Condensed states of nucleic acids. II. Effects of molecular size, base composition, and presence of intercalating agents on the Ψ transition of DNA

Abstract Circular dichroism spectroscopy has been used to investigate the influence of DNA molecular size, base composition, and the presence of intercalating agents upon the Ψ transition of DNA brought about by high concentrations of poly(ethylene oxide) and salt (Lerman (1971) Proc. Natl. Acad. Sci. ( U.S. ) 68 , 1886–1890). A molecular weight of 0.15–3.0 × 10 6 daltons yields maximum formation of Ψ‐DNA. Both the amplitude of the large negative CD band at 265 nm—a chief characteristic of the Ψ state—and the thermal stability of Ψ‐DNA increase linearly with increasing mole fraction of guanine plus cytosine in the DNA sample. Either ethidium or proflavine, at concentrations where approximately one dye is bound per 5–10 nucleotide residues, can prevent the transition completely. Striking similarities between the Ψ‐DNA produced by poly(ethylene oxide) + salt and the complexes formed between DNA and lysine‐rich histone f1 suggest the presence of similar nucleic acid–nucleic acid interactions in both types of condensed phase.