Heteroatom-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 139

Table 7. 4. Absorption maxima and molar absorption coefficients of cyclic disulfide radical
anions and their protonated forms. According to von Sonntag (1990). The lipoic acid value
has been taken from Hoffman and Hayon (1972). Note that the equilibria (cf. apparent p K a
values) include the ring-open forms which are not detectable

p K a

λ max ε λ max ε

500 2900 460 240 6.7

400 6800 380 4910 6.6

410 9200 385 6900 5.85

400 4540 380 290 7.0

390 6600 380 450 5.2

425 3830 370 155 9.4

systems. This is in keeping with a weaker three-electron bond due to ring strain
(Asmus 1979).
The equilibria of a series of sulf hydryl groups containing amino acids have
been investigated (Mezyk 1995, 1996a,b), and details governing these equilibria
are now reasonably well understood (Mezyk and Armstrong 1999).
Metallothioneins are small ubiquitous oligopeptides containing a high pro-
portion of cysteine residues but no disulfide bonds (Tsunoo et al. 1978; Suzuki
and Maitani 1983). Mammalian metallothionins are made up of 61 amino acids,
20 of which are cysteine (Hamer 1986; for their protective effects see Chap. 12.10).
Despite this, disulfide radical anion formation is mostly bimolecular (Fang et al.
1995).
The disulfide radical anions are also formed upon the reduction of the di-
sulfide by CO 2 • − [reaction (41)] and the thymine radical anion (Willson 1970;
Elliot et al. 1984). For example, the reducing hydroxymethyl radical can reduce
the disulfide (Anderson et al. 1986), but the rate is only fast when the hydroxy-
methyl radical is deprotonated (Akhlaq et al. 1989) and thus its redox potential
decreased.