D₂O as an Imperfect Replacement for H₂O: Problem or Opportunity for Protein Research?

D₂O is commonly used as a solvent instead of H₂O in spectroscopic studies of proteins, in particular, in infrared and nuclear-magnetic-resonance spectroscopy. D₂O is chemically equivalent to H₂O, and the differences, particularly in hydrogen-bond strength, are often ignored. However, replacing solvent water with D₂O can affect not only the kinetics but also the structure and stability of biomolecules. Recent experiments have shown that even the mesoscopic structures and the elastic properties of biomolecular assemblies, such as amyloids and protein networks, can be very different in D₂O and H₂O. We discuss these findings, which probably are just the tip of the iceberg, and which seem to call for obtaining a better understanding of the H₂O/D₂O-isotope effect on water-water and water-protein interactions. Such improved understanding may change the differences between H₂O and D₂O as biomolecular solvents from an elephant in the room to an opportunity for protein research.