title: Counteraction of urea by trimethylamine N-oxide is due to direct interaction.
abstract: 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.
authors.affiliation: Department of Chemistry, Katholieke Universiteit Leuven, Leuven, Belgium. Filip.Meersman@chem.kuleuven.be
authors.forename: Filip
authors.initials: F
authors.lastname: Meersman
authors.forename: Daniel
authors.initials: D
authors.lastname: Bowron
authors.forename: Alan K
authors.initials: AK
authors.lastname: Soper
authors.forename: Michel H J
authors.initials: MH
authors.lastname: Koch
mesh_terms: 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
chemical_list: D008744:Methylamines; D010455:Peptides; D012996:Solutions; D012997:Solvents; D014867:Water; D006859:Hydrogen; D014508:Urea; C005855:trimethyloxamine; D010100:Oxygen