Chemistry Reference
In-Depth Information
15
N CT-CPMG Relaxation Dispersion Experiment
4.2
The CPMG
R
2
experiment has long been applied to detect chemical exchange
phenomena [
95
,
96
,
102
]. A simple case is when the exchange rate is larger than
the intrinsic relaxation rate,
R
2
(determined by dipolar and CSA relaxation, details
described below), and the observed
R
2
is the sum of
R
2
and the chemical exchange
contribution,
R
ex
,[
98
,
103
]:
R
2
þ
R
2
t
CP
¼
R
ex
ðt
CP
Þ:
(4)
t
CP
is a half duration of CPMG interpulse delay, and the effective field
strength is defined by
Here,
t
CP
). CPMG experiments have also been
applied to characterize protein conformational equilibria and kinetics on the
milli- to microsecond time scale [
103
-
108
]. The current form of the constant-
time (CT) version of the experiment (Fig.
3
) consists of the following two
distinct steps.
One step averages the contributions to
R
2
from the inphase and the antiphase
(N
XY
and N
XY
H
Z
, respectively) components by having tandem CPMG periods
linked by an rc-INEPT [
109
]. Averaging is achieved because one CPMG period
starts from the inphase coherence and the other from the antiphase coherence for all
values of
n
CP
¼
1/(4
t
CP
[
109
]. This insures that
R
2
remains the same at all values of
t
CP
.
Recently, use of a relatively strong
1
H CW field (B
1
>
15 kHz) that decouples the
inphase and the antiphase terms has been introduced [
110
]. The approach is
advantageous because the maximum value of
t
CP
that can be achieved increases
twofold, using the same transverse relaxation duration. However, application of a
strong
1
H CW field may cause heating by dielectric or inductive losses for samples
containing high salt at high magnet field strength [
111
-
114
].
The other step in the CPMG relaxation dispersion experiment is determination of
R
2
values by two-point intensity measurement: one is the magnetization at the
initial time and the other is the magnetization at time
T
CP
[
115
,
116
]. Although
Fig. 3 Schematic flow chart of the experiment and analysis of constant-time Curr-Purcell-
Meiboom-Gill (CT-CPMG) R
2
dispersion
