Design and Synthesis of a Functional Derivative of the Triazinium Cation and Its Rhodium Complex that Shows Photoinduced DNA Cleavage Activity and Photocytotoxicity

Abstract

AbstractThe design and synthesis of an intensely blue rhodium(III) complex [3]+ of a new N,N‐donor ligand, 8‐(quinolin‐8‐ylamino)pyrido[2,1‐c][1,2,4]benzotriazin‐11‐ium, [2]+, which contains a planar pendant triazinium arm, is described. Structural characterization for [3]+ was carried out by using various spectroscopic techniques and single‐crystal X‐ray crystallography. The organometallic rhodium(III) compound shows a ligand‐based reversible reduction at –0.65 V. The electrochemically reduced compound displays a single‐line EPR spectrum that signifies the formation of ligand‐based free radicals. Compound [3]+ shows a binding propensity to calf thymus DNA to give a Kapp value of 6.05 × 105 M–1. The parent triazinium salt, pyrido[2,1‐c][1,2,4]benzotriazin‐11‐ium [1]+ and the ligand salt [2]+ exhibit photoinduced cleavage of DNA in UV‐A light, whereas the reference Rh complex [3]+ photocleaves DNA with red light (647.1 nm). The compounds show photonuclease activities under both aerobic and anaerobic conditions. Mechanistic investigations under aerobic conditions with several inhibitors indicate the formation of hydroxyl radicals by means of a photoredox pathway. Under anaerobic conditions, it is believed that a photoinduced oxidation of DNA mechanism is operative. Compound [3]+ exhibits photocytotoxicity in HeLa cervical cancer cells to give IC50 values of (12 ± 0.9) μM in UV‐A light at 365 nm and (31.4 ± 1.1) μM in the dark.