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HO
CH 3
CH 3
OH
NH
O
NH
Reaction 19
O
HN
HN
CH 3
CH 3
CH 3
CH 3
CH 3
-H 2 O
OH
C
NH
N
NH
O
O
O
Reaction 20
Reaction 21
HN
HN
HN
OH
CH 3
CH 3
CH 3
O 2
O O
CH 3
CH 3
C
NH
N
O
O
HN
HN
OH
CH 3
CH 3
O 2 •- / H +
O OH
O
CH 3
CH 3
NH
O
N
HN
O
H
HN
CH 3
CH 3
O
FIG. 1.8 Reactions of tryptophan with hydroxyl radicals result in an initial addition
to the benzene ring ( reaction 19 ) or to the pyrole moiety ( reaction 20 ). In the absence
of O 2 , the radicals derived from the benzene ring form low amounts of 4-, 5-, 6-, and
7-hydroxytryptophans ( reaction 20 ) or the neutral indolyl radical ( reaction 21 ). With a
slow rate, indolyl radicals react with O 2 , and with a very fast rate with superoxide (O 2 ),
forming a hydroperoxide ( reaction 23 ). The peroxyl radicals from the reaction with the
initial C3 pyrrole ring (from reaction 20 ) can react with O 2 , resulting in N-formylkynurenine
after a ring opening ( reaction 22 ) (according to Clare et al., Biochim. Biophys. Acta
1504: 196-219, 2001).
Electron transfer reactions resulting in the formation of radical anions and
radical cations are also common with aromatic side chains. Thus, the reaction
of the solvated electron with Phe generates a transient radical anion which
rapidly protonates to give a cyclohexadienyl radical (Fig. 1.9, reaction 24). Ring
radical cations are generated with all the aromatic amino acids on reaction
with powerful oxidants, such as SO 4 •− (Fig. 1.9, reaction 25), and on direct
photoionization (89). The charge of these species is rapidly lost by a number
of processes, including hydration (thereby yielding hydroxylated products)
and loss of a proton from an adjacent C-H (Phe), N-H (with His or Trp), or
O-H bond (with Tyr) (90).
1.1.1.4 Transfer between Sites Transfer reactions between side chains
and from side chain to backbone and vice versa may occur. Several transfer
 
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