Biomedical Engineering Reference
In-Depth Information
The other way to avoid the long longitudinal relaxation time (T
1
) of low-c
nuclei is to start the experiment from
1
H. Using
1
H polarisation as the
starting magnetisation also enhances the S/N of the resultant spectrum, uptoa
certain molecular weight. Since the sensitivity is proportional to the square
root of the number of scans,
54
in theory the S/N of such spectra in comparison
to
experiments
that
exclusively
use
low-c nuclei
should
be
proportional
r
T
1
s
T
1H
c
H
c
s
:
:
e
{D
=T
2
(
H
)
where, D is the polarisation transfer period from H to
to
the directly attached heteronuclei. Several experiments have already been
developed for C9-detected experiments as well as for the
15
N-detected
experiment to utilise this effect.
25,26,29,43,55,56
For example, an approximately
2.2-fold sensitivity gain was reported for the
13
C9-detected hCBCACON vs.
CBCACON with a 22 kDa unfolded protein.
43
The sensitivity of nitrogen-
detected CAN experiment was roughly five times enhanced when starting with
proton polarisation in an hCAN experiment compared to the original CAN
sequence.
56
Some of the proton-start low-c detection experiments have been
further
improved
to
minimise
the
effective
recycle
delay
by
longitudinal
relaxation enhancement.
29,55,57,58
2.3 Management of One-Bond Couplings in Low-c
Detection Experiments
Managing homo- and heteronuclear couplings in both indirect evolution and
direct detection periods is critical for obtaining simple and high-resolution
spectra. In particular, large one-bond couplings are most problematic as they
cause large splitting or severe broadening. With the advances in sequence
design,
evolution,
59,60
which
include
CT/SCT
and
adiabatic
broadband
pulses,
61,62
as well as band-selective pulses,
63
decoupling in indirect evolution
periods can be achieved satisfactorily.
As for direct detecting periods, heteronuclear decoupling is relatively easy to
achieve as long as the frequencies of targeted nuclei do not interfere with each
other, and most modern NMR spectrometers satisfy this criterion. The cases
where spins to be irradiated for decoupling have large chemical shift dispersion
are the major concern left for heteronuclear decoupling. Many attempts have
been made to cope with this problem. Among them, a recent advance is the
MODE sequence (multiply MODulatEd rf field), which utilises the multiple
rotating frame technique for designing modulated rf fields and was developed
with optimal control theory methods.
64
The MODE sequence greatly
improved the field homogeneity for a wider chemical shift range that is
enough to cover carbon resonances even in the highest field magnets available.
Therefore, a nucleus that forms bonds only with different types of nuclei, such
as
15
N, is easier to decouple. On the other hand, nuclei that have the same
atom
types
as
their
neighbour(s)
are
more
problematic.
Carbons
have
13
C-labelling
neighbouring carbon(s) in most situations; thus, under uniform
