The binding curves of 1-, 2-, 3-, and 9-aminoacridine and proflavine on native DN.4 and the circular dichroism (CD) spectra of the bound cations have been determined under the same conditions. The variation of the CD spectra with the amount ( r ) of aminoacridine bound per DNA phosphorus waa of two main kinds: (1) the rotational strength of those aminoacridines which possess a 3-amino group depended markedly on T and decreased to relatively small values (or zero) a t zero r ; or, (3) the rotational strength changed relatively little with r and tended to a finite value at zero r. The relevance of these observations is discussed with respect to intercalation models of the complexes and with respect to possible explanatioiis of the basis of this induction of optical activity.
Previous reports from this lab~ratoryl-~ have been concerned mainly with the binding of proflavine (3,6-diaminoacridine) to nucleic acids and polynucleotides, with an emphasis on the optical rotatory dispersion (ORD) of the complexes formed. These studies suggested that a single molecule of proflavine bound to a polynucleotide does not itself possess optical activity, but that the presence of an adjacent group of bound proflavine molecules was essential. The size of the group necessary for this induction of optical activity was thought to be between 3 and 4 proflavine molecules with native DNA, and 2 with denatured DNA and RNA.
This study attempts to clarify these results, by measuring circular dichroism (CD) rather than optical rotatory dispersion so that the two overlapping bands, which give rise to the Cotton effect a t 450 mp, can be separated and investigated separately. The binding and CD of complexes of DNA with other aminoacridines was also examined in order to elucidate the effect of the position of the amino group in the aminoacridine molecule upon these properties.
Experimental
Calf thymus DNA and proflavine (Pf) were obtained as described by Blake and Peacocke.' 1-, 2-, 3-, and 9-AA (1-, 2-, 3-, and 9-aminoacridine, respectively) were obtained from Professor A. Albert,