NMR Water Artifact Removal - Biomedical Signal Analysis: Contemporary Methods and Applications - page 386

14.4 NMR Spectra

Modern multidimensional NMR spectroscopy [76] is a versatile tool
for the determination of the native 3-D structure of biomolecules in
their natural aqueous environment. Proton NMR (i.e. the observation
of the magnetization of the 1 H nuclei in the probe), is an indispensable
contribution to this structure determination process but is hampered
by the presence of the very intense water (H 2 O) proton signal. Since
it is the most intense signal in two-dimensional spectra, it causes the
most trouble with baseline distortions and t 1 noise, and it can obscure
weak signals lying under its edges. Because of its intensity it also
causes severe dynamic range problems; hence sophisticated experimental
protocols have been developed to suppress the water signal as far as
possible. All these procedures introduce spectral distortions that can be
neither avoided nor removed, and prevent the analysis of the spectral
region close to the water resonance. Hence equivalent spectra of the
molecules dissolved in heavy water (D 2 O) also have to be taken which
raises additional problems not the least being that heavy water differs
suciently in its physical-chemical properties from light water to cast a
doubt on a direct comparison of both spectra. Hence it is interesting to
consider whether (BSS) techniques can contribute to the removal of the
water artifact in such spectra without regard to any sophisticated water
suppression pulse protocols except a simple presaturation to reduce the
dynamic range problem. However, even a long, weak pulse on the water
resonance can bleach nearby solute proton resonances and can also affect
other signals through crossrelaxation or chemical exchange.
Concerning structure determination, homonuclear 2-D NOESY spec-
tra are a must. They rely on the nuclear Overhauser effect, the change
in the intensity of the resonance of one spin species upon saturation of
an adjacent spin with which it has an appreciable dipole-dipole inter-
action. They provide information about crossrelaxation rates, which for
protons depend mainly on magnetic dipolar interactions. The latter vary
with distance as r −6 , and hence allow distances to neighboring nuclei to
be determined. Loosely speaking, one can consider the NOE effect an
atomic ruler, which allows the 3-D structure to be determined if enough
NOEs are available experimentally. A two-dimensional NMR time do-
main signal, called free induction decay (FID), is modeled by a sum of