Linear dichroism studies of nucleic acids. II. Calculation of reduced dichroism curves of A‐and B‐form DNA

We have calculated the uv linear dichroism for the A‐ and B‐forms of DNA using π‐π* transition moments and band components determined from the free DNA bases. The reduced dichroism (LDR) as a function of wavelength is estimated is in the 220–300‐nm region, for both the oriented‐gas model and a simple exciton model. For B‐form DNA, LDR is obtained to −1.48S (S being the orientation factor) over the whole wavelenth region by both models. For A‐form DNA, LDR is not constant, but changes monotonically from about −1.15S at 220 nm to about −1.35S to −1.45S at 300 nm, depending on base combination and degree of interaction (−1.35S for the oriented gas). It is emphasized that a common assumption of a single “effective” transition moment of the principal band at 260 nm may not generally be made because of the extensive overlap of differently polarized bands. The possibility of using the reduced dichroism curve for characterizing the secondary structure of DNA is discussed.