Chemistry Reference
In-Depth Information
R
RH
+
3324.8
3325.2
Field, G
(a)
1.0
0.5
0
6
7
8
9
10
11
pH
(b)
Figure 16.14
High field components of the absorption EPR spectra of 50
MaTAM
4
radical in 1.5mM
pyrophosphate buffer at various pH values, 37
◦
C (a) which demonstrate isosbestic point characteristics for the
chemicalequilibriumbetweenprotonatedandunprotonatedaTAM
4
radicals(Scheme16.7).Thearrowsindicate
thedirectionofsignalintensitychangesuponacidificationofthesample.(b)ThepHdependenceofthefraction
ofaTAM
4
radicalprotonatedformobtainedfromtheEPRspectra.Linerepresentsthebestfitsoftitrationequation
totheexperimentaldatayieldingpK
a
value8
µ
.
0
±
0
.
1.
e
−
, H
+
+
N
OH
N
O
−
e
−
, H
+
Scheme 16.8
Illustration of the nitroxide/hydroxylamine redox couple. In general, for the most biologically
relevant samples one-electron reduction of the nitroxides prevails and the equilibrium is strongly shifted towards
hydroxylamine form
34
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