Photoactive ruthenium nitrosyls: Effects of light and potential application as NO donors

Abstract

In recent years, various exogenous nitric oxide (NO) donors have been synthesized to modulate NO concentrations in cellular environments and control physiological processes that are regulated by NO. Transition metal complexes of NO (metal nitrosyls) are one such class of NO donors. Since complexes of ruthenium are in general more stable, a variety of ruthenium nitrosyls have been isolated and studied in detail in terms of their NO donating capacities. A large number of {Ru–NO} 6 type of nitrosyls release NO upon exposure to UV light. Several research groups have studied their photochemistry to evaluate their potential as NO donors under the control of light. In general, the nitrosyls with non-porphyrin ligands (such as amines, Schiff bases, thiolates and ligands with carboxamide groups) readily release NO upon illumination and generate Ru(III) photoproducts. In contrast, NO photorelease from ruthenium nitrosyls derived from porphyrins remains limited due to rapid recombination. In some cases, the {Ru–NO} 6 nitrosyls are photochemically converted to nitrite species (especially in water at neutral pH) while a few afford Ru(II) photoproducts. UV irradiation of selected ruthenium nitrosyls in the solid state results in NO linkage isomerism. To date, notable progress has been made in the area of nitrosyl-polymer hybrids that could be used for site-specific delivery of NO. Various strategies have also been developed to make these nitrosyls release NO under the influence of visible and/or near IR light. Although some ruthenium nitrosyls are stable under physiological conditions and are capable to NO delivery to proteins such as myoglobin and cytochrome c oxidase, so far success has been limited in using these nitrosyls as light-activated NO donors in cellular and tissue models. In this review, the effects of light on ruthenium nitrosyls derived from a wide variety of ligands (reported so far) have been summarized and their utility as NO donors have been discussed.