Semiempirical SCF—LCAO—MO Calculation of the Electronic Structure of the Guanine—Cytosine Base Pair: Possible Interpretation of the Mutagenic Effect of Radiation

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Abstract
The π‐electronic structure of the guanine—cytosine nucleotide base pair has been investigated in the semiempirical SCF—LCAO—MO approximation. An examination of the energy levels with respect to the single bases showed that the transition between the highest filled and lowest unfilled π orbitals (n*→n*+1) takes place between the levels originating from two different single bases. This statement follows from considering the n* and n*+1 levels of the base pair as perturbed ones of the guanine, cytosine single bases, respectively. This view is substantiated by the structure of the eigenvectors of the respective states. The transition between these states is associated with a transfer of charge, which in the investigated case involves the transfer of a unit charge from guanine to cytosine. Since in the charge‐transfer state the bases are essentially in an ionic form, a considerable increase in the probability of a single‐proton tunnelling, proposed by Löwdin, may be expected. The interpretation of radiation‐induced mutations in terms of the charge‐transfer state and single‐proton tunneling is further discussed.