Microanalysis of a single droplet produced by optical tweezers in an aqueous solution of bovine serum albumin

Abstract

The liquid–liquid phase separation of proteins is associated with the generation of membraneless organelles as well as of crystals and amyloid fibrils. The phase separation proceeds through spontaneous concentration fluctuations, and is therefore challenging to control. Here, we demonstrate a spatially and temporally controlled phase separation in an aqueous bovine serum albumin solution using optical tweezers. The protein solution contains trivalent ions and separates into two phases at elevated temperatures. Upon focusing a near-infrared (NIR) laser beam into the solution, a single microdroplet is produced at the laser focus site due to the photothermal effect and optical trapping. Taking advantage of this controllability, the droplets are individually characterized in terms of protein concentration, secondary structure, and growth modes. The droplets exhibit two distinct growth behaviors, depending on their size and secondary structure. This approach offers a platform for probing phase behavior at the single-droplet level with potential applications in crystallization and amyloidogenesis.