Abstract
The changes of π-electron distribution in atoms in purine bases participating in the hydrogen-bond formation in Watson-Crick model of deoxyribonucleic acid are calculated by the simple linear combination of atomic orbital-molecular orbital (LC-AO-MO) method assuming that the alkylating agents attack the guanine at its 7-N position and adenine at its 7-N and 3-N positions. The changes in electron density are believed to make an alteration of the hydrogen bond with respect to the mode of pairing. It is assumed that the tautomeric change of guanine-cytosine pair results and weakens the hydrogen bond strength of adenine-thymine pair. These changes are discussed in relation to the biological effect of alkylating agents such as mutagenic, carcinogenic, and carcinostatic activities.
