Counteraction of urea by trimethylamine N-oxide is due to direct interaction.
Trimethylamine N-oxide (TMAO) is a naturally occurring osmolyte that stabilizes proteins, induces folding, and counteracts the denaturing effects of urea, pressure, and ice. To establish the mechanism behind these effects, isotopic substitution neutron-scattering measurements were performed on aqueous solutions of TMAO and 1:1 TMAO-urea at a solute mole fraction of 0.05. The partial pair distribution functions were extracted using the empirical potential structure refinement method. The results were compared with previous results obtained with isosteric tert-butanol, as well as the available data from spectroscopy and molecular-dynamics simulations. In solution, the oxygen atom of TMAO is strongly hydrogen-bonded to, on average, between two and three water molecules, and the hydrogen-bond network is tighter in water than in pure water. In TMAO-urea solutions, the oxygen atom in TMAO preferentially forms hydrogen bonds with urea. This explains why the counteraction is completed at a 2:1 urea/TMAO concentration ratio, independently of urea concentration. These results strongly support models for the effect of TMAO on the stability of proteins based on a modification of the simultaneous equilibria that control hydrogen bonding between the peptide backbone and water or intramolecular sites, without any need for direct interaction between TMAO and the protein.
Meersman
Filip
F
Department of Chemistry, Katholieke Universiteit Leuven, Leuven, Belgium. Filip.Meersman@chem.kuleuven.be
Bowron
Daniel
D
Soper
Alan K
AK
Koch
Michel H J
MH
D001703:Biophysics / Q000379:methods; D003198:Computer Simulation; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D008744:Methylamines / Q000737:chemistry*; D008956:Models, Chemical; D009502:Neutrons; D010100:Oxygen / Q000737:chemistry; D010455:Peptides / Q000737:chemistry; D012542:Scattering, Radiation; D012996:Solutions; D012997:Solvents / Q000737:chemistry; D014508:Urea / Q000737:chemistry*; D014867:Water / Q000737:chemistry
D008744:Methylamines; D010455:Peptides; D012996:Solutions; D012997:Solvents; D014867:Water; D006859:Hydrogen; D014508:Urea; C005855:trimethyloxamine; D010100:Oxygen