DNA binding by fagaronine and ethoxidine, inhibitors of human DNA topoisomerases I and II, probed by SERS and flow linear dichroism spectroscopy

Raman, surface-enhanced Raman scattering (SERS), and flow linear dichroism (FLD) spectroscopies were employed to study the potent anticancer agent fag aronine (FGR, NSC 157995) and its derivative ethoxidine (ETX)-inhibitors of DNA topoisomerases (topos) I and II (Figure 1)-and their complexes with DN A, The FLD data obtained suggest that both compounds are strong major groov e intercalators with stoichiometries 1 FGR/2.0 DNA bp and 1 ETX/4.0 DNA bp The SERS spectra of both compounds were recorded at the concentrations down to 10(-8) M for FGR and 10(-6) M for ETX, and the SERS-active modes were a ssigned by comparison of Raman and SERS spectra of the drugs following the changes induced by deuteration and pH environment. The SERS-active surface was proved not to affect the drug/DNA interactions, since the DNA binding c onstants calculated from the SERS experiments were found to be practically the same as those determined previously by viscosimetric measurements. The SERS study of the FGR/DNA complex showed that the OH group of FGR plays a k ey role in DNA binding, most probably because of formation of the H bond wi th DNA. Cooperative use of Raman, SERS, and FLD techniques enabled us to pr opose a molecular model for drug/DNA interactions. The differences in DNA b inding by FGR and ETX are discussed in terms of different topoisomerases in hibitory activities of these drugs.