Photochemical and photobiological studies on methylthioangelicins

4,4',5'-Trimethyl-1'-thioangelicin (1) and 4,6,4',5'-tetramethyl-1'-thioangelicin (2), two newly synthesised isosters of furocoumarins having a sulfur atom in their five-membered ring, were studied in terms of interactions with DNA, both in the ground state and after UVA light absorption. The compounds were able to intercalate the macromolecule and to photobind efficiently, forming C4-cycloadducts with thymine. The antiproliferative effect of this binding was shown in Ehrlich and HeLa cells and by T2 phage inactivation. Tests on Salmonella typhimurium indicated low mutagenic activity. In particular, compound 1 has photobiological activity comparable with that of 4,6,4'-trimethylangelicin, but is less mutagenic.