Linear dichroism studies of nucleic acids. III. Reduced dichoism curves of DNA in ethanol–water and in poly(vinyl alcohol) films

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Abstract
The uv linear dichroism of calf thylus DNA has been studied at different degrees of orientation both in flow‐oriented ethanol–water solutions and in a stretched aqueous host of poly(vinyl alcohol) (PVA). The reduced dichroism (LDR) curves in the region 250–290 nm for DNA in PVA films at 75 and 100% relative humidity (r.h.) are in fair agreement with the curves calculated for the A‐ and B‐forms of DNA, based on the fiber structures and the π‐π* transitions of the free bases. This suggests that DNA adopts its A and B conformations in PVA at 75 and 100% r.h. In ethanol, on the other hand, a deviation from the A‐form spectrum shows that the conformation of DNA in the solution can differ from the fiber structure. At shorter wavelenghts, a positive contribution to LDR is explained in terms of an out‐of‐plane polarized n‐π* transition.