Neutral polymer conformations with attractive bridging crowder interactions: role of crowder size

Extensive molecular dynamics simulations were conducted to explore the conformational phase diagram of a neutral polymer in the presence of attractive crowders of varying sizes. For weakly self-attractive polymers, larger crowders induce a transition from an extended state to a bridging-crowder-induced collapse (CB phase) at lower polymer-crowder interaction strengths. Similarly, for strongly self-attractive polymers, a transition from a crowder-excluded collapse (CI phase) to the CB phase occurs with larger crowders and stronger polymer-crowder attraction. The simulations reveal that the collapsed states in both weakly and strongly self-attractive polymers are identical when small bridging crowders are involved. Near the transition threshold for weakly self-attractive polymers, increasing crowder size leads to complex transitions, including collapse, extension, and re-collapse. Additionally, a confined collapse (CC phase) is observed, where the polymer is confined to interstitial spaces among large crowders. Strongly self-attractive polymers show similar transitions, but only with smaller crowders, as larger crowders suppress these changes. These findings underscore the crucial influence of crowder size on the conformational behavior of neutral polymers, especially under varying degrees of polymer-crowder interaction.