Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 263

The acetone-sensitized photolysis of Thy in the presence of l l-cysteine (R = cys-

teinyl), yields mainly 5- S - l -cysteinyl-5,6-dihydrothymine (see above), but also

minor amounts of 5- S - -cysteinylmethyl-5,6-

dihydrouracil are formed (Shetlar and Hom 1987; for an earlier study see Var-
ghese 1973).

l -cysteinylmethyluracil and 5- S - l

The formation of 5- S - -cysteinylmethyluracil may be taken as an indication that

under these conditions photoexcited acetone also abstracts an H-atom from the
methyl group of Thy or (less likely) that RS • is a lso capable of undergoing this re-
action. The observation of 5- S -

l

-cysteinylmethyl-5,6-dihydrouracil is less easily
explained. An exomethylene precursor as discussed above for the reaction of
• CH 2 OH with Thy could, in principle, account for it. Detailed mechanistic stud-
ies are, however, missing.

l

Purines. The reactions of purines with -alkoxyalkyl radi-

cals have been most intensively investigated (Elad et al. 1969; Steinmaus et al.
1969, 1971; Elad and Salomon 1971; Leonov et al. 1973; Salomon and Elad 1973;
Leonov and Elad 1974a,b; Moorthy and Hayon 1975; Frimer et al. 1976; Aravin-
dakumar et al. 1994), but the reactions of radicals derived from amino acids
(Elad and Rosenthal 1969; Elad et al. 1969; Elad and Salomon 1971; Poupko et al.
1973; Salomon and Elad 1974) and amines (Elad and Salomon 1971; Salomon and
Elad 1973) and of the methyl radical (Maeda et al. 1974) are also reported (for a
review of the older work see Elad 1976).
Alkyl radicals are nucleophilic radicals, and for this reason protonated pu-
rines react faster with alkyl radicals than the purines themselves (Table 10.22).
Their rate also increases with increasing nucleophilicity of the radical, and
hence • C(CH 3 ) 2 OH reacts much faster than • CH 2 OH (Table 10.23). In fact, the

α -hydroxyalkyl and α