All of the rings of the four heterocyclic bases are aromatic. This is more apparent when the polar resonance forms of the amide groups are drawn, as is done for thymine at left. Redraw the hydrogen-bonded guanine-cytosine and adenine-thymine pairs shown in figure 23-24, using the polar resonance forms of the amides. Show how these forms help to explain why the hydrogen bonds involved in these pairings are particularly strong. Remember that a hydrogen bond arises between an electron-deficient hydrogen atom and electron-rich pair of nonbonding electrons.

Hydrogen bond arises between an electron-deficient hydrogen atom and electron-rich pair of non-bonding electrons. Adenine and guanine are purine bases whereas thymine and cytosine are pyrimidine bases. Electronegative atoms present in these bases have a negative charge or lone pair which is involved in hydrogen bonding with hydrogen and in each pair, one N-H is polarized more strongly because the nitrogen atom possesses a positive charge which further enhances the electronegativity of nitrogen.

Hydrogen bonding between guanine and cytosine

Hydrogen bonding between adenine and thymine

Hydrogen bonds are created when hydrogen atom which is bonded to an electronegative atom approaches a nearby electronegative atom. These are characterised by strong intermolecular forces and more the electronegativity of hydrogen bond acceptor, more will be the hydrogen bond strength. In between the purine and pyrimidine base pairs, nitrogen atom possess positive charge and this will highly increase hydrogen bond acceptor strength and hydrogen bond strength. Negative charge on oxygen also increases hydrogen bond strength.