The unusual behavior of proteins in saliva droplets could explain a long-puzzling property of airborne viruses. Actually, pathogens like Sars-CoV-2 should be destroyed very quickly in dry air because the water evaporates from the droplets in the aerosol – but exactly the opposite is observed. Respiratory pathogens seem to spread very well in dry air, such as that found indoors in winter. A working group led by Ryan Davis from Trinity University in Texas now proposes that proteins in saliva droplets solidify into a kind of glass that protects the viruses contained from being damaged by dehydration.
When droplets of saliva become airborne, the water in them begins to evaporate. The concentration of salts increases, and too much salt destroys the outer membrane of viruses such as Sars-CoV-2 or influenza. One would therefore expect that viruses in aerosols would survive for a particularly long time in humid air and would become inactive more quickly the drier the air is. In fact, however, this relationship only seems to hold up to a relative humidity of 50 to 80 percent; if the air gets drier, the viruses in the aerosol survive longer again.
The working group now explains this with a solid capsule made of a protein gel, which forms in the droplets in dry air. As she reports in the journal »PNAS«, aerosol droplets from simulated saliva then no longer behave like droplets, but more like elastic or even solid bodies. The microscope showed that the liquid in the droplets had split into a liquid and a solid phase. The team interprets this as evidence of a glass transition. Calcium ions connect the molecular chains of the proteins with each other so that they separate from the liquid in the droplet as solid material.