Local effects of partial adenine‐ N 1‐oxidation on poly A · poly U and poly A · 2 poly U helix conformations

We prepared helices with noncomplementary bases by N 1‐oxidation of poly A, followed by reaction with poly U. Mixing curves indicate that doubly and triply helical structures form, with only the unmodified adenines involved in base pair formation. Circular dichroism spectra were examined particularly at the absorbance maximum of the adenine N 1‐oxide (A*). In the single strand poly (A,A*), there is a relatively strong pair of positive and negative CD bands from the A*. These are greatly reduced in the double helix, and abolished in the triple helix. We conclude that A* stacks in a conventional manner with A in the single strand, but is rotated out of the double and triple helix. In the double helix the A* probably maintains a preferred orientation with respect to the helix, but rotates randomly in the triple helix.