Chemistry Reference
In-Depth Information
4.4.2.2 Oxidation of Amino Acid Side Chains.
Aliphatic Side Chains.
The effect of radiation on aliphatic amino acids has
been performed [291-293]. Near-edge x-ray absorption fine structure
(NExAFS) measurements and DFT calculation showed a complex, a multi-
step decomposition of gly [291]. The reaction of
•
OH with gly in solution was
also found complex [294-296]. The reaction of
•
OH with the glycine anion
(NH
2
-cH
2
-cOO
−
) immediately produced (∼10-ns experimental time resolu-
tion) three radicals, glycyl (NH
2
-
•
cH-cOO
−
), aminoethyl (NH
2
-
•
cH
2
), and
aminyl (
•
NH-cH
2
-cOO
−
), which were also confirmed by time-resolved EPR
measurements [297-301]. A
scheme
has been proposed to demonstrate the
pathways to the formation of these radicals (Fig. 4.24). Initially, the precursor
species, aminium radical zwitterions (
+•
NH
2
-cH
2
-cOO
−
and HO : NH
2
-cH
2
-
cOO
−
), were proposed to yield different radicals (reaction a) [297, 302]. Direct
hydrogen atom transformation from the precursor species produced the glycyl
and aminyl radicals (reactions b-d). The aminyl species could be formed from
either decarboxylation or deprotonation (reaction e). Recently, the scheme
given in Figure 4.24 was examined using the H/D kinetic isotope effects (KIE)
to distinguish H abstraction and electron/proton transfer mechanisms [294].
The second-order rate constants for the reactions of
•
OH with various glycine
anions are provided in Table 4.12 [294]. The partial rate constants, calculated
by multiplying
k
overall
with the relative yield of the each radical, for the forma-
tion of individual radicals are also presented in Table 4.12. The values of
k
overall
were similar and decreased to some extent with an increasing degree of the
gly deuteration.
The reaction to yield the glycyl radical by a direct abstraction of hydrogen
from −cH
2
- occurred with a 37% probability, irrespective of
•
OH or
•
OD as
reactive species and H
2
O or D
2
O solvent. The reaction was independent of the
substitution of the H/D on the amino group of gly (Table 4.12). The yields
were 30% and 24% for the reactions of
•
OH or
•
OD with −cD
2
-, respectively
NH
2
-CH
2
-CO
2
+ OH
(a)
NH
2
-CH
2
-CO
2
----HO
HO NH
2
-CH
2
-CO
2
HO ... NH
2
-CH
2
-CO
2
(c)
(b)
-H
2
O
-H
2
O
(e)
NH
2
-CH-CO
2
NH-CH
2
-CO
2
-H
2
O
-CO
2
-OH
(d)
NH
2
-CH
2
Figure 4.24.
Radical products in the reaction of
•
OH with glycine (adapted from
Štefani
ć
et al. [294]).
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