Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

Radiation-induced bioradicals: physical, chemical and biological aspects
damage,... Such a Local multiply damaged site (Steel 1996) may be too complex to
allow enzymatic repair. DNA damage is often investigated in theoretical (Michalik
1995) and experimental (Spotheim-Maurizot 1995) model systems, e.g. dried or frozen
DNA to study the direct effect or in dilute aqueous systems to study the indirect effect.
Under aerobic conditions, the main source of DNA damage are OH° radicals from
water radiolysis (indirect effect) (Mee 1987; von Sonntag 1994) and one-electron
oxidation (direct effect) (von Sonntag 1987). In Figure 2, reaction pathways for the
formation of altered sugars in DNA leading to chain breakage are given as an example
(Mee 1987). Radical 1 is formed due to hydrogen abstraction with a OH° radical. The
C4’ position of the sugar is considered as the main site of sugar attack by OH" radicals
(Spotheim-Maurizot 1995). Abstraction at the C4’ position is preferred over other
positions due to sterical hindrance (Obe 1992). The radical 1 removes first the
phosphate ester anion thus breaking the chain. Further interaction with OH° radicals
produces either radicals 3 or 6. For radical 3 the second phosphate group can be
eliminated. Radical 4 is terminated in disproportionation reactions with other radicals,
followed by ring opening and the elimination of the unaltered base-forming sugar. For
radical 6 the second phosphate group cannot be removed, but in an analogous series of
reactions, the ring opens and an unaltered base is released. The base radicals may also
lead to strand breaks by radical transfer to the sugar moieties (Becker 1993).
Fast neutron and photon-irradiation of double- and single-stranded synthetic
oligonucleotides have shown that radiation-induced damages occur at all nucleotide
sites (Isabelle 1995). In all structures, the probabilities of inducing a modification are
almost identical for adenine, thymine and cytosine, but this probability is higher for
guanine, in accordance with the known higher reactivity of guanine base towards the
OH" radicals. An almost complete description of the radical reactions mediated by OH"
radicals and also of direct one-electron oxidation has been developed recently for the
guanine compounds, guanine having the lowest ionisation potential (Cadet 1999). The
radiosensitivity of each nucleotide is the same in the single- and double-stranded DNA.
The similarity of radiosensitivity of the single and of the double stranded DNA strongly
suggests that the stacking and the helical conformation, governing the accessibility of
the sites to radical attack, are the determinant factors and not the interstrand H bonding
(Spotheim-Morizot 1995). The radiosensitivity of DNA is also modulated by its
interactions with proteins, the presence of bound proteins protecting the DNA in
regions of contact (Isabelle 1993).
5.1.2. Radiation damage to proteins
Proteins generally have very high rate constants for reaction with the reactive species of
water. The possible sites of interaction with proteins can be predicted from pulse
radiolysis experiments (see next chapter) on amino acids and peptides, but the intrinsic
reactivity of these sites are influenced by the structure of the proteins, governing the
accessibility of the sites to radical attack (Houée-Levin 1994; Sharpatyi 1995).
Based on known protein structural factors and kinetic constants of radical
attachments it can be estimated that most hydrated electrons should react with the
surface peptide carbonyl groups, and to a lesser extent with disulfide bridges and
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