- S. P. Brown, X. X. Zhu, K. Saalwächter, and H. W. Spiess.
An Investigation of the Hydrogen-Bonding Structure in Bilirubin
by 1H Double-Quantum Magic-Angle
Spinning Solid-State NMR Spectroscopy.
J. Am. Chem. Soc.
123, 4275-4285 (2001).
The complex hydrogen-bonding arrangement in the biologically important molecule
bilirubin IXa is probed by 1H double-quantum
(DQ) magic-angle spinning (MAS) NMR spectroscopy. Employing fast MAS (30 kHz)
and a high magnetic field (16.4 Tesla), 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 by means of the proton-proton proximities identified by a
two-dimensional rotor-synchronised 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 of 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°. 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.