Optimum geometries and relative energies for guanine, the imino-enol tautomer of guanine, the enol tautomer of guanine, adenine, and the imino tautomer of adenine as found by the MINDO/2 SCF MO method

Abstract

A MINDO/2 SCF MO geometry optimization of guanine (G), the imino-enol tautomer of guanine (G*), the enol tautomer of guanine (G**), adenine (A), and the imino tautomer of adenine (A*) was made. The optimized geometries for guanine and adenine agree well with crystallographic data. The optimized geometries for the tautomers show the correct trends in bond lengthening and bond angle adjustment. The energies of all five molecules were lowered relative to the initial assumed geometry by the optimization procedure. The energies of tautomerization were found to be 0.4, − 18.7, and 12.5 kcal for G ⇌ G*, G ⇌ G**, and A ⇌ A*, respectively. When solvent effects are included the energies of tautomerization are 5.2, − 10.0, and 8.7 kcal for G ⇌ G*, G ⇌ G**, and A ⇌ A*, respectively.