Biomedical Engineering Reference
In-Depth Information
Table 6.1
Paramagnetic properties of various metals. The columns RDC,
PRE and PCS indicate the suitability of the metal to obtain these
restraints and the distance range in which significant PREs or
PCSs are obtained is indicated. HS and LS refer to high spin and
low spin, respectively. Reprinted with permission from ref. 4. #
John Wiley & Sons, 2011.
Distance
range/A
˚
Diamagnetic
analogue
S/J
a
t
s
/s
b
Paramagnet
RDC
PRE
PCS
Ref.
Fe
3+
HS
10
29
-10
211
5-12
c
Fe
2+
LS
9
5/2
2
+
+
Fe
3+
LS
10
211
-10
213
5-17
c
Fe
2+
LS
10
1/2
+
+
+
Mn
2+
10
28
Ca
2+
,Mg
2+
11
5/2
2
++
2
16-25
Co
2+
LS
10
29
-10
210
Zn
2+
,Cd
2+
12
3/2
+
2
+
4-13
Ni
2+
10
210
-10
212
12-22
c
Zn
2+
,Cd
2+
13
2/2
2
+
+
Cu
2+
10
28
-10
29
Cu
+
,Zn
2+
14
1/2
2
+
2
5-25
Gd
3+
10
28
-10
29
La
3+
,Lu
3+
,Y
3+ 15
7/2
2
++
2
20-30
Er
3+
10
212
-10
213
La
3+
,Lu
3+
,Y
3+ 16
15/2
+
2
++
10-40
Ho
3+
10
212
-10
213
La
3+
,Lu
3+
,Y
3+ 17
16/2
+
2
++
10-40
Yb
3+
10
212
-10
213
La
3+
,Lu
3+
,Y
3+ 18
7/2
+
2
++
10-40
Dy
3+
10
212
-10
213
La
3+
,Lu
3+
,Y
3+ 19
15/2
++
2
++
15-60
Tb
3+
10
212
-10
213
La
3+
,Lu
3+
,Y
3+ 20
12/2
++
2
++
15-60
Tm
3+
10
212
-10
213
La
3+
,Lu
3+
,Y
3+ 21
12/2
++
2
++
15-60
reduced
radical,
MTS
d
10
27
22
Nitroxide
1/2
2
++
2
12-25
a
J is the quantum number used for lanthanides, which is a vectorial addition of S and L, the total
orbital angular momentum quantum number.
23 b
t
s
is the electronic relaxation time which is the
inverse of the equivalent rate.
c
Based on PRE.
d
MTS (1-acetyl-2,2,5,5-tetramethyl-3-pyrroline-3-
methyl-methanethiosulfonate), diamagnetic analogue of MTSL.
24
also offer great flexibility in experimental design as they can be interchanged in
binding sites due to their chemical similarity. In particular, this allows for fine
tuning of effects with differently sized x-tensors.
26,27
The effects of the larger
lanthanide x-tensors are also greater than those of transition metals.
28
Furthermore, suitable diamagnetic references are available with La
3+
and
Lu
3+
. Finally, Gd
3+
can be used to induce only PRE.
29
Several methods have been developed to introduce paramagnetic centers
experimentally. Free paramagnetic centers in solution can be used to study
surface exclusion during complex formation,
30,31
but site-specific tagging
provides more detailed structural information. Some work has been done with
non-covalent tag attachment,
32,33
but paramagnetic centers are most often
attached via fusion peptides, surface thiol groups or unnatural amino acids
that have been engineered at specific sites.
28
The distance dependence of
paramagnetic effects must be considered when choosing an attachment site and
the tag must be sufficiently rigid in order to observe anisotropic effects.
34,35
Nevertheless,
PRE
can be
obtained
with
flexible
tags
provided
that
the
calculations represent the paramagnetic center as multiple conformers.
36
For
