Note on the preference of aliphatic epoxides for the N-7 position of guanine in DNA.

The reactivity of aliphatic epoxides like ethylene oxide, chloroethylene oxide or propylene oxide towards N-3 and N-7 in both purine bases was calculated by the MNDO method and correlated with their HOMO coefficients. A greater chemical susceptibility of the N-3 position in both purines to aliphatic epoxides when compared with the N-7 position was found. On this background the preference of aliphatic epoxides for the N-7 position of guanine in DNA was explained by the existence of intramolecular hydrogen bonds in adenine and guanine and the steric orientation of the N-3 (minor groove) and N-7 (major groove) positions of both purines in alpha-helical DNA.