title: An investigation of the hydrogen-bonding structure in bilirubin by 1H double-quantum magic-angle spinning solid-state NMR spectroscopy.
abstract: The complex hydrogen-bonding arrangement in the biologically important molecule bilirubin IXalpha is probed by using 1H double-quantum (DQ) magic-angle spinning (MAS) NMR spectroscopy. Employing fast MAS (30 kHz) and a high magnetic field (16.4 T), three low-field resonances corresponding to the different hydrogen-bonding protons are resolved in a 1H MAS NMR spectrum of bilirubin. These resonances are assigned on the basis of the proton-proton proximities identified from a two-dimensional rotor-synchronized 1H DQ MAS NMR spectrum. An analysis of 1H DQ MAS spinning-sideband patterns for the NH protons in bilirubin allows the quantitative determination of proton-proton distances and the geometry. The validity of this procedure is proven by simulated spectra for a model three-spin system, which show that the shortest distance can be determined to a very high degree of accuracy. The distance between the lactam and pyrrole NH protons in bilirubin is determined to be 0.186 +/- 0.002 nm (corresponding to a dominant dipolar coupling constant of 18.5 +/- 0.5 kHz). The analysis also yields a distance between the lactam NH and carboxylic acid OH protons of 0.230 +/- 0.008 nm (corresponding to a perturbing dipolar coupling constant of 9.9 +/- 1.0 kHz) and an H-H-H angle of 122 +/- 4 degrees. Finally, a comparison of 1H DQ MAS spinning-sideband patterns for bilirubin and its dimethyl ester reveals a significantly longer distance between the two NH protons in the latter case.
authors.affiliation: Max-Planck-Institut für Polymerforschung, Postfach 3148, D-55021 Mainz, Germany.
authors.forename: S P
authors.initials: SP
authors.lastname: Brown
authors.forename: X X
authors.initials: XX
authors.lastname: Zhu
authors.forename: K
authors.initials: K
authors.lastname: Saalwächter
authors.forename: H W
authors.initials: HW
authors.lastname: Spiess
mesh_terms: D001663:Bilirubin / Q000737:chemistry*; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation
chemical_list: D001663:Bilirubin