Singlet Oxygen Formation vs Photodissociation for Light- Responsive Protic Ruthenium Anticancer Compounds: The Oxygenated Substituent Determines Which Pathway Dominates

Ruthenium complexes bearing protic diimine ligands are cytotoxic to certain cancer cells upon irradiation with blue light. Previously reported complexes of the type [(N,N)(2) Ru(6,6'-dhbp)]Cl-2 with 6,6'-dhbp = 6,6'-dihydroxybipyridine and N,N = 2,2'-bipyridine (bipy) (1(A)), 1,10-phenanthroline (phen) (2(A)), and 2,3-dihydro-[1,4]-dioxino[2,3-f][1,10]phenanthroline (dop) (3(A)) show EC50 values as low as 4 mu M (for 3(A)) vs breast cancer cells upon blue light irradiation (Inorg. Chem. 2017, 56, 7519). Herein, subscript A denotes the acidic form of the complex bearing OH groups, and B denotes the basic form bearing O- groups. This photo-cytotoxicity was originally attributed to photodissociation, but recent results suggest that singlet oxygen formation is a more plausible cause of photocytotoxicity. In particular, bulky methoxy substituents enhance photodissociation but these complexes are nontoxic (Dalton Trans 2018, 47, 15685). Cellular studies are presented herein that show the formation of reactive oxygen species (ROS) and apoptosis indicators upon treatment of cells with complex 3A and blue light. Singlet oxygen sensor green (SOSG) shows the formation of O-1(2) in cell culture for cells treated with 3(A) and blue light. At physiological pH, complexes 1(A)-3(A) are deprotonated to form 1(B)-3(B) in situ. Quantum yields for O-1(2) (phi Delta) are 0.87 and 0.48 for 2(B) and 3(B), respectively, and these are an order of magnitude higher than the quantum yields for 2(A) and 3(A). The values for phi(Delta) show an increase with 6,6'-dhbp derived substituents as follows: OMe < OH < O-. TD-DFT studies show that the presence of a low lying triplet metal-centered ((MC)-M-3) state favors photodissociation and disfavors O-1(2) formation for 2(A) and 3(A) (OH groups). However, upon deprotonation (O- groups), the (MLCT)-M-3 state is accessible and can readily lead to O-1(2) formation, but the dissociative (MC)-M-3 state is energetically inaccessible. The changes to the energy of the (MLCT)-M-3 state upon deprotonation have been confirmed by steady state luminescence experiments on 1(A)-3(A) and their basic analogs, 1(B)-3(B). This energy landscape favors O-1(2) formation for 2(B) and 3(B) and leads to enhanced toxicity for these complexes under physiological conditions. The ability to convert readily from OH to O- groups allowed us to investigate an electronic change that is not accompanied by steric changes in this fundamental study.

Automated summary

Ruthenium complexes with protic diimine ligands exhibit cytotoxicity to cancer cells under blue light irradiation, likely due to singlet oxygen formation rather than photodissociation. Cellular studies have shown the generation of reactive oxygen species and apoptosis indicators upon treatment with specific complexes and blue light, highlighting their potential for targeted cancer therapy.