Salt-induced protein phase transitions in drying drops.

Protein phase transitions in drying sessile drops of protein-salt-water colloidal systems were studied by means of optical and atom-force microscopy. The following sequence of events was observed during drop drying: attachment of a drop to a glass support; redistribution of colloidal phase due to hydrodynamic centrifugal stream; protein ring formation around the edge; formation of protein spatial structures inside a protein ring that pass into gel in the middle of the drop; salt crystallization in the shrinking gel. It was assumed that rapid drying of a protein ring over the circle of high colloidal volume fraction and low strength of interparticle attraction leads to formation of colloidal glass, whereas gel forms only in the middle of the drop at very low protein volume fraction and strong attraction between the particles. Before gelation, colloidal particles form fractal clusters. In dried drops of salt-free protein solutions, no visual protein structures were observed. Structural evolution of protein in sessile drying drops of protein-salt aqueous colloidal solutions is discussed on the basis of experimental data.