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The geometrical and electrical properties of four tautomers of 2-OH-adenine (2-OH-A) and 8-oxoadenine (8-oxo-A) and three tautomers of fapy-adenine (fapy-A) have been estimated on the basis of a 6-31G ab initio RHF method followed by single-point 6-3 1G** and MP2 calculations. The resulting relative stability of different tautomeric forms suggests that the most favorable in vacuum are the enol-amino forms of 2-OH-A, the keto-amino tautomer of 8-oxo-A, and the diamino form of fapy-A. After taking into account for the polarization functions and electron correlation contribution the energetical succession of the studied tautomers was not changed. The smaller contribution of the electron correlation was observed for the most stable tautomers and the highest for enol-imino isomers. The dipole moment strongly depends on the tautomeric form and has the lowest value for all enol-amino forms of adenine derivatives. The nonplanarity of all studied tautomers was noticed. The amino groups are nonsymmetrical pyramids with out of plane hydrogen atoms.For 2-OH-A and 8-oxo-A atom H62 is tumed above and atom &I below the ring plane. The opposite orientation is observed in the case of fapy-A. However, the 2-OH-A tautomers are characterized by a much smaller deviation from the strict planar structure then 8-oxo-A. The use of the prefix oxo for Cs-substituted adenine and hydroxyl for the derivative modified at the C2 position is justified by the most stable tautomeric forms.

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