Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces
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610 Charged Particle and Photon Interactions with Matter
(Goldman, 1995). Melatonin reacts with
•
OH radicals, peroxyl radicals, and other oxidizing radicals
with rate constants comparable to many important antioxidants like ascorbate. The
•
OH radical
reacts
with melatonin through addition followed by elimination to produce indolyl radicals absorb-
ing
at 330 and 520nm with radical pK
a
at ∼4.5 (Roberts et al., 1988; Stasica et al., 1998; Mahal
et al., 1999). No direct reaction of melatonin with
O
2
i−
radicals was observed by pulse radiolysis.
Some reports indicate that melatonin induces formation of
O
2
i−
radicals, a reaction considered to be
responsible for its tumorigenic response. Melatonin reacts with reducing radicals like
e
aq
−
, H atom
with rate constants less than diffusion-controlled limits. The rate constants for the regeneration of
melatonin from the melatonin radical by ascorbate and urate were determined to be 5 7 × 10
7
and
4×10
7
M
−1
s
−1
, respectively. Melatonin reacts with guanosine radicals with a rate constant of 3 × 10
9
M
−1
s
−1
, indicating its ability to repair damaged bases in DNA. Scheme 22.14 shows the structures
of
one-electron oxidation of melatonin.
22.6.2.2
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