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

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 (LD R ) 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 LD R is explained in terms of an out‐of‐plane polarized n ‐π* transition.