Polymer conformational entropy as the driver of complex coacervation.

Understanding the mechanisms leading to complex coacervation is a fundamental challenge in biomolecular sciences. Here, we explored the condensation transition and pretransitional behavior of an archetypal system consisting of polyuridylic acid chains (PolyU) and short cationic peptides in buffered aqueous solutions. By combining static and dynamic light scattering with confocal microscopy, we determined the location of the transition and the partitioning of PolyU and peptides between the coexisting phases as a function of the total peptides/PolyU ratio. We find that upon adding peptides in the system, the size of PolyU coils in the single-phase region is progressively reduced by peptide-mediated intrachain bonds. Such conformational constraints become less severe in the dense phase, where part of the bonds become interchain as indicated by the sticky-reptation-type kinetics observed by dynamic light scattering. We propose a phenomenological model in which we include the loss in PolyU conformational entropy induced by peptide decoration and show that this quantity is large enough to be the main driver of the condensation transition. The model well reproduces the observed molecular partitioning between phases. We argue that the role of coil conformational relaxation, here demonstrated for a specific system, might be of general relevance in complex coacervation.