The surface charge of an open water surface is crucial for solvation phenomena and interfacial processes in aqueous systems. However, the magnitude of the charge is controversial, and the physical mechanism of charging remains incompletely understood. Here we identify a previously overlooked physical mechanism determining the surface charge of water. Using accurate charge measurements of water microdrops, we demonstrate that the water surface charge originates from the electrostatic effects in the contact line vicinity of three phases, one of which is water. Our experiments, theory, and simulations provide evidence that a junction of two aqueous interfaces (e.g., liquid–solid and liquid–air) develops a pH-dependent contact potential difference Δϕ due to the longitudinal charge redistribution between two contacting interfaces. This universal static charging mechanism may have implications for the origin of electrical potentials in biological, nanofluidic, and electrochemical systems and helps to predict and control the surface charge of water in various experimental environments.

One key feature of proteins that form liquid droplets by phase separation inside a cell is multivalency—the presence of multiple sites that mediate interactions with other proteins. We know little about the variation of multivalency on evolutionary time scales. Here, we investigated the long-term evolution (∼600 million years) of multivalency in fungal mRNA decapping subunit 2 protein (Dcp2), and in the FET protein family. We found that multivalency varies substantially among the orthologs of these proteins. However, evolution has maintained the length scale at which sequence motifs that enable protein-protein interactions occur. That is, the total number of such motifs per hundred amino acids is higher and less variable than expected by neutral evolution. To help explain this evolutionary conservation, we developed a conformation classifier using machine-learning algorithms. This classifier demonstrates that disordered segments in Dcp2 and FET proteins tend to adopt compact conformations, which is necessary for phase separation. Thus, the evolutionary conservation we detected may help proteins preserve the ability to undergo phase separation. Altogether, our study reveals that the length scale of multivalent interactions is an evolutionarily conserved feature of two classes of phase-separating proteins in fungi and vertebrates.

One key feature of proteins that form liquid droplets by phase separation inside a cell is the presence of multiple sites – multivalency – that mediate interactions with other proteins. We know little about the variation of multivalency on evolutionary time scales. Here, we investigated the long-term evolution (∼600 million years) of multivalency in fungal mRNA decapping subunit 2 protein (Dcp2), and in the FET protein family. We found that multivalency varies substantially among the orthologs of these proteins. However, evolution has maintained the length scale at which sequence motifs that enable protein-protein interactions occur. That is, the total number of such motifs per hundred amino acids is higher and less variable than expected by neutral evolution. To help explain this evolutionary conservation, we developed a conformation classifier using machine-learning algorithms. This classifier demonstrates that disordered segments in Dcp2 and FET proteins tend to adopt compact conformations, which is necessary for phase separation. Thus, the evolutionary conservation we detected may help proteins preserve the ability to undergo phase separation. Altogether, our study reveals that the length scale of multivalent interactions is an evolutionarily conserved feature of two classes of phase-separating proteins in fungi and vertebrates.