Interaction of poly‐5‐bromouridylic acid. I. Formation of helical complexes with various adenine and 2‐aminoadenine derivatives

Complexes of poly(BU) with various adenine derivatives were investigated by circular dichroism (CD) and absorption spectroscopy. A 1:2 stoichiometry was indicated on CD mixing curves for typical complexes of 9‐substituted adenine and 2‐aminoadenine derivatives with poly(BU). The CD spectrum of adenosine·2poly(BU) is characterized by well‐resolved bands in the range of 210–350 nm. Other adenine derivative–poly(BU) complexes also afford similar CD spectra, while 2‐aminoadenine derivative–poly(BU) complexes give quite different spectra. Attempts to assign representative CD spectra were made using the transition of helical poly(BU) and the respective purine polynucleotides. The similarity of the CD spectra suggests that poly(A)·2poly(BU) and adenine derivative–poly(BU) complexes are nearly identical in structure except for the ribose–phosphate linkage. The fact that the uv isosbestic point of adenosine·2poly(BU) falls in close proximity to that of the corresponding polymer complex also supports this conclusion. In the formation of stable helices, the ribose moiety is dispensable in the “strand” of purine. The T m of 9‐methyladenine·2poly(BU) is somewhat higher than that of adenosine·2poly(BU) under equivalent conditions. The T m difference with the monomer–poly(U) system was found to be about 20°C in 0.4 M NaCl–0.02 M Na–cacodylate–5 × 10 −4 M EDTA (pH 7.0). Further, it was noted that the monomer–poly(BU) complexes are formed even when the T m is lower than that of self‐folded poly(BU).