The in-cell environment is crowded
with macromolecules,
and the
consequent reduction in free volume, based on the hard-sphere paradigm,
is central to understanding macromolecular motions. A much-used model
crowder, Ficoll, often assumed to be a compact, if not rigid, colloidal
particle, is studied by rheology, small-angle neutron scattering,
nuclear magnetic resonance diffusometry, and relaxometry. We find
that the Ficoll suspension viscosity scales linearly with concentration c
F in the dilute limit and as
at high c
F,
i.e, consistent with the 15/4 (de Gennes) scaling for a reptating
polymer. The form factor of Ficoll, obtained via small-angle neutron
scattering (SANS), resembles either a Gaussian polymer or a soft polymer
blob. From NMR diffusion measurements, we obtain an effective volume
fraction for Ficoll that accounts for Ficoll-bound water in two ways
and show that each results in a volume occupancy of 60% to 70% in
the crowding limit, much larger than the traditionally reported values
of around 30%. If we persist with the colloid paradigm and examine
the dependence of the zero-q structure factor obtained
via SANS in terms of this effective volume fraction, we find that
only a combination of particle softness and interparticle attractions,
quantified using a computational model, can replicate the experimental S(0). The stark difference between effective and traditional
volume occupancies affects the interpretation of previous experiments
on macromolecular crowding and might explain the intriguing non-monotonicity
observed in the dependence of protein relaxation rates on crowder
concentration.
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