Circular dichroism of acridine orange bound to DNA

Abstract

AbstractThe circular dichroism (CD) spectra of DNA–acridine orange (DNA–AO) complex in the visible region were measured at DNA phosphate‐to‐dye ratios (P/D) from 1 to 550. The CD spectrum of DNA–AO complex in the P/D ratio between 1 and approximately 40 consists of four components, i.e., positive CD bands centered at 510 and 480 mμ, and negative CD bands at 497 and 468 mμ. The CD bands at 510 and 468 mμ are optimum at P/D = 4, and the change of ε1 − εr with P/D suggests that both of them are induced from the interaction between dye molecules bound to adjacent DNA binding sites, each of which is composed of four nucleotides. This is supported by the fact that the values of ε1 − εr for both decrease with increasing temperature or increasing methylene blue concentration added to the complex. The negative Cotton effect at, 497 mμ is most favored at larger P/D ratio (∼8), and the suggested assignment is to the interaction between two dye molecules bound with an empty site between them. A positive Cotton effect at 480 mμ is observed at P/D ratio of less than 4 and is optimum at 1. Above P/D ratio of 40, the CD spectrum of the complex can not be resolved into its components and even at sufficiently high P/D ratio (550) the complex shows a small Cotton effect.