Quantitative phase microscopy enables precise and efficient determination of biomolecular condensate composition

Many cellular processes rely on condensed macromolecular phases termed biomolecular condensates. Despite recent progress in measurements and theoretical descriptions of several condensate properties, an understanding of their most basic feature, composition, remains elusive. Here we combined quantitative phase microscopy and the physics of sessile droplets to measure the shape and composition of individual model condensates. This technique requires 1000-fold less material than traditional approaches, achieves a precision of better than 2 %, and does not rely on fluorescent dyes or tags, which we show can significantly alter protein phase behavior. We find that condensed-phase protein concentrations in three model condensates span a broad range, from 80 to 500 mg/ml, pointing to a natural diversity in condensate composition specified by protein sequence. In addition to salt- and temperature-dependent binodals, we also report time-resolved measurements revealing that PGL3 condensates undergo a contraction-like process during aging. This leads to doubling of the internal protein concentration coupled to condensate shrinkage. We anticipate that this new approach will enable understanding the physical properties of biomolecular condensates and their function.