Bruker BioSpin Solid State NMR, Руководство пользователя

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User Manual Version 002 BRUKER BIOSPIN 143 (327)
11
Proton Driven Spin
Diffusion (PDSD)
11
PDSD is a 2D experiment that correlates a spin 1/2 nucleus to another spin of the
same species via homonuclear dipolar coupling or chemical exchange. The ex-
periment resembles the NOESY (Nuclear Overhauser Effect SpectroscopY) pulse
sequence in the liquid state by replacing the initial 90° pulse with a cross polariza-
tion scheme. Since spin diffusion between X-nuclei is measured, cross peak in-
tensities depend on the probability of interaction between different sites, which is
low with low natural abundance of NMR-active nuclei. Therefore, these experi-
ments usually require enrichment for nuclei like
13
C or
15
N in order to allow sensi-
ble measurement times.
The pulse program
cpspindiff
allows to run several types of PDSD experiments.
The CP preparation period excites the X nuclei. During the evolution time the X
magnetization evolves under the effect of the chemical shift interaction. The evo-
lution time ends with an X 90° pulse that stores the chemical shift information
along the z axis and marks the beginning of the mixing time. The X spins commu-
nicate through chemical exchange or spin diffusion, depending on the properties
of the material and the duration of the
mixing time. At the end of the mixing period,
another X 90° (read) pulse and the data acquisition follow. The usually strong
1
H-
X dipolar interaction is removed by high power
1
H decoupling during the prepara-
tion and the acquisition time. The
1
H decoupling is switched off during the mixing
to dephase the residual X transverse magnetization. Spin diffusion between X-nu-
clei is usually very slow and requires very long mixing times since the dipolar cou-
pling between all X-nuclei is small. However, turning the proton decoupling field
off during the mix period allows another process to take place: spin diffusion via
the proton spin system. Since the rare nuclei are strongly coupled to protons and
all protons are strongly coupled to each other, the flip flop transition rate is high
along the X
1
-H
1
-H
2
-X
2
-pathway. In fact, the spin exchange is almost solely due to
proton mediated mechanisms except when chemical exchange is present. At high
spin rates, spin diffusion may however still be slow since the X-H spins are decou-
pled. A simple procedure to recouple the X-H interaction is to irradiate the protons
at an RF field of n times the spin rate. These modified sequences are DARR (Di-
polar Assisted Rotational Resonance, T. Terao et al.) or RAD (Rf Assisted Diffu-
sion, see C.R. Morcombe et al).
The setup for all these sequences is rather robust, requiring only the
1
H to X Hart-
mann-Hahn condition and the X 90° hard pulse to be set. For RAD and DARR, it
is usually sufficient to calculate an RF power level corresponding to n times the
spin rate, which is then applied during the mixing period. Rotor synchronization of
the
mixing period is recommended in some cases, where cross peaks due to side-
bands need to be suppressed (de Jong et al.) or where spin diffusion is enhanced
by matching the spin rate with the chemical shift difference between the sites to
be correlated (M. Ernst et al.). PDSD is typically applied to high abundance nuclei
or labeled materials to detect through space proximity between spins. This exper-
iment has been often used on proteins as an alternative to Radio Frequency
DRiven spin diffusion (see
"RFDR" on page 137
). RFDR provides similar infor-
mation to PDSD but with a different mixing period. Here the term “frequency driv-