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The real success of this method has been its ability to permit protein structure determination almost entirely
from the EPR results. That is, it is possible to determine local rigidity and proximity to other structural
elements, accessibility to solvent or the non-polar interior, and quantitative conformational movements form
double labeling studies. In fact, the trends and cycles in increasing and decreasing exposure (accessibility)
can trace an alpha-helical backbone or beta-sheet structure. By using MTSL labels of varying tether length
and rigidity, these results can be confirmed unambiguously.
15.7 Other spin labeling applications
Spin-label-based distance measurements can be applied to other macromolecular systems, such as polymers,
dendrimers, and the like but, as noted in the Introduction (Section 15.1), the topics in this chapter focus on
biochemical/biological applications. The field of lipids and membranes, which is technically a spin probe
approach, is vast and is not covered here. An excellent topic has appeared recently that covers both the
theory and applications.
27
+
N
NH
+
+
H
+
•
• •
N
N
•
•
•
•
−
O
O
•
5-H
+
•
• •
5
16.2
6
7
15.8
5
11
10
8
8
15.4
9
40*
15.0
9
14.6
0
2
4
6
8
10
12
14
pH
Figure15.8
Imidazolinenitroxideprotonationreaction(top)andaplotofpHinducedchangesinthehyperfine
splittingforeightdifferentimidazolinenitroxides(bottom).(Reprintedwithpermissionfrom[28].Copyright1998
PlenumPress.)
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