UV-Vis absorption spectroscopy is an effective method for studying small molecule interactions with DNA, irrespective of binding mode. Whether through intercalation, groove binding or external binding to the DNA backbone, these interactions with DNA should lead to changes in the absorption spectra of both the small molecule and DNA. Figure shows absorption spectra of free DNA from calf thymus (ctDNA) and ctDNA at increasing concentrations of COSAN ([COSAN]/[ctDNA(per base, pb)] ratios ranging from 0.125 to 1). Because the absorption band of COSAN overlaps with that of ctDNA, all measured spectra were adjusted to the corresponding concentration of COSAN. ctDNA has a single, broad absorption band, with an absorbance maximum at 258 nm. After adding COSAN, the absorption spectra of ctDNA remained unchanged.