Protein stability at negative pressure

We record proton NMR spectra of the protein ubiquitin at 1 atmosphere pressure and at negative pressures (under tension), under conditions where the native and denatured states are nearly equally populated. Analysis of the unique histidine aromatic resonance of ubiquitin shows that negative pressure destabilizes the protein, in accord with a quadratic free energy dependence on pressure and temperature previously suggested in the literature. Our molecular dynamics simulations at negative pressure agree with the experimental result. In addition, molecular dynamics predicts a turnaround of the folding free energy at very low pressure. An 'island of stability' may exist at very negative pressures, where the protein is likely to fold into low density fluctuations of the solvent.