  Depletion forces and macromolecular crowding govern the structure and
function of biopolymers in biological cells and the properties of polymer
nanocomposite materials. To isolate and analyze the influence of polymer shape
fluctuations and penetrability on depletion-induced interactions and crowding
by nanoparticles, we model polymers as effective penetrable ellipsoids, whose
shapes fluctuate according to the probability distributions of the eigenvalues
of the gyration tensor of an ideal random walk. Within this model, we apply
Monte Carlo simulation methods to compute the depletion-induced potential of
mean force between hard nanospheres and crowding-induced shape distributions of
polymers in the protein limit, in which polymer coils can be easily penetrated
by smaller nanospheres. By comparing depletion potentials from simulations of
ellipsoidal and spherical polymer models with predictions of polymer field
theory and free-volume theory, we show that polymer depletion-induced
interactions and crowding depend sensitively on polymer shapes and
penetrability, with important implications for bulk thermodynamic phase
behavior.
