Liquid-Liquid Phase Separation of Peptide/Oligonucleotide Complexes in Crowded Macromolecular Media

The membraneless organelles (MLOs) and coacervates of oppositely charged polyelectrolytes are both formed by liquid-liquid phase separation. To reveal how the crowded cell interior regulates the MLOs, we chose the coacervates formed by peptide S5 and single-stranded oligonucleotide (ss-oligo) at 1:1 charge ratio and investigated the phase separation processes in polyacrylamide (PAM) and poly(ethylene oxide) (PEO) media at varying concentrations. Results show that the droplet formation unit is the neutral primary complex, instead of individual S5 or ss-oligo. Therefore, the coacervation process can be described by the classic theory of nucleation and growth. The dynamic scaling relationships show that S5/ss-oligo coacervation undergoes in sequence the heterogeneous nucleation, diffusion-limited growth, and Brownian motion coalescence with time. The inert crowders generate multiple effects, including accelerating the growth of droplets, weakening the electrostatic attraction, and slowing down or even trapping the droplets in the crowder network. The overall effect is that both the size and size distribution of the droplets decrease with increasing crowder concentration, and the effect of PEO is stronger than that of PAM. Our study provides a further step toward a deeper understanding of the kinetics of MLOs in crowded living cells.