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1 H- 1 H dipolar couplings by means of selective radiofrequency
pulses is achieved by “MAS in spin space”. 18,20 In high magnetic
fi elds, a regular MAS NMR probe is often suffi cient while in
lower magnetic fi elds (£ 400 MHz, 9.4 T) specially optimized
MAS probes are necessary limiting widespread application of
these combined line-narrowing techniques. Besides, reduction
(or removal) of the homonuclear dipolar couplings also scales
the chemical shift and hence requires careful calibration.
Figure 8: (left) Schematic representation of the LEE-GOLDBURG irradiation:
the combination of a certain radiofrequency field with an offset frequency
produces an effective radiofrequency field that is aligned at the magic angle.
(right) 1 H MAS NMR spectra at 850.1 MHz of α-glycine at 30 kHz MAS
(bottom) and wPMLG at 15 kHz MAS (top). Note the significant narrowing of
the line-shapes. 37
2.4 HETERONUCLEAR MULTI-QUANTUM NMR
Double-quantum or more generalized multi-quantum NMR is
also possible for heteronuclear spin pairs such as 13 C- 1 H or
15 N- 1 H, and offers signifi cant advantages for obtaining more
specifi c distance constraints while exploiting the much higher
site selectivity of 13 C and 15 N chemical shifts. Once specifi c
spin pairs of interest are identifi ed, precise measurement
of their corresponding dipole-dipole couplings via spinning
sidebands, similar to the 1 H- 1 H homonuclear case mentioned
above, provides an effi cient way to study molecular dynamics
in analogy to 2 H-NMR experiments, 15 but without the need of
elaborate isotope labeling. Rotational-Echo Double-Resonance
(REDOR) 29,42 currently is most popular for determining heteronuclear
dipole-dipole couplings in solids. It comprises a particularly
simple but versatile approach of recoupling, i.e. the
application of equally spaced p-pulses twice per rotor period
on one of the isotopes. If, however, one of the considered isotopes
has a rather low natural abundance (like e.g. 13 C or 15 N),
isotope-enriched samples are often required for the sake of
an appropriate signal-to-noise ratio and NMR measurement
time requirements. Though this can be tedious in some cases,
isotope labeling of specifi c sites (e.g. introduction of 13 C spin
pairs) offers the advantage of background suppression (e.g.
unwanted signals due to natural abundance 13 C spins), when
the initial magnetization is prepared by a double-quantum fi lter,
as recently exploited in the study of DNA packaging in bacteriophages
(DQF-REDOR). 43
In fact, the REDOR technique can be seen as the basis of many
more sophisticated ways of reintroducing homo- and heteronuclear
dipole-dipole as well as anisotropic chemical shift interactions
under MAS. 36,44,45 Pictorially speaking the mechanical
rotation by MAS in real space is partially offset by counter
rotations in spin space. REDOR can be used in two alternative
ways: on one hand, the signal intensity can be followed
as a function of the dipolar recoupling time thereby producing
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BUNSEN-MAGAZIN · 11. JAHRGANG · 2/2009
build-up curves, which are also termed dephasing curves. This
approach has been described in detail in a previous issue of
this series 19 and is not repeated here. On the other hand, we
can utilize the rotor encoded signal that evolves when an incremented
time period t 1 is inserted between two REDOR-type
pulse trains 46 (cf. fi gure 9).
Figure 9. Basic REDOR 29 pulse sequence (top) and the rotor-encoded version
(RE-REDOR) 46 (bottom).
The fi rst recoupling period creates a signal that corresponds
to the conventional REDOR sequence. Similar to the DQ case
detailed above, when using increments in t 1 of less than a rotor
period, rotor encoding yields characteristic MAS sideband patterns
comprised of (ideally) only odd sidebands. In fact, the frequency
distribution of the sideband pattern (that is observing
suffi cient intensity of sidebands of nth order) critically depends
on the underlying heteronuclear dipolar coupling D ij and can be
adjusted by setting appropriate recoupling times (cf. fi gure 7).
In conventional REDOR build-up curves, however, perturbing
multi-spin effects damp out the oscillatory behaviour of the polarization
transfer curve so that typically only the initial slope
of the curve is considered. 19 Such multi-spin effects appear as
additional even sidebands in RE-REDOR curves and are thus
separated from the spin pair signal rendering distance constraints
obtained from RE-REDOR curves more accurate. 47
Figure 10: Comparison between REDOR build-up curve and RE-REDOR sideband
pattern. Note that positions (ii) and (iii) are rather difficult to account for in the
build-up curve but can be exploited in the respective sideband pattern. 46
When applied under fast MAS in order to effi ciently reduce
the otherwise perturbing homonuclear dipole-dipole coupling,
REDOR can be also combined with multiple-quantum spec-