Reversing the direction of unforced polymer translocation by macromolecular crowding

Polymer translocation is a ubiquitous phenomenon in crowded biological systems. Here, we investigate polymer translocation processes through a pore in a free, one-sided (asymmetric), and two-sided (symmetric) crowded environment using extensive Langevin dynamics simulation. We observed a location-dependent translocation rate of monomers showcasing counterintuitive behavior in stark contrast to the bead velocity along the polymer backbone. Free energy calculations for asymmetrically placed polymer indicate a critical number of segments directing to receiver-side translocation. For one-sided crowding, we have identified a critical crowding size revealing a non-zero probability of translocation towards the crowded side rather than the free side. Moreover, we have observed that shifting the polymer towards the crowded side compensates for one-sided crowding, yielding an equal probability akin to a crowder-free system. Unlike its one-sided crowding counterpart, under two-sided crowding, we propose a mechanism of how a slight variation in crowder size and packing fraction induces a polymer to switch its translocation direction, aligning well with the equal probability scenario in the crowd-free case. These conspicuous yet counter-intuitive phenomena are rationalized by simple theoretical arguments based on osmotic pressure and radial entropic forces.