Mellitic Acid as a Stable Abiotic Precursor for Liquid-Liquid Phase Separation: An RNA-Independent Pathway for...

Apart from playing the well-recognized roles in the self-assembly of intracellular machinery liquid-liquid phase separation (LLPS) constitutes a hypothetical, yet compelling scenario for prebiotic compartmentalization. One challenge facing this idea is the absence (or scarcity) of typical biopolymer precursors of liquid droplets, such as RNA, on early Earth. We demonstrate that mellitic acid (MAc), an abiotic component found in certain minerals and a marker of organic matter in astrobiology, forms liquid droplets with poly-L-lysine (PLL) across a broad pH range and various mixing ratios. The chain-length threshold of PLL capable of forming droplets with MAc ((L-Lys)₆) is significantly lower than when ATP, the primary intracellular energy currency, is used to induce LLPS in PLL. MAc-PLL droplets are highly dynamic systems that respond reversibly to added ATP through an efficient mixing and demixing behavior. The observed effects of pH, ionic strength, temperature, and pressure, as well as isothermal titration calorimetry (ITC) data, are all consistent with coulombic interactions being the key contributor to maintaining the droplet state in the MAc-PLL system. We propose that the capacity to induce LLPS with short oligocations, along with abiotic synthesis routes, positions mellitic acid as a viable candidate for an ingredient of early prebiotic membraneless protocells.