Partial charges obtained
from NPA (at B3LYP/6-31G+(d,p) level)
are reported as a fraction of elementary charge (e) for (A) neutral,
(B) N3 protonated and (C) N7 protonated guanine. (D) Conjugation between
the lone pair of electrons of the exocyclic amino group and the ring
π electrons. Red, blue, and black colors are used for oxygen,
nitrogen, and hydrogen atoms, respectively. Pyramidalization of the
exocyclic amino group has been characterized by the improper dihedral
angle (ψ) between C2, HS, HW, and N2,
where, HS and HW represent the hydrogen atoms
of the exocyclic amino group that belongs to the sugar edge and the
WC edge, respectively. The improper dihedral angle (ψ) and the
C2–N2 bond length (dC2–N2) are reported in degrees and Å, respectively.