Induction of Circular Dichroism in Acridine Orange with Opposite Signs by DNA and t-RNA respectively

Both DNA and t-RNA induce circular dichroism in the symmetric dye acridine orange (AO). While AO enhances the UV CD of DNA, that of t-RNA is progressively reduced and eventually reversed in sign with increasing AO. In the visible range strong CD bands are observed in the presence of both DNA and t-RNA but with opposite signs. It is obvious that AO is aggregated with systematic constraints on binding to both the nucleic acids, and thus the 263 nm UV band of the dye as well as the 460 nm band of its aggregate becomes optically active. But AO is aggregated with right handed chirality on binding to DNA and in the reversed sense with RNA as the polyanion. The slightly tilted base pairs in the double stranded regions of t-RNA, as contrasted from the right angled orientation to the DNA axis, may cause this reversal. The CD spectra of the non-intercalating dye pinacyanol induced by both the nucleic acids have, however, the same signs. Thus induced CD in the dye AO capable of intercalation within base pairs differentiates between DNA and t-RNA regarding the signs of the CD band in the UV and visible range.