Optical Activity of n–π* Transitions in Polynucleotides

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Abstract
Although the source of most of the large optical activity of polynucleotides may be explained by the interaction of the strong π–π* electronic transitions in the ultraviolet, a number of recent experiments have been interpreted as evidence for Cotton effects due to n–π* transitions in nucleotides. In this study, we propose a method for calculating the rotational strengths of n–π* transitions in polynucleotides. We use the Kirkwood–Tinoco approach, which assumes no electron exchange among the nucleotides in the polymer. The azine n–π* transition is described by an all-valence-electron molecular orbital theory, and the interactions among the nucleotides are calculated by a distributed dipole-charge monopole approximation. The results of a sample calculation on polyadenylic acid, using extended Hückel orbitals to describe adenine, yield oscillator strengths consistent with absorption in the 280-nm-wavelength region. Rotational strengths computed for a polymer of DNA-type geometry are small. This is in agreement with the circular dichroism band of polyadenylic acid at 280 nm. The dependence of the rotational strength on polymer geometry is shown to differ from that of π–π* transitions due to the greater spatial localization of the n–π* transition in the ring.