Wa. Bernhard et al., THE INFLUENCE OF PACKING ON FREE-RADICAL YIELDS IN CRYSTALLINE NUCLEIC-ACIDS - THE PYRIMIDINE-BASES, Radiation research, 140(2), 1994, pp. 199-214
The total free radical yield has been measured for crystals of five py
rimidine derivatives: thymine (T), 1-methylthymine (1MeT), 1-methylura
cil (1MeU), 1-methylcytosine (1MeC) and cytosine monohydrate (C:HOH).
Q-band EPR measurements were made on samples X-irradiated between 4 an
d 12 K. The G values in units of 10(-7) mol/J are 1MeC < 0.01, T < 0.0
4, 1MeU = 0.15, 1MeT = 0.25, and C:HOH = 0.8. The types of free radica
ls formed in these crystals are known through previous EPR investigati
ons. A model is presented that attempts to identify the salient variab
les behind the large range in G values and, simultaneously, explain th
e variation in radical types. It is concluded that packing is a critic
al variable. Hydrogen-bonding networks promote the trapping of radical
s through reversible proton transfer. In the absence of such a network
less probable radical types are observed, such as radicals formed by
irreversible protonation/deprotonation, higher-order reactions and hom
olytic bond cleavage. Crystals with low G values trap radicals formed
predominantly by irreversible protonation/deprotonation at carbon posi
tions and by excitation-spawned homolytic bond cleavage. In contrast,
crystals with high G values trap radicals formed predominantly by reve
rsible protonation/deprotonation at heteroatom positions. This model i
s extended to polynucleotides irradiated at low temperatures, where G
values are typically 2-6. The high trapping efficiency seen in polynuc
leotides reflects highly efficient proton transfer. Consistent with th
is is the predominance of radicals formed by reversible protonation/de
protonation compared to those formed by irreversible protonation/depro
tonation.