Biomedical Engineering Reference
In-Depth Information
Figure 10.13
Five-site NMR exchange parameters for the metal in the scheme of
Figure 10.11. In the cartoons,
113
Cd(
II
) is a green circle, Cys-SH
groups are yellow circles, Glu-COO
2
groups are red circles and Lys
NH
3
+
groups are blue circles. The relevant transition probabilities p
are related to the kinetic constants in Figure 10.11 as indicated.
where p
BZ
is the sum of all transitions leading away from species b. It thus
appears that the model of Figure 10.11, representing five-site metal exchange,
can account for both the decrease in intensity of the bound signal as well as
progressive line broadening when excess free Cd
2+
is present.
Why the other lines are not observed in the experiment may be understood
as follows. As we already know, the signal of
113
Cd
2+
ion in water is broadened
beyond detection. Hence, it would not be unexpected that the Cd
2+
signals of
the solvent exposed sites c and e are also broadened away. But why would the
Cd
2+
signal of internal site d in structure S be invisible? We associate this
species as being destabilised for the following reasons. The peptides contain
Lys and Glu residues that stabilise the structure through salt bridges. We have
evidence that excess Cd can interfere with these salt bridges and hence
destabilise the bundle, leading to destabilisation of the central site. This likely
causes excessive exchange broadening of the signal d. Moreover,
113
Cd
2+
is also
known to have a large CSA, which can reach to 100 ppm in asymmetric
environments such as one may expect for this destabilised situation.
20
The
associated line-broadening effects, especially at higher magnetic fields, would
further exacerbate the broadening of Cd
2+
in the site d in species S.
