Infrared dichroism of polynucleotides of repeated sequence. I. Poly(dA)·poly(dT) and poly[d(A‐T)]·poly[d(A‐T)]

Infrared dichroism measurements of oriented films of poly(dA)·poly(dT) and poly[d(A‐T)]·poly[d(A‐T)] have been made under the conditions of low salts content and high humidity for which the geometry is known. The angles which the transition moments make with the helix axis are compared with the orientations of the corresponding bonds. Except for the in‐plane base model of poly[(A‐T)]·poly[d(A‐T)], there is no agreement. This may imply either that a model which assumes bonds and transition moments to be colinear is not acceptable or that x‐ray data are inaccurate. These possibilities are discussed especially with respect to phosphate group orientation. An appendix gives the derivations of dichroic‐ratio expressions for helical molecules of different symmetry types.