To test the third hypothesis, we conducted measurements using a double-cuvette system in which one cuvette (closer to the light source) contained various concentrations of free COSAN (0, 0.2, and 0.4 mM), and the second cuvette contained free ctDNA (0.4 mM (pb)) (Figure and). In that setup, the light was first absorbed by the cuvette with COSAN and then passed through the cuvette with ctDNA, which allowed us to assess the COSAN light absorption effect on the ctDNA CD spectrum. We observed the same effect as in the original measurements, that is, a decreased intensity of the DNA CD bands (Figure ). These changes in the DNA CD spectrum resulted from DNA CD signal quenching caused by COSAN light absorption. Thus, our CD measurements show that COSAN does not affect the secondary structure of DNA.