Primary source information:
Title: Impact of wet-dry cycling on the phase behavior and compartmentalization properties of complex coacervates
Authors: Hadi M. Fares, Alexander E. Marras, Jeffrey M. Ting, Matthew V. Tirrell, and Christine D. Keating.
Journal: Nat Commun 11, 5423 (2020)
Think about combining oil and water: droplets of oil might become interspersed in the water, but the oil and water do not mix and will soon separate into distinct regions. Similarly, when oppositely charged polymers are dissolved in water at high enough concentrations, those polymers can form dense droplets that do not mix with the surrounding solution (Fig 1). Droplet formation is dependent on electrostatic interactions between the charged polymers, and the strength of these electrostatic interactions depends on the polymer concentration and the salt concentration.
Charged polymers may have formed billions of years ago on Earth, before the formation of the first cells. Cycles of dehydration and rehydration are helpful for forming polymers via condensation reactions, and so many researchers favor the hypothesis that life originated in shallow pools on land. Would these charged polymers still be able to form droplets during cycles of dehydration and rehydration? And what roles might these droplets have had during the origin of life on Earth? These are big questions that Fares et al. seek to understand with their research (Fig 2).
First, the researchers selected polymers that plausibly could have been available on the early Earth, and then measured a phase diagram for these polymers (Fig 3, top). We often use phase diagrams to determine whether a compound (for example, water) will be a solid, liquid, or gas at a given temperature and pressure. In exactly the same way, these researchers created a phase diagram to learn at what salt and polymer concentrations would their charged polymers mix into the surrounding solution (1 phase), and at what concentrations would the polymers form droplets (2 phase). The researchers observed that dehydration would change a sample’s position within the phase diagram (Fig 3, bottom): for example, a sample that had droplets initially might mix (2 phase –> 1 phase) during the course of dehydration. But these changes are completely reversible! As soon as the researchers rehydrated their samples, the droplets re-formed. So during periods of cyclic dehydration/rehydration on the early Earth, droplets readily dissolve and re-form.
Next, the researchers added RNA molecules, and observed that most of the RNA enters the droplets (Fig 4)! As the researchers dehydrated their sample, they observed that the concentration of RNAs inside the droplets remained constant. Next, the researchers intentionally disabled the florescence of RNAs within a defined spatial region (Fig 5, white circle). This technique allowed the researchers to measure the diffusion of new RNAs into that region. They observed that RNA diffusion into the droplets increased as the sample dried. So as the environment became more dehydrated, the surrounding solution became more similar to the droplets. Upon rehydration, the droplets and surrounding solution returned to their initial state.
These researchers have used a variety of techniques to understand the properties of charged polymer droplets during cycles of dehydration and rehydration. Their results suggest that droplet properties do change during dehydration, but upon rehydration the droplets return to their initial state. Additionally, the droplets are able to sequester high concentrations of RNA, which might have been a critical step towards the origin of life.