension viscosity help to quantify the softness of Ficoll 400.