Mi Hee Lim

Nitric Oxide-Induced Fluorescence Enhancement by Displacement of Dansylated Ligands from Cobalt

The cobalt complexes [Co(Ds‐AMP)2] (1) and [Co(Ds‐AQ)2] (2), where Ds‐AMP and Ds‐AQ are the conjugate bases of dansyl aminomethylpyridine (Ds‐HAMP) and dansyl aminoquinoline (Ds‐HAQ), respectively, were synthesized in two steps as fluorescence‐based nitric oxide (NO) sensors and characterized by X‐ray crystallography. The fluorescence of the two complexes was significantly quenched in CH3CN or CH3OH compared to that of the free Ds‐HAMP or Ds‐HAQ ligands. Addition of NO to a CH3CN solution of 1 or 2 enhanced the integrated fluorescence emission by factors of 2.1(±0.3) or 3.6(±0.4) within 35 or 20 min, respectively. Introduction of NO to methanolic solutions of the complexes similarly increased the fluorescence by 1.4(±0.1) for 1 or 6.5(±1.4) for 2 within 1 h. These studies demonstrate that 1 and 2 can monitor the presence of NO with turn‐on emission and that their fluorescence responses are more rapid than those of previously reported cobalt systems in coordinating solvents such as CH3CN and CH3OH. 1H NMR and IR spectroscopic data revealed the formation of a {Co(NO)2}10 cobalt–dinitrosyl adduct, with concomitant dissociation of one ligand from the cobalt center, as the metal‐containing product of the NO reactions, a result indicating NO‐induced ligand release to be the cause of the fluorescence increase.

Fluorescence-based Nitric Oxide Detection

Nitric oxide (NO) is a neutral free radical gas. The NO molecule has long been recognized as an environmental contaminant and a potential health hazard in the atmosphere. It was not until recently that beneficial roles for NO were discovered in biological systems.1, 2, 3 In the years since the groundbreaking discovery of NO signaling in biology, further work has shown that NO is a ubiquitous messenger in the cardiovascular, immune, and nervous systems.1,4, 5, 6, 7, 8, 9, 10, 11, 12 Even though NO is a relatively stable species with reported half-lives under physiological conditions of up to five seconds,5, 13, 14 it readily reacts with a variety of species commonly found in living organisms. The targets of NO include dioxygen, oxygen radicals, thiols, amines, and transition metal ions. The reactions of NO with dioxygen and superoxide result in formation of reactive nitrogen oxide species (RNOS) NO2 and ONOO−, respectively. Both products are more reactive than NO itself In aqueous environments, the reactions of NO with dioxygen can also yield NO2.