Computer Analysis and Reconstruction of E.S.R. Spectra of Gamma-irradiated DNA

Summary

Previous results suggest that in DNA γ-irradiated in the dry state at 25°c an electronic migration occurs which transforms the primary random radioproducts into new ones, localized essentially on the pyrimidine nucleotides, dTMP and dCMP. According to this mechanism, the E.S.R. spectrum of irradiated DNA should consist of the weighted superimposition of pyrimidine patterns only. Conversely, starting from pyrimidine patterns only, it should be possible to reconstruct the E.S.R. spectrum of DNA.

To demonstrate this assumption we attempted first to isolate the constituent patterns from the dTMP and dCMP experimental spectra. While the dCMP spectrum, C, behaves like a single entity, the spectrum of dTMP was found to consist of the superimposition of three main patterns which we called τ₁, τ₂ and τ₃.

A programme was then prepared to perform a large number of linear combinations following the formula Δ = h₁τ₁ + h₂τ₂ + h₃τ₃ + kC, where the variable coefficients represent the relative intensity of the four elementary patterns in the resulting spectra Δ. We obtained in this manner an atlas of reconstructed spectra, all built up from the same pyrimidine patterns, but at different relative intensity. According to our hypothesis, for each experimental spectrum of DNA there should be an identical one among the reconstructed spectra. A series of DNA spectra obtained in our laboratory or published in the literature were successfully subjected to this test. For each of them the relative yields of the different pyrimidine radicals were hence determined by comparison.