  Macromolecular diffusion in strongly confined geometries and crowded
environments is still to a large extent an open subject in soft matter physics
and biology. In this paper, we employ large-scale Langevin dynamics simulations
to investigate how the diffusion of a tracer is influenced by the combined
action of excluded-volume and weak attractive crowder-tracer interactions. We
consider two species of tracers, standard hard-core particles described by the
Weeks-Chandler-Andersen (WCA) repulsive potential and core-softened (CS)
particles, which model, e.g., globular proteins, charged colloids and
nanoparticles covered by polymeric brushes. These systems are characterized by
the presence of two length scales in the interaction and can show water-like
anomalies in their diffusion, stemming from the inherent competition between
different length scales. Here we report a comprehensive study of both diffusion
and structure of these two tracer species in an environment crowded by quenched
configurations of polymers at increasing density. We analyze in detail how the
tracer-polymer affinity and the system density affect transport as compared to
the emergence of specific static spatial correlations. In particular, we find
that, while hardly any differences emerge in the diffusion properties of WCA
and CS particles, the propensity to develop structural order for large crowding
is strongly frustrated for CS particles. Surprisingly, for large enough
affinity for the crowding matrix, the diffusion coefficient of WCA tracers
display a non-monotonic trend as their density is increased when compared to
the zero affinity scenario. This water-like anomaly turns out to be even larger
than what observed for CS particle and appears to be rooted in a similar
competition between excluded-volume and affinity effects.
