A Direct Method for Oxidizing Quinoxaline, Tetraazaphenanthrene, and Hexaazatriphenylene Moieties Using Hypervalent λ³‑Iodinane Compounds

An efficient oxidation reaction of various electron-poor quinoxaline-core-containing compounds, such as quinoxalines, 1,4,5,8-tetraazaphenanthrenes, and 1,4,5,8,9,12-hexaazatriphenylene, using [bis­(trifluoroacetoxy)­iodo]­benzene is reported. These compounds are converted into the corresponding quinoxalinediones in good to high yields at room temperature using an acetonitrile/water solvent mixture. This unprecedented reaction should enable the synthesis of a wide variety of compounds useful in several fields of chemistry

Synthesis and Electrochemical and Photophysical Properties of Calixarene-Based Ruthenium(II) Complexes as Potential Multivalent Photoreagents

The grafting of photoreactive and photooxidizing Ru^II(TAP) (TAP = 1,4,5,8-tetraazaphenanthrene) complexes on calix­[4 or 6]­arene molecular platforms is reported. Thus, either [Ru­(TAP)₂(phen)]²⁺ (phen = 1,10-phenanthroline) or [Ru­(TAP)₂(pytz)]²⁺ [pytz = 2-(1,2,3-triazol-4-yl)­pyridine] complexes are anchored to the calixarenes. The data in electrochemistry, combined with those in emission under steady state and pulsed illumination and the determination of the associated photophysical rate constants, indicate the presence of intramolecular luminescence quenching by the phenol moieties of calixarene. From transient absorption studies under pulsed laser irradiation, it is concluded that the quenching originates from a par proton-coupled electron transfer (PCET) process. Such an intramolecular quenching is absent when the phenol groups of the calixarene platform are derivatized by azido arms

pH Dependence of Photoinduced Electron Transfer with [Ru(TAP)₃]²⁺

The quenching of the excited state of [Ru­(TAP)₃]²⁺ (TAP = 1,4,5,8-tetraazaphenanthrene) by guanosine-5′-monophosphate (GMP), <i>N</i>-acetyltyrosine (<i>N</i>-Ac-Tyr), and hydroquinone (H₂Q) has been studied in aqueous solution over a wide range of pH values including, for the first time, strongly acidic media. This quenching by electron transfer was examined by steady-state ¹H photochemically induced dynamic nuclear polarization (photo-CIDNP) as well as by more conventional techniques, among which are pulsed laser-induced transient absorption and emission experiments. A deeper knowledge of the photochemical behavior of [Ru­(TAP)₃]²⁺ has been gained thanks to the combined use of these two approaches, photo-CIDNP and electronic spectroscopies, highlighting their complementarity. In contrast to what was believed, it is found that the protonated excited state of [Ru­(TAP)₃]²⁺ may give rise to an electron transfer with <i>N</i>-Ac-Tyr and H₂Q. Such a photoinduced electron transfer does not occur with protonated GMP, however. ¹H photo-CIDNP experiments are expected to be particularly promising for characterization of the reductive quenching of excited-state ruthenium­(II) polypyridyl complexes comprising several nonequivalent protonation sites

Mesoscale DNA Structural Changes on Binding and Photoreaction with Ru[(TAP)₂PHEHAT]²⁺

We used scanning force microscopy (SFM) to study the binding and excited state reactions of the intercalating photoreagent Ru­[(TAP)₂PHEHAT]²⁺ (TAP = 1,4,5,8-tetraazaphenanthrene; PHEHAT = 1,10-phenanthrolino­[5,6-<i>b</i>]­1,4,5,8,9,12-hexaazatriphenylene) with DNA. In the ground state, this ruthenium complex combines a strong intercalative binding mode via the PHEHAT ligand, with TAP-mediated hydrogen bonding capabilities. After visible irradiation, SFM imaging of the photoproducts revealed both the structural implications of photocleavages and photoadduct formation. It is found that the rate of photocleaving is strongly increased when the complex can interact with DNA via hydrogen bonding. We demonstrated that the photoadduct increases DNA rigidity, and that the photo-biadduct can crosslink two separate DNA segments in supercoiled DNA. These mechanical and topological effects might have important implications in future therapeutic applications of this type of compounds

Revisited Photophysics and Photochemistry of a Ru-TAP Complex Using Chloride Ions and a Calix[6]crypturea

The effects of the nonprotonated and protonated calix[6]­crypturea <b>1/1^•</b><b>H⁺</b> on the PF₆^– and Cl^– salts of a luminescent Ru-TAP complex (TAP = 1,4,5,8-tetraazaphenanthrene) were investigated. Thus, the phototriggered basic properties of this complex were examined with <b>1^•H⁺</b> in acetonitrile (MeCN) and butyronitrile (BuCN). The Ru excited complex was shown to be able to extract a proton from the protonated calixarene, accompanied by a luminescence quenching in both solvents. However, in BuCN, the Cl^– salt of the complex exhibited a surprising behavior in the presence of <b>1/1^•</b><b>H⁺</b>. Although an emission decrease was observed with the protonated calixarene, an emission increase was evidenced in the presence of nonprotonated <b>1</b>. As the Cl^– ions were shown to inhibit the luminescence of the complex in BuCN, this luminescence increase by nonprotonated <b>1</b> was attributed to the protection effect of <b>1</b> by encapsulation of the Cl^– anions into the <i>tris</i>-urea binding site. The study of the luminescence lifetimes of the Ru-TAP complex in BuCN as a function of temperature for the PF₆^– and Cl^– salts in the absence and presence of <b>1</b> led to the following conclusions. In BuCN, in contrast to MeCN, in addition to ion pairing, because of the poor solvation of the ions, the luminescent metal-to-ligand charge transfer (³MLCT) state could reach two metal-centered (³MC) states, one of which is in equilibrium with the ³MLCT state during the emission lifetime. The reaction of Cl^– with this latter ³MC state would be responsible for the luminescence quenching, in agreement with the formation of photosubstitution products

A Toolkit for Engineering Proteins in Living Cells: Peptide with a Tryptophan-Selective Ru-TAP Complex to Regioselectively Photolabel Specific Proteins

Using a chemical approach to crosslink functionally versatile bioeffectors (such as peptides) to native proteins of interest (POI) directly inside a living cell is a useful toolbox for chemical biologists. However, this goal has not been reached due to unsatisfactory chemoselectivity, regioselectivity, and protein selectivity in protein labeling within living cells. Herein, we report the proof of concept of a cytocompatible and highly selective photolabeling strategy using a tryptophan-specific Ru-TAP complex as a photocrosslinker. Aside from the high selectivity, the photolabeling is blue light-driven by a photoinduced electron transfer (PeT) and allows the bioeffector to bear an additional UV-responsive unit. The two different photosensitivities are demonstrated by blue light-photocrosslinking a UV-sensitive peptide to POI. Our visible light photolabeling can generate photocaged proteins for subsequent activity manipulation by UV light. Cytoskeletal dynamics regulation is demonstrated in living cells via the unprecedented POI photomanipulation and proves that our methodology opens a new avenue to endogenous protein modification