Planar triazinium cations from vanadyl-mediated ring cyclizations: the thiazole species for efficient nuclear staining and photocytotoxicity.

Planar triazinium cationic species from vanadyl-assisted cyclization of 1-(2-thiazolylazo)-2-naphthol (H-TAN, 1), 1-(2-pyridylazo)-2-naphthol (H-PAN, 2), 2-(2'-thiazolylazo)-p-cresol (H-TAC, 3) and 6-(2'-thiazolylazo)-resorcinol (H-TAR, 5) were prepared and characterized. A dioxovanadium(V) species [VO(2)(TAR)] (4) was also isolated. Compounds 1, 2 and 4 were structurally characterized. Both 1 and 2 have planar structures. Complex 4 has V(V)O(3)N(2) coordination geometry. The cyclised triazinium compound forms a radical species within -0.06 to -0.29 V vs. SCE in DMF-0.1 M tetrabutylammonium perchlorate with a second response due to formation of an anionic species. A confocal microscopic study showed higher nuclear uptake for 1 having a fused thiazole moiety than 2 with a fused pyridine ring. The compounds showed a partial intercalative mode of binding to calf thymus DNA. Compound 1 showed plasmid DNA photo-cleavage activity under argon and photocytotoxicity in HeLa and MCF-7 cells with IC(50) values of 15.1 and 3.4 μM respectively in visible light of 400-700 nm, while being essentially non-toxic in the dark with IC(50) values of 90.4 and 21.9 μM. A TDDFT study was done to rationalize the experimental data.