Biomedical Engineering Reference
In-Depth Information
Recent advances in paramagnetic NMR spectroscopy have provided the
theoretical and computational tools necessary to study these states in
populations as low as 0.5%.
2
Paramagnetic NMR spectroscopy relies on the
effects of unpaired electrons from paramagnetic centers to supply long-range
distance information and/or bond vector orientations. These effects include
pseudo-contact shifts (PCS), paramagnetic relaxation enhancement (PRE)and
partial alignment resulting in residual dipolar coupling (RDC).
3
NMR spectroscopy offers two distinct approaches for the study of lowly
populated states (accounting for 0.5-50% of the total population). First, the
three paramagnetic effects mentioned above, PCS, RDC and PRE, can provide
structural restraints. These restraints describe the time-average of a set of
conformations that can be used to construct a model of such an ensemble.
4
Second, both structural and kinetic information of lowly populated states can
be obtained with relaxation dispersion (RD). This technique relies on exchange
broadening due to a difference in resonance frequency for nuclei in the major
and minor states.
1
The resulting data are fit to an exchange model using a
series of equations representing a two- (or more) state system.
5
This chapter focuses on paramagnetic approaches for studying lowly
populated states. The theory behind paramagnetic effects will be explained,
followed by several recent examples to illustrate their applications.
Furthermore, new developments combining paramagnetism and RD will be
discussed briefly. This chapter uses examples from solution NMR and does not
aim to provide a comprehensive overview; for further reading, several in-depth
reviews on paramagnetic NMR are available.
2-4,6,7
6.1.2 Paramagnetic Centers
Paramagnetic NMR spectroscopy relies on observable effects induced by
paramagnetic centers. These effects are dependent on two main properties of
the center; the electronic longitudinal relaxation rate and the time-averaged
anisotropic component of the magnetic susceptibility tensor (x-tensor). Of the
most commonly observed paramagnetic effects, PREs are dependent on the
former whereas PCS and RDCs are dependent on the latter. Paramagnetic
centers with an isotropic electron distribution (nitroxide radicals, Mn
2+
,Gd
3+
),
which generally have a low electronic relaxation rate (10
7
-10
10
s
21
), produce
large PREs but no PCS or RDCs. Alternatively, centers with anisotropic
electron distribution (Co
2+
,Fe
3+
and most lanthanides), which generally have a
high electronic relaxation rate (10
10
-10
13
s
21
), cause PCS and RDCs and only
limited PRE.
8
See Table 6.1 for a summary of the paramagnetic effects of
transition metals and lanthanides commonly used for paramagnetic NMR.
Until now, most studies have used either a nitroxide spin-label or a
transition metal as the paramagnetic center. However, lanthanides are
particularly well suited for paramagnetic NMR spectroscopy. First, their
unpaired electrons are in the inner f orbitals so they tend not to delocalizeto
bound ligands. This limits effects to through-space dipolar interactions.
25
They
