Bruker BioSpin Solid State NMR, Руководство пользователя

Страница: 210 / 328

210 (327) BRUKER BIOSPIN User Manual Version 002
Relaxation Measurements
1. Start from standard CP parameters. The only additional calibration required is
the carbon RF field strength of the spin-lock pulse. This can be set indepen-
dently of the field strength for the cross-polarization. In principle, the strength
of this field can be set to any value (within probe limits) to probe motions on a
range of time scales. However, only at relatively large field strengths is true T
1r
relaxation the only significant relaxation pathway.
2. Set
pulprog
to cp90 and measure the required power level for a 70 kHz RF
field (3.57 µs 90 degree pulse).
3. Make a new data set with
iexpno
, change
pulprog
to cpxt1rho and set this
measured power as
pl11
.
4. Set up a variable pulse list for the incrementation of the spin-lock, with the
command
edlist vp
. Check that this list is set as the parameter
vplist
. Re-
member that this is a high-power pulse, so the duration should not be too long.
For the glycine sample, a possible set of times would be: 1ms, 2ms, 5ms,
10ms, 15ms, 20ms, 25ms, 30ms, 40ms, 50ms. This will not allow the signals
to decay completely, so is not ideal, but should not place undue stress on the
probe. Often a compromise must be reached between recording an ideal de-
cay curve and avoiding the risk of probe damage.
5. Change
parmode
to 2D and set other 2D parameters as for the other relax-
ation experiments.
6. Acquire spectrum with
zg
.
Data Processing
The data can be processed in the same way as the other relaxation experiments.
The slice with shortest spin-lock time contains most signal, so this slice should be
used for processing. The fitting function should be set to expdec, and
vplist
should be selected as the list file name, in the fitting function dialogue.
At 500 MHz, with a 60 kHz spin-lock field, the T
1
ρ
values should be approximately
400 ms and 48 ms for the carbonyl and alpha carbons respectively. The data for
the alpha carbon does not give a perfect fit to a single exponential, but this may
result from the relatively low spin-lock field allowing non-T
1
ρ
relaxation.
Indirect Relaxation Measurements 16.3
If proton relaxation measurements are desired, the considerable broadening of
the proton resonances seen at even high spinning speeds can make resolution of
individual components impossible. In such cases, indirect observation of proton
relaxation by X-nucleus observation can be used. A typical example would be at-
tempting to observe the proton relaxation of two components of a mixture or multi
phase material. In general, the proton spins within a single molecule are suffi-
ciently strongly coupled by the homonuclear dipolar coupling that different relax-
ation is not seen for the different sites. If the experiments are set up with short
contact times, the individual carbon signals will be derived only from directly bond-
ed protons, and thus any differences in proton relaxation within a molecule could
be isolated.
Such indirect observation can be implemented conveniently for both T
1
and T
1
ρ
relaxation. For T
1
, a proton saturation-recovery step is inserted prior to the cross-
polarization step in a standard CP sequence. The proton magnetization immedi-
ately prior to CP, and thus the observed carbon signal, depends on the extent of