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(BIJ). The CD spectrum of adenosine2poly(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 simiiarity 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 adenosine2poly(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,,, of 9-methyladenine-2poly(BU) is somewhat higher than that of adenosine-2poly(BU) under equivalent conditions. The T, difference with the monomer-poly(U) system was found to be about 20Β°C in 0.4M NaCI-0.02M Nacacodylate-5 X 10-4M EDTA (pH 7.0). Further, it was noted that the monomer-poly(BU) complexes are formed even when the T,,, is lower than that of self-folded poly(BU).
1. NTRODUCTION
Many experimental and theoretical studies have been carried out on the interaction of pyrimidine polynucleotides with purine monomers (base, nucleoside, and n~cleotide).l-"~ The resulting helical complexes were well-documented by making use of ir,1,21 nmr,5,16 and CD spectroscopy.3,6,23,[26][27][28][29] The principal conclusions derived were that the interactions are stoichiometric with complementary base-pairing schemes and the monomer-polymer complex structures are very similar to the corresponding polymer-polymer helices. The absence of a ribose-phosphate linkage had little effect on the structures. Furthermore, analysis of the thermal stability revealed the characteristics of the interactions in terms of precise
We describe here our study of the monomer-polyribo-5-bromouridylic acid [poly(BU)] interaction, using CD and absorption spectroscopy. The CD extrema of poly(BU) [(BU) = bromouracil] shift to wavelengths longer