Biomolecular condensates perform their cellular functions via the exclusion or enrichment of molecules. Analyzing the composition of condensates is important for understanding their properties and functions. A considerable amount of work has gone into understanding the nucleic acid and protein components of condensates; however, much less is known about partitioning of small molecules and the determinants that govern their enrichment or exclusion under the conditions where they form. Now Thody et al. have measured the partitioning of approximately 1,700 biologically relevant small molecules into 4 different condensates formed through phase separation of unrelated proteins and protein–DNA mixtures.

To ensure they analyzed the partitioning of a wide range of structures, the team used a diverse library of metabolites and drugs that were not constructed using a common molecular core. Accurately measuring the partition coefficients required the team to develop two assays — one based on centrifugation followed by separation and then mass spectrometry; the other assay was based on fluorescence. These showed that partition coefficients of the small molecules varied by nearly six orders of magnitude but showed similarities between different condensates, suggesting that condensates formed from different compositions of macromolecules produce a physically similar internal environment.