Bruker BioSpin Solid State NMR, User Manual

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Double-CP
User Manual Version 002 BRUKER BIOSPIN 259 (327)
one might think, since the transfer efficiency depends strongly on the timing
and RF fields of the HH match. The proton T
1 ρ
of the glycine NH
2
protons
(from which the nitrogen is polarized) is fairly short, so the polarization transfer
is not efficient. On a fully labelled sample, a maximum enhancement factor of
8.3 is possible (5 protons transfer to one nitrogen). Comparing the cross polar-
ized
15
N spectrum to the directly observed spectrum (using
hpdec
and 90 de-
gree pulses at 4 sec repetition) one can measure the enhancement factor
rather easily. Without optimization, the enhancement factor may be as low a
5fold. It should be at least 6.5fold, more than 7.5fold is hard to achieve. To
achieve a good result, the HH RF-fields should be set as high as possible with
a contact time of 4 msec (higher proton RF fields yield a longer proton T1
ρ and
allow longer contact times). Of course, the RF field is limited by transmitter
power and probe breakthrough limits. Note the optimum power levels (
sp0
and
pl1
) and contact time (
p15
).
Setup of the Double CP Experiment 20.2.3
1. Read the reference carbon data set and generate a new data set using
edc
or
iexpno
.
2. Select the pulse program doubcp. Set the optimum
15
N cp parameters as
found in the previous step (set
sp0
,
p15
and
pl3
for proton to
15
N cp). Set
o3
close to the
15
N peak position.
3. Now we have to select the appropriate parameters for the nitrogen to carbon
magnetization transfer. In the standard
doubcp
pulse program,
p16
is used as
the second contact time, and the shapes
sp1
and a square pulse at
pl5
are
specified for the
13
C and
15
N contact, so
p16
,
sp1
(
13
C power level) and
pl5
(
15
N power level) are the relevant parameters. The C-N contact consists of a
square pulse on one channel and a ramp or adiabatic shape on the other
channel. Using a square pulse on the
15
N channel is preferred, but the se-
quence can be rewritten to use a
13
C square pulse and have the shape on the
15
N channel.