title: Isotopic fractionation in proteins as a measure of hydrogen bond length.
abstract: If a deuterated molecule containing strong intramolecular hydrogen bonds is placed in a hydrogenated solvent, it may preferentially exchange deuterium for hydrogen. This preference is due to the difference between the vibrational zero-point energy for hydrogen and deuterium. It is found that the associated fractionation factor Φ is correlated with the strength of the intramolecular hydrogen bonds. This correlation has been used to determine the length of the H-bonds (donor-acceptor separation) in a diverse range of enzymes and has been argued to support the existence of short low-barrier H-bonds. Starting with a potential energy surface based on a simple diabatic state model for H-bonds, we calculate Φ as a function of the proton donor-acceptor distance R. For numerical results, we use a parameterization of the model for symmetric O-H⋯O bonds [R. H. McKenzie, Chem. Phys. Lett. 535, 196 (2012)]. We consider the relative contributions of the O-H stretch vibration, O-H bend vibrations (both in plane and out of plane), tunneling splitting effects at finite temperature, and the secondary geometric isotope effect. We compare our total Φ as a function of R with NMR experimental results for enzymes, and in particular with an earlier model parametrization Φ(R), used previously to determine bond lengths.
authors.affiliation: School of Mathematics and Physics, University of Queensland, Brisbane 4072, Australia.
authors.forename: Ross H
authors.initials: RH
authors.lastname: McKenzie
authors.affiliation: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India.
authors.forename: Bijyalaxmi
authors.initials: B
authors.lastname: Athokpam
authors.affiliation: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India.
authors.forename: Sai G
authors.initials: SG
authors.lastname: Ramesh
mesh_terms: D003903:Deuterium / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D011506:Proteins / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D011506:Proteins; D012997:Solvents; D006859:Hydrogen; D003903:Deuterium