Biomedical Engineering Reference
In-Depth Information
In the spin-diffusion limit, the relaxation of the side-chain methyl is
dominated by the intra-methyl dipolar interactions.
60
The double and zero
quantum coherences of the methyl group, quite amazingly, do not relax due to
intra-methyl dipole-dipole interactions. Furthermore, the methyl dipole-
dipole TROSY effect is independent of static magnetic field strength, provided
the spin-diffusion limit is satisfied, and therefore more modest B
0
fields can be
used. Unlike the HSQC sequence, the HMQC sequence for methyl groups
naturally avoids mixing the slow- and fast-relaxing methyl coherences and
provides a high-resolution, high signal-to-noise spectrum that can be used to
characterise methyl side-chain dynamics. Kay and co-workers termed this
effect 'methyl-TROSY' and subsequently exploited this in a CPMG relaxation
dispersion experiment that monitors the effects of conformational exchange on
the methyl MQ coherence. Additionally, because
13
C-
1
H multiple-quantum
coherences are monitored during the relaxation period, the observed dispersion
will depend on both the
13
C and
1
H chemical shift differences between
conformational states. Thus it is possible to extract Dv values for both
13
C and
1
H from fits of the multiple-quantum dispersion data, whereas chemical shift
data
for
only
a
single
nucleus
can
be
obtained
from
single-quantum
experiments.
In this experiment, once MQ coherence is generated, a pair of CPMG pulse
trains (t-180
o
-t)
n
follows, separated by a single
1
H 180u pulse, which serves to
refocus
1
H
chemical
shift
evolution
in
which
2t 5 t
cp
.
The
transverse
relaxation rate, R
2,MQ
, is given as,
1
2nt
cp
R
2
(1
=
t
cp
)~Re(l
1
){
ln(Q)
ð
7
:
11
Þ
1
2
1
t
cp
cosh
{1
l
1
~R
2,MQ
z
k
ex
{
D
z
cosh(g
z
){D
{
cos(g
{
)
ð
7
:
12
Þ
2
3
Yz2Dv
C
2
Y
2
zj
2
1
2
4
5
D
+
~
1
=
2
+1
ð
7
:
13
Þ
h
i
1
=
2
t
cp
2
p
+Yz(Y
2
zf
2
)
1
=
2
g
+
~
ð
7
:
14
Þ
Þ
2
{Dv
C
z4p
A
p
B
k
ex
Y~ iDv
H
z p
A
{p
B
ð
ð
Þ
k
ex
ð
7
:
15
Þ
f~{2Dv
C
iDv
H
z p
A
{p
B
ð
ð
Þ
k
ex
Þ
ð
7
:
16
Þ
