Patterns produced by dried droplets of protein binary mixtures suspended in water

- Salt-free mixtures of two proteins show complex patterns when drying.
- Protein-protein interactions define the formation of small aggregates and crystal clusters.
- Narrow coffee rings are formed at a relative protein concentration 1:1.
- The size of the coffee ring increases with protein concentration.

Patterns formed by the evaporation of a drop containing biological molecules have provided meaningful information about certain pathologies. In this context, several works propose the study of protein solutions as a model to understand the formation of deposits of biological fluids. Generally, dry droplets of proteins in a saline solution create complex aggregates. Here, we present an experimental study on the formation of patterns produced by the evaporation of droplet suspensions containing a protein binary mixture. We explore the structural aspect of such deposits by using optical and atomic force microscopy. We found that salt is unnecessary for the formation of complex structures such as crystal clusters, dendritic and undulated branches, and interlocked chains. Such structural features allow us to differentiate among protein binary mixtures. Finally, we discuss the potential use of this finding to reveal the presence of a protein suspensions, the folded and unfolded state of a protein, as well as their structural changes.

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