Biomedical Engineering Reference
In-Depth Information
Figure 10.10
113
Cd NMR spectra of a solution containing the tripeptide (Ac-G
LKALEEK LKALEEK LKALEEK LKAAEEK CKALEEK G-
NH
2
)
3
at pH 8.5 in the presence of 0.2 to 1.7 equivalents of added
113
Cd(NO
3
)
2
. The number in the centre identifies the peak position in
ppm, the numbers on the left the linewidth in Hz. Equivalences are
with respect to tripeptide. Adapted from ref. 21.
chemical shift change occurred when excess equivalents of
113
Cd(
II
) were added
to the peptides, indicating slow exchange, which is not surprising given the high
affinity. However, no NMR signal for the excess free, Cd
2+
was observed. The
aqueous
113
Cd
2+
NMR signal is expected to occur around 50-100 ppm relative
to
113
Cd(ClO
4
)
2
, depending on counterions and ionic strength.
19
The failure to
observe the NMR signal of
113
Cd
2+
in water is likely due to broadening caused
by the sizeable
113
Cd
2+
chemical shift anisotropy (CSA) relaxation
20
and/or
conformational broadening due to fluctuations in hydration. Hence the lack of a
'free' signal can be explained. But that is not all that is strange. The spectra show
an anomalous increase in linewidth and decrease in intensity for the 'bound'
113
Cd(
II
) signal when excess equivalents of
113
Cd
2+
are present. The signal
intensity loss and progressive broadening is inconsistent with a two-site slow-
exchange mechanism. In two-site slow exchange, the intensity of the 'bound'
resonance should not change when excess free metal is present (dilution is
negligible in these experiments). In addition, the linewidth R
2
/p of a 'bound'
resonance (when excess is present) is independent of the ratio metal free/bound
and is determined by the intrinsic linewidth R
2
=
p at that site, augmented with the
constant lifetime broadening k
off
/p:
R
2
=
p~R
2
=
pzk
off
=
p
ð
10
:
32
Þ
