D(2)O as an Imperfect Replacement for H(2)O: Problem or Opportunity for Protein Research?

[Image: see text] D(2)O is commonly used as a solvent instead of H(2)O in spectroscopic studies of proteins, in particular, in infrared and nuclear-magnetic-resonance spectroscopy. D(2)O is chemically equivalent to H(2)O, and the differences, particularly in hydrogen-bond strength, are often ignored. However, replacing solvent water with D(2)O 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 D(2)O and H(2)O. 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 H(2)O/D(2)O-isotope effect on water–water and water–protein interactions. Such improved understanding may change the differences between H(2)O and D(2)O as biomolecular solvents from an elephant in the room to an opportunity for protein research.