The Effect of Polymer Length in Phase Separation

Understanding the thermodynamics that drives liquid-liquid phase separation (LLPS) is quite important given the many numbers of diverse biomolecular systems undergoing this phenomenon. Regardless of the diversity, the processes underlying the formation of condensates exhibit physical similarities. Many studies have focused on condensates of long polymers, but very few systems of short polymer condensates have been observed and yet studied. Here we study a short polymer system of poly-Adenine RNA (polyA_n) and peptide ([RGRGG]_m) to understand the underlying thermodynamics of LLPS. We carried out MD simulations using the recently developed COCOMO coarse-grained (CG) model which revealed the possibility of condensates for lengths as short as 5-10 residues, which was then confirmed by experiment, making this one of the smallest LLPS systems yet observed. Condensation depends on polymer length and concentration, and phase boundaries were identified. A free energy model was also developed. Results show that the length dependency of condensates is driven solely by entropy of mixing and identifies a negative free energy (-ΔG) of phase separation, indicating the stability of the condensates. The simplicity of this system will provide the basis for understanding, more biologically realistic systems.