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 fagaronine (FGR, NSC 157995) and its derivative ethoxidine (ETX)-inhibitors of DNA topoisomerases (topos) I and II (Figure 1)-and their complexes with DNA, The FLD data obtained suggest that both compounds are strong major groove 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 assigned 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 constants 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 key role in DNA binding, most probably because of formation of the H bond with DNA. Cooperative use of Raman, SERS, and FLD techniques enabled us to propose a molecular model for drug/DNA interactions. The differences in DNA binding by FGR and ETX are discussed in terms of different topoisomerases inhibitory activities of these drugs.