  We use molecular dynamics simulations to investigate the tracer diffusion in
a sea of polymers with specific binding zones for the tracer. These binding
zones act as traps. Our simulations show that the tracer can undergo normal yet
non-Gaussian diffusion under certain circumstances, e.g, when the polymers with
traps are frozen in space and the volume fraction and the binding strength of
the traps are moderate. In this case, as the tracer moves, it experiences a
heterogeneous environment and exhibits confined continuous time random walk
(CTRW) like motion resulting a non-Gaussian behavior. Also the long time
dynamics becomes subdiffusive as the number or the binding strength of the
traps increases. However, if the polymers are mobile then the tracer dynamics
is Gaussian but could be normal or subdiffusive depending on the number and the
binding strength of the traps. In addition, with increasing binding strength
and the number of the polymer traps, the probability of the tracer being
trapped increases. On the other hand, removing the binding zones does not
result trapping, even at comparatively high crowding. Our simulations also show
that the trapping probability increases with the increasing size of the tracer
and for a bigger tracer with the frozen polymer background the dynamics is only
weakly non-Gaussian but highly subdiffusive. Our observations are in the same
spirit as found in many recent experiments on tracer diffusion in polymeric
materials and questions the validity of Gaussian theory to describe diffusion
in crowded environment in general.
