Tyrosine or Tryptophan? Modifying a Metalloradical Catalytic Site by Removal of the Cys–Tyr Cross-Link in the Galactose 6-Oxidase Homologue GlxA

The concerted redox action of a metal ion and an organic cofactor is a unique way to maximize the catalytic power of an enzyme. An example of such synergy is the fungal galactose 6-oxidase, which has inspired the creation of biomimetic copper oxidation catalysts. Galactose 6-oxidase and its bacterial homologue, GlxA, possess a metalloradical catalytic site that contains a free radical on a covalently linked Cys–Tyr and a copper atom. Such a catalytic site enables for the two-electron oxidation of alcohols to aldehydes. When the ability to form the Cys–Tyr in GlxA is disrupted, a radical can still be formed. Surprisingly, the radical species is not the Tyr residue but rather a copper second-coordination sphere Trp residue. This is demonstrated through the introduction of a new algorithm for Trp-radical EPR spectra simulation. Our findings suggest a new mechanism of free-radical transfer between aromatic residues and that the Cys–Tyr cross-link prevents radical migration away from the catalytic site.     

Tyrosine Conjugation Methods for Protein Labelling

Over the last two decades, the development of chemical biology and the need for more defined protein conjugates have fostered active research on new bioconjugation techniques. In particular, a wide range of biorthogonal labelling strategies have been reported to functionalise the phenol side chain of tyrosines (Tyr). Tyr occur at medium frequency and are partially buried at the protein surface, offering interesting opportunities for site-selective labelling of the most reactive residues. Tyr-targeting has proved effective for designing a wide range of important biomolecules including antibody–drug conjugates, fluorescent or radioactive protein probes, glycovaccines, protein aggregates, and PEG conjugates. Innovative methods have also been reported for site-specific labelling with ligand-directed anchors and for the specific affinity capture of proteins. This review will present and discuss these promising alternatives to the conventional labelling of the nucleophilic lysine and cysteine residues.     

Ischemia/reperfusion injury of primary porcine cardiomyocytes in a low-shear microfluidic culture and analysis device

Ischemia/reperfusion (I/R) injury was induced in primary porcine cardiomyocytes in a low-shear microfluidic culture chip. The chip was capable of sustaining the cardiomyocyte culture and inducing I/R injury by subjecting the cells to periods of hypoxia lasting 3-4 hours followed by normoxia. Mitochondrial membrane potential was assayed using MitoTracker Red to follow mitochondrial depolarization, the earliest stage of apoptosis. Cell adhesion and morphology were also determined simultaneously with fluorescence measurements. Changes in membrane potential were observed earlier than previously reported, with mitochondria becoming depolarized as early as 2 hours into the ischemia period. The cells with depolarized mitochondria were deemed apoptotic. Out of 38-61 cells per time frame, the fraction of apoptotic cells was found to be similar to control samples (3%) at two hours of ischemia, which increased up to 22% at the end of the ischemia period as compared to 0% in the control samples. Morphological analysis of cells showed that 4 hours of ischemia followed by reperfusion produced blebbing cells within 2 hours of restoring oxygen to the chip. This approach is a versatile method for cardiomyocyte stress, and in future work additional analytical probes can be incorporated for a multi-analyte assay of cardiomyocyte apoptosis.     

Selective and Quantitative Oxidation of Xanthate End‐Groups of RAFT Poly(n‐butyl acrylate) Latexes by Ozonolysis

Although various successful strategies have been reported in the past for the postpolymerization modification of the reversible addition‐fragmentation chain transfer (RAFT) terminal group in homogeneous media, no solution is proposed for the tedious case of aqueous polymer dispersions where most of the thiocarbonylthio terminal group is buried into the core of the polymer particle. In this work, ozone is proposed to tackle this important academic and industrial challenge. After preliminary model ozonolysis reactions performed on a xanthate RAFT agent and a derived low molar mass poly(n‐butyl acrylate) (PBA) in dichloromethane solution, it is shown that the hydrophobic nature and strong oxidant properties of ozone are responsible for its efficient diffusion in aqueous PBA latex particles obtained by RAFT and selective and complete transformation of the xanthate terminal group into a thiocarbonate end‐group. In addition to the beneficial total discoloration of the final product, this chemical treatment does not generate any volatile organic compound and leaves the colloidal stability of the polymer particles unaffected, provided that a PBA latex with a sufficiently high M_n of 5000 g mol⁻¹ is selected.

     

A novel and easy two-step,microwave-assisted method for the synthesis of halophenyl pyrrolo[2,3-b]quinoxalines via their pyrrolo precursors. Evaluation of their bioactivity

A novel, two-step, facile route for the synthesis of pyrrolo[2,3-b]quinoxalines via 2,3-dioxopyrroles, enhanced by microwave irradiation, is presented. The newly synthesized 2,3-dioxo-5-halophenyl pyrrolo precursors 4a–c as well as the non-aromatized ethyl 2-(4-halophenyl)-1-methyl-2,4-dihydro-1H-pyrrolo[2,3-b]quinoxaline-3-carboxylates 6a–c and the aromatized ethyl 2-(4-halophenyl)-1-methyl-1H-pyrrolo[2,3-b]quinoxaline-3-carboxylates 7a–c were evaluated for their antioxidant, cytostatic, and antiviral properties. Most of them proved to be potent hydroxyl radical scavengers and inhibited in vitro lipid peroxidation. The compounds showed moderate antiproliferative activity, while 6a inhibited vaccinia virus at an EC₅₀ value of 2 μM, and 4c and 6c inhibited Sindbis virus at EC₅₀ values of 4 μM.