Chemical labeling of proteins with synthetic low-molecular-weight probes is an important technique in chemical biology. To achieve this, it is necessary to use chemical reactions that proceed rapidly ...under physiological conditions (i.e., aqueous solvent, pH, low concentration, and low temperature) so that protein denaturation does not occur. The radical reaction satisfies such demands of protein labeling, and protein labeling using the biomimetic radical reaction has recently attracted attention. The biomimetic radical reaction enables selective labeling of the C-terminus, tyrosine, and tryptophan, which is difficult to achieve with conventional electrophilic protein labeling. In addition, as the radical reaction proceeds selectively in close proximity to the catalyst, it can be applied to the analysis of protein-protein interactions. In this review, recent trends in protein labeling using biomimetic radical reactions are discussed.
Polymerization of N‐carboxy anhydrides (NCAs) is the primary process used to prepare polypeptides. The synthesis of various pure NCAs is key to the efficient synthesis of polypeptides. The only ...practical method that can be used to synthesize NCAs requires harsh acidic conditions that make acid‐labile substrates unusable and results in an undesired ring opening of NCAs. Basic‐to‐acidic flash switching and subsequent flash dilution technology in a microflow reactor was used to demonstrate the synthesis of NCAs. It is both rapid (0.1 s) and mild (20 °C) and includes substrates containing acid‐labile functional groups. The basic‐to‐acidic flash switching enabled both an acceleration of the desired NCA formation and avoided the undesired ring opening of NCAs. The flash dilution precluded the undesired decomposition of acid‐labile functional groups. The developed process allowed the synthesis of various NCAs which cannot be readily synthesized using conventional batch methods.
In a flash: Rapid (0.1 s) and mild (20 °C) synthesis of N‐carboxy anhydrides (NCAs) using basic‐to‐acidic flash switching and subsequent flash dilution was demonstrated. This process enabled the synthesis of acid‐labile NCAs, which are impossible to access using conventional batch conditions. The developed process requires simple phase separations and recrystallization to yield pure NCAs. This microflow approach could pave the way to on‐demand, on‐site NCA synthesis.
A photocatalyst (Ru(bpy)32+) bound to a protein ligand was essential for the title method. Local single‐electron transfer from the catalyst resulted in the formation of tyrosyl radicals. ...N′‐Acetyl‐N,N‐dimethyl‐1,4‐phenylenediamine was used as the tyrosyl radical trapping agent and used in a radical addition to afford selective modification of the target protein.
The widespread applications of benzophenone (BP) photochemistry in biological chemistry, bioorganic chemistry, and material science have been prominent in both academic and industrial research. BP ...photophores have unique photochemical properties: upon n−π* excitation at 365 nm, a biradicaloid triplet state is formed reversibly, which can abstract a hydrogen atom from accessible C–H bonds; the radicals subsequently recombine, creating a stable covalent C–C bond. This light-directed covalent attachment process is exploited in many different ways: (i) binding/contact site mapping of ligand (or protein)–protein interactions; (ii) identification of molecular targets and interactome mapping; (iii) proteome profiling; (iv) bioconjugation and site-directed modification of biopolymers; (v) surface grafting and immobilization. BP photochemistry also has many practical advantages, including low reactivity toward water, stability in ambient light, and the convenient excitation at 365 nm. In addition, several BP-containing building blocks and reagents are commercially available. In this review, we explore the “forbidden” (transitions) and excitation-activated world of photoinduced covalent attachment of BP photophores by touring a colorful palette of recent examples. In this exploration, we will see the pros and cons of using BP photophores, and we hope that both novice and expert photolabelers will enjoy and be inspired by the breadth and depth of possibilities.
Chronic active Epstein–Barr virus (EBV) infection (CAEBV) is a rare syndrome characterized by prolonged infectious mononucleosis-like symptoms and elevated peripheral blood EBV DNA load in apparently ...immunocompetent persons. CAEBV has been primarily reported in East Asia and Latin America, suggesting a genetic predisposition in its pathogenesis. In most cases of CAEBV, EBV induces proliferation of its unusual host cells, T or natural killer (NK) cells. The clinical course of CAEBV is heterogeneous; some patients show an indolent course, remaining in a stable condition for years, whereas others show an aggressive course with a fatal outcome due to hemophagocytic lymphohistiocytosis, multiple organ failure, or progression to leukemia/lymphoma. The pathogenesis of CAEBV is unclear and clinicopathological investigations suggest that it has aspects of both malignant neoplasm and immunodeficiency. Recent genetic analyses of both viral and host genomes in CAEBV patients have led to discoveries that are improving our understanding of the nature of this syndrome. This article summarizes the latest findings on CAEBV and discusses critical unsolved questions regarding its pathogenesis and disease concept.
Tyrosyl radical generation is one of the major factors for hemin/peroxide-induced oxidative stress. A method for trapping tyrosyl radical directly was developed using N-methyl luminol derivative, a ...tyrosine labeling reagent. N-Methyl luminol derivative selectively forms a covalent bond with a tyrosine residue under the single-electron oxidation condition. This reaction labels oxidative stress hotspots not only at the protein level but also at the level of tyrosine residues undergoing oxidation. Human serum albumin complexed with hemin was labeled at Tyr138, the tyrosine residue closest to the hemin binding site and most strongly subjected to oxidative stress caused by hemin/H2O2. Oxidatively damaged proteins were visualized in protein mixtures.
The arachidonic acid (AA) cascade is regulated mainly by the actions of two rate-limiting enzymes, phospholipase A2 (PLA2) and inducible cyclooxygenase-2 (COX-2). PLA2 acts to generate AA, which ...serves as the precursor substrate for COX-2 in the metabolic pathway leading to prostaglandin production. Amongst more than 30 members of the PLA2 family, cytosolic PLA2α (cPLA2α, group IVA) plays a major role in releasing AA from cellular membranes. Sphingolipids are a novel class of bioactive lipids that play key roles in the regulation of several cellular processes including growth, differentiation, inflammatory responses, and apoptosis. Recent studies implicated a regulatory function of sphingolipids in prostaglandin production. Whereas ceramide-1-phosphate and lactosylceramide activate cPLA2α directly, sphingosine-1-phosphate induces COX-2 expression. Sphingomyelin has been shown to inhibit the activity of cPLA2α. In addition, several sphingolipid analogs including a therapeutic agent currently used clinically are also reported to be inhibitors of cPLA2α. This review explores the role of sphingolipids in the regulation of cPLA2α and COX-2.
The zinc(II)-catalyzed redox cross-dehydrogenative coupling (CDC) of propargylic amines and terminal alkynes proceeds to afford N-tethered 1,6-enynes. In the current CDC reaction, a C(sp)–C(sp3) bond ...is formed between the carbon adjacent to the nitrogen atom in the propargylic amine and the terminal carbon of the alkyne with reduction of the C–C triple bond of the propargylic amine, which acts as an internal oxidant.
Sphingolipids (SLs), such as ceramide, glucosylceramide (GlcCer), and sphingomyelin, play important roles in the normal development/functions of the brain and peripheral tissues. Disruption of SL ...homeostasis in cells/organelles, specifically up-regulation of ceramide, is involved in multiple diseases including Alzheimer’s disease (AD). One of the pathological features of AD is aggregates of amyloid beta (Aβ) peptides, and SLs regulate both the formation/aggregation of Aβ and Aβ-induced cellular responses. Up-regulation of ceramide levels via de novo and salvage synthesis pathways is reported in Aβ-treated cells and brains with AD; however, the effects of Aβ on ceramide decomposition pathways have not been elucidated. Thus, we investigated the effects of the 25–35-amino acid Aβ peptide (Aβ25–35), the fundamental cytotoxic domain of Aβ, on SL metabolism in cells treated with the fluorescent nitrobenzo-2-oxa-1,3-diazole-labeled C6-ceramide (NBD-ceramide). Aβ25–35 treatment reduced the formation of NBD-GlcCer mediated by GlcCer synthase (GCS) without affecting the formation of NBD-sphingomyelin or NBD-ceramide-1-phosphate, and reduced cell viability. Aβ25–35-induced responses decreased in cells treated with D609, a putative inhibitor of sphingomyelin synthases. Aβ25–35-induced cytotoxicity significantly increased in GCS-knockout cells and pharmacological inhibition of GCS alone demonstrated cytotoxicity. Our study revealed that Aβ25–35-induced cytotoxicity is at least partially mediated by the inhibition of GCS activity.
A palladium‐catalyzed N−H/B−H double activation of 1,2‐dihydro‐1,2‐benzazaborines proceeded via cycloaddition with vinyl ethylene carbonate to produce polycyclic oxazaborolidines in 31–96 % yield. ...The key step in this process is the release of molecular hydrogen from a borate intermediate. Using a SPINOL‐derived phosphoramidite as a chiral ligand, chiral oxazaborolidines were synthesized in good to high yields with excellent enantioselectivity (up to 95 % ee). The vinyl group of the resulting oxazaborolidine underwent metathesis, Heck reaction, and Wacker oxidation without affecting the oxazaborolidine framework.
Catalytic N−H/B−H double activation of 1,2‐azaborines has been demonstrated. A wide variety of 1,2‐azaborines underwent cycloaddition with vinylethylene carbonate through N−H/B−H bond cleavage to produce unique polycyclic oxazaborolidines under mild conditions. Furthermore, highly enantioselective direct functionalization of 1,2‐azaborines afforded chiral tricyclic oxazaborolidines.
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