Near-identical macromolecules spontaneously partition into concentric circles.

Although separation is entropically unfavourable, it is often essential for our life1,2. The separation of very similar macromolecules such as deoxyribonucleic acids (DNAs) and their single nucleotide variants is difficult but holds great advantage for the progress of life science3. Here we report that a particular liquid-liquid phase separation (LLPS) at a solid-liquid interface led to the partitioning of DNAs with nearly identical structures. We found this intriguing phenomenon when we did drop-casting onto a glass plate an aqueous ammonium sulfate dispersion of phase-separated droplets comprising a homogeneous mixture of poly(ethylene glycol) (PEG) samples with different termini. Even when the molecular weights of their PEG parts were identical to each other, terminally different PEGs spread competitively at the solid-liquid interface and partitioned into micrometre-scale concentric circles. We found that this competitive spreading was induced by an ammonium sulfate layer spontaneously formed on the glass surface. We successfully extended the above mechanism to partitioning a mixture of nearly identical DNAs into concentric circles followed by their selective extraction using the salting-in effect. We could isolate a human cancer-causing single nucleotide variant in 97% purity from its 1:1 mixture with the original DNA.