Specific complexes between oligoribonucleotides have been investigated using circular dichroism. The following sets of compounds were prepared enzymatically: G(pU)n, (Up)nG, C(pA)n, (Ap)nC, where n = 5, 6, and 7. The uridine-containing oligomers were mixed with the adenosine-containing oligomers of equal total chain length under conditions designed to maximize specific complex formation. Both melting curves and circular dichroism difference spectra showed evidence of complex formation but only at relatively high concentrations, about 10–2 M in nucleotide residues. Complexes formed from mixtures of antiparallel complements have higher melting temperatures than those formed from mixtures of parallel complements. Ultracentrifuge studies on one oligonucleotide mixture indicate that the complex formed is low molecular weight, consistent with a double- or triple-strand structure. As would be expected, the stability of the complexes formed is strongly concentration and chain-length dependent. Substantial differences in melting temperature are found when two antiparallel complexes containing the same base composition but different sequences are compared. Several simple tests were devised using circular dichroism difference spectra to investigate the stoichiometry of the complexes formed. These indicate that 1:1 complexes will form between the complementary oligonucleotides that have been studied.
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