The main photosensitized oxidation product in aqueous
solution of 8-oxo-7,8-dihydro-2‘-deoxyguanosine
(1) has been isolated by HPLC and characterized as
1-(2-deoxy-β-d-erythro-pentofuranosyl)-cyanuric
acid (7) and
its precursor, 1,3,5-triazine-1(2H)-carboximidamide,
3-(2-deoxy-β-d-erythro-pentofuranosyl)tetrahydro-2,4,6-trioxo-
(6). This was achieved by carrying out extensive
spectroscopic measurements including FAB mass
spectrometry,
1H and 13C NMR analyses. The formation of
the photooxidized nucleoside 7 is accounted for by a type II
mechanism
through initial [2+2] cycloaddition across the C-4−C-5 ethylenic
bond of 1. The 1,2 bond cleavage of the
transient
dioxetanes gives rise to the modified nucleoside 6 through a
nine-membered ring oxidized intermediate nucleoside.
The derivative 6 further decomposes into the stable
cyanuric acid nucleoside 7 by the hydrolysis of the
guanidino
residue with the concomitant release of urea 8.
Photosensitizers such as methylene blue and rose Bengal
were
found to efficiently photooxidize 8-oxodGuo 1
via
a type II mechanism (1O2), whereas riboflavin
and benzophenone,
which act mostly by a type I mechanism, were far less
efficient.
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