NMR and small-angle neutron scattering examination of dynamics and structure in a polymer-colloid model system directed at understanding macromolecular crowding in cells*

The inside of a living cell is crowded (30 - 40% volume fraction) with many macromolecular components, so that each individual macromolecule is not necessarily highly concentrated. This unique system calls for a colloidal perspective to tease out the competing roles of size, shape, flexibility, charge, hydrodynamic interactions, and chemical specificity.

In this work [1], we study long-time self diffusion (using pulsed-gradient NMR) and size (via small-angle neutron scattering) of a linear polymer, polyethylene glycol (PEG, radius of gyration $R_g$, and a more compact crowder (polysucrose, radius $R_c$) of varying size ratio $R_g/R_c$. We find that polymer diffusion exhibits a universal exponential concentration dependence at all polysucrose volume fractions, suggestive of an entropic origin [2]. The crowder exhibits reversible clustering at a volume fraction of 5 - 10%, depending on crowder charge. In the crowding limit, the flexible PEG is up to 100 times mobile than the compact Ficoll.

[1] S. Palit et al., Phys. Rev. Lett, 118, 097801 (2017); J. Chem. Phys., 147, 114902 (2017).
[2] Y. Rosenfeld, Phys. Rev. A15, 2545 (1977)
[3] S. Palit, A. Yethiraj, J. Chem. Phys. 147, 074901 (2017).