User Manual Version 002 BRUKER BIOSPIN 193 (327)
15
PISEMA 15
Polarization Inversion Spin Exchange at the Magic Angle is an experiment that
correlates the chemical shift of a spin 1/2 X nucleus with the heteronuclear dipolar
coupling to another spin 1/2 nucleus. Most of the applications so far reported have
been in the field of structural biology, therefore, the X nucleus is normally
13
C or
15
N and the other hetero-nucleus
1
H. The experiment provides orientation infor-
mation on the vector connecting the
13
C or
15
N and the
1
H nucleus. The achiev-
able high resolution of the CS as well as the dipole coupling makes the
experiment well suited for 3D NMR experiments on aligned systems or single
crystals.
Unlike normal FSLG experiments, where the dipolar and CS interactions are
scaled by,
the scaling factor for the heteronuclear dipolar interaction is
because the coupling takes place in the transverse plane of the rotating frame,
the spin locked state. (The projection is from the tilted frame (locked
1
H spin sys-
tem) to the transverse plane of the rotating frame system (spin locked
15
N spin
system), i.e.,
the scaling of the heteronuclear dipolar coupling strength.
Through the combination of spin exchange (dipolar flip flop term) and the homo-
nuclear decoupling using FSLG, PISEMA achieves a line width that is an order of
magnitude better than its predecessor, the separated local field experiment.
The central line in the dipolar dimension can be caused among other things by a
proton frequency offset introducing a constant term in the time domain signal.
That offset frequency makes also the splitting larger. See additional test proce
-
dures in the paper about „experimental aspects of multidimensional solid state
correlation spectroscopy“.
PISEMA is not very sensitive to the exact Hartmann-Hahn condition. A mismatch
has only little effect on the dipolar coupling. The scaling factor in the indirect di
-
mension depends of the
1
H resonance offset and a wrong
1
H carrier frequency
can cause besides a wrong scaling factor some intensity loss and, as mentioned
above, a zero frequency contribution. Diligent adjustment of the LG condition
and the rf-carrier is critical for accurate measurement of the dipolar coupling as
the splitting increases quadratic with increasing (proton) frequency offset.
Simulations of the spin dynamics show that the heteronuclear term in the Hamilto-
nian leads to a complicated spectrum for small heteronuclear dipolar couplings
(usually introduced by remote protons), see Z. Gan's paper for more information.
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