Water's Second Critical Point Tunes Proteins

01The evidence

• Direct observation of liquid-liquid phase coexistence in deeply supercooled water with a polarizable multipole model provides the strongest simulation evidence yet that a second critical point exists.PNAS, 2026
• Experimental work has been closing in on a second critical point of water, with the debate moving from whether it exists to where it sits.Physics Today

02The tension

The liquid-liquid critical point is a contested feature of pure water, usually discussed in the context of water's anomalies and almost never in the context of biology. Protein hydration is modelled with a continuum or with a few layers of structured water, neither of which has a critical point.

03The theory

If water has two liquid structures in equilibrium, then a hydration shell can switch between them. Proteins are covered in surfaces of different chemistry, so different patches of a protein would favour different structures, and the shell would be a mosaic rather than a uniform layer.

A critical point in the bulk liquid means the correlation length of that mosaic diverges near the critical temperature. That would show up as a maximum in the heat capacity of hydration, which is exactly the quantity that dominates the thermodynamics of protein unfolding.

So the prediction is specific: the anomalous heat capacity of unfolding should peak at a temperature tied to the critical point, and osmolytes that shift the critical point should shift the anomaly by a matching amount. Pressure should move both along the same line.

The theory is attractive because it converts a debate in physical chemistry into a biological measurement. If the coupling exists, the critical point is not a curiosity but a parameter that evolution has tuned against, and the hydration of proteins becomes a many-body problem rather than a surface one.

04What this is not

05Predictions

• P1The heat capacity of protein unfolding should show a maximum near the liquid-liquid critical temperature, not a monotonic dependence.
• P2Osmolytes that shift the critical point should shift the unfolding anomaly by a matching amount.
• P3Pressure denaturation should follow the critical line rather than a generic pressure dependence.

06What would refute it

No heat capacity anomaly, or an anomaly that does not track the critical point under osmolytes and pressure, would leave hydration continuous.

07What it would change

It would make the water debate biologically consequential and give protein stability a physical parameter instead of an empirical one.