Heteroatom-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 133

et al. 2001; for further studies on the free-radical-induced decarboxylation of
amino acids see Mönig et al. 1985].

The formation of the reducing aminoalkyl radical formed in reaction (23) that
had escaped detection in the earlier studies (Neta and Fessenden 1971) has now
also been confirmed by EPR (Hug et al. 2000b).
As purine free-radical chemistry is concerned, the 1,2-H-shift reaction (25)
is of special interest.

• NR

− CHR 2 → HNR − • CR 2 (25)

The rate of such an (exothermic) reaction is not yet known. In the amino ac-
ids, -fragmentation [reactions (22)−(24)] compete successfully (Bonifacic et al.

2000a; for DFT calculations see therein).
Tryptophan and its derivatives such as the Hoechst compounds (Adhikary
et al. 2000) have reduction potentials below that of G • (tryptophan: E 7 = 1.0 V;
Jovanovic and Simic 1985) and thus are capable of repairing some of the DNA
damage (for a review on indol free-radical chemistry see Candeias 1998; for the
thermochemistry of N -centered radicals see Armstrong 1998). In these reac-
tions, radical cations and N -centered radicals are formed. Similar to phenoxyl
radicals, these radical react with O 2 • − mainly by addition despite the large differ-
ence in the redox potential which would allow an ET as well (Fang et al. 1998).

β

7. 4
Sulfur-Centered Radicals

The chemistry of S -centered radicals is so different from that of C -centered
radicals and so surprisingly varied that it has found considerable attention (for
reviews besides the one mentioned above see von Sonntag and Schuchmann
1980a,b; Anklam and Margaretha 1989; Asmus 1990a,b; Armstrong 1990; for the
thermochemistry of sulfur radicals see Griller et al. 1990; Armstrong 1999; for
their structures see Armstrong and Chipman 1999; for thiyl radicals in biology
see Wardman 1999). In the present context of free-radical-induced DNA damage,
there is a special interest, because thiols, notably glutathione which is present in
cells at rather high concentrations, achieves a reduction of free-radical-induced
DNA damage caused by ionizing radiation in cellular systems (Alper 1979; for
more recent reviews, see Quintiliani 1983; von Sonntag and Schuchmann 1990;
Wardman and von Sonntag 1995; see also Chap. 12.11).