The CD spectrum of ctDNA is characteristic of the right-handed B form and consists of a positive band at 275 nm due to base stacking and a negative band at 245 nm due to helicity. Both bands are highly sensitive to the intercalation of small molecules with DNA. However, CD spectra of DNA show no or only very little alteration in electrostatic binding and minor groove binding. At [COSAN]/[ctDNA(pb)] ratios ranging from 0.125 to 0.375, no changes were identified in the CD spectra. At a [COSAN]/[ctDNA(pb)] ratio of 0.5, the intensity of the positive band at 275 nm decreased. At a [COSAN]/[ctDNA(pb)] ratio of 1, the intensity of both the positive band at 275 nm and the negative band at 245 nm further decreased. These changes in the DNA CD spectrum may be explained by at least three reasons: (i) perturbation of the DNA structure, (ii) appearance of the induced CD (ICD) signal of COSAN, and (iii) quenching of the DNA CD signal by light absorption by COSAN. The first explanation is inconsistent with the results of UV-Vis absorption measurements because the perturbation of DNA structure would also entail changes in the DNA absorption spectrum.