On the dissociation of DNA enhanced by radical cations

This concerns the heat of reaction, ΔHreact∘=ΔHf∘(K·)+ΔHf∘(L·)-ΔHf∘(KL), accompanying the cleavage of deoxyribonucleosides (KL) to give sugar radicals (K·) and (adenine or guanine) base radicals (L·) under the influence of radical cations R+=H+,CH3+ and C2H5+. (The cited ΔHf∘'s are the appropriate standard enthalpies of formation at 298.15 K.) Addition of R+ onto one or another nitrogen of the bases does indeed perturb the zero-point + heat-content energies Z, namely, Z(KL)→Z(KL′),Z(L·)→Z(L·′), as well as the “reorganizational energies” Re, i.e., Re(L·)→Re(L·′). (Re describes how a free radical differs from its hypothetical electroneutral counterpart found in a symmetrical molecule L-L.) Finally, the “Charge Neutralization Energy” CNE describes the energetics accompanying charge transfers between the fragments K and L in the original molecule, linked to one another by the bond with energy εkl(k∈K,l∈L). It turns out that CNE and Z have little (or no) saying in the decomposition KL→K·+L·. The driving force is clearly in the weakening of εkl under the influence of added R+ cations. The gas-phase Re energies, whose evaluation is reported in detail, would magnify this effect but are unlikely to be of any relevance in in vivo depurinations of DNA molecules.