Guanine electrophilic reactivity

These examples show the complementary roles of /s(r) and Vs(r) with respect to reactivity towards electrophiles. When it is a radical that is approaching, however, then 7S (r) would be expected to be the determining factor. Indeed, C5 has been observed to be a favoured site for the interactions of guanine and cytosine with radicals [114-116],... 

The Fukui Equations (4.16), (4.17), and (4.18) reactivity indices are considered a measure of the reactivity of the molecule (as nucleophil, electrophil, or radical) when the reaction center is the atom for which the indices have been calculated. The values of the Fukui indices, calculated for different atoms, allow the comparison of the atoms and the identification, as a rule, of the reaction center in different types of chemical reactions. The reactivity estimated in this way does not consider steric factors. Table 4.4 presents the values of the Fukui reactivity indices for the isomers of guanine that are analyzed here. The indices 1, Ie, and 1r do not reflect the deformation of the electron clouds of two molecules that are approaching in order to react 1 therefore they are called static. The reactivity indices that consider this factor also are called dynamic. Their calculation requires much more complicated formulas (Roothaan 1951) and involves the comparison of the energy of the molecular orbitals of the isolated molecules to the energy of the molecular orbitals of the supermolecule (i.e., the ensemble of the A and B molecules, situated at very short distances). 

---