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The post-translational modification of histones by the small ubiquitin-like modifier (SUMO) protein has been associated with gene regulation, centromeric localization, and double-strand break repair in eukaryotes. Although sumoylation of histone H4 was specifically associated with gene repression, this could not be proven due to the challenge of site-specifically sumoylating H4 in cells. Biochemical crosstalk between SUMO and other histone modifications, such as H4 acetylation and H3 methylation, that are associated with active genes also remains unclear. We addressed these challenges in mechanistic studies using an H4 chemically modified at Lys12 by SUMO-3 (H4K12su) and incorporated into mononucleosomes and chromatinized plasmids for functional studies. Mononucleosome-based assays revealed that H4K12su inhibits transcription-activating H4 tail acetylation by the histone acetyltransferase p300, as well as transcription-associated H3K4 methylation by the extended catalytic module of the Set1/COMPASS (complex of proteins associated with Set1) histone methyltransferase complex. Activator- and p300-dependent in vitro transcription assays with chromatinized plasmids revealed that H4K12su inhibits both H4 tail acetylation and RNA polymerase II-mediated transcription. Finally, cell-based assays with a SUMO-H4 fusion that mimics H4 tail sumoylation confirmed the negative crosstalk between histone sumoylation and acetylation/methylation. Thus, our studies establish the key role for histone sumoylation in gene silencing and its negative biochemical crosstalk with active transcription-associated marks in human cells.

The Cu(i)-mediated click reaction of proteins with affinity tags enables their selective isolation from complex mixtures. However, irreversible protein modification limits the interpretation of results from subsequent biophysical and biochemical assays. We report a facile and modular chemical strategy to reversibly modify peptides and proteins with biotin and FLAG affinity tags at a clickable glutamine (CliQ) residue.

Protonation of an iron C–H activated complex containing pendant amines in the presence of N2 generated a cis-(H)FeII–N2 complex. Addition of acid protonates the pendant amines. Reduction of the protonated complex results in N2 loss and H2 formation, followed by N2 binding. The origin of H2 formation in this Fe system is compared to proposed mechanisms for H2 loss and N2 coordination in the E4 state of nitrogenase.

Synthetically useful radical thiol-ene reactions can be initiated by visible light irradiation in the presence of transition metal polypyridyl photocatalysts. The success of this method relies upon the use of p-toluidine as an essential additive. Using these conditions, high-yielding thiol-ene reactions of cysteine-containing biomolecules can be accomplished using biocompatibile wavelengths of visible light, under aqueous conditions, and with the thiol component as the limiting reagent. We present evidence that p-toluidine serves as a redox mediator that is capable of catalyzing the otherwise inefficient photooxidation of thiols to the key thiyl radical intermediate. Thus, we show that co-catalytic oxidants can be important in the design of synthetic reactions involving visible light photoredox catalysis.

α,β-Unsaturated 2-imidazolyl ketones undergo [2 + 2] cycloaddition with a variety of Michael acceptors upon irradiation with visible light in the presence of Ru(bpy)(3)(2+). Cleavage of the imidazolyl auxiliary from the cycloadducts affords cyclobutane carboxamides, esters, thioesters, and acids that would not be accessible from direct cycloaddition of the corresponding unsaturated carbonyl compounds.

Access to protein substrates homogenously modified by ubiquitin (Ub) is critical for biophysical and biochemical investigations aimed at deconvoluting the myriad biological roles for Ub. Current chemical strategies for protein ubiquitylation, however, employ temporary ligation auxiliaries that are removed under harsh denaturing conditions and have limited applicability. We report an unprecedented aromatic thiol-mediated N–O bond cleavage and its application towards native chemical ubiquitylation with the ligation auxiliary 2-aminooxyethanethiol. Our interrogation of the reaction mechanism suggests a disulfide radical anion as the active species capable of cleaving the N–O bond. The successful semisynthesis of full-length histone H2B modified by the small ubiquitin-like modifier-3 (SUMO-3) protein further demonstrates the generalizability and compatibility of our strategy with folded proteins., Chemical approaches to site-specifically ubiquitylate a target protein allow investigation of the biochemical effects of this modification, but they often destabilize the protein. Here, the authors report on a synthetic conjugation strategy that leads to protein ubiquitylation in non-denaturing conditions.

Synthetically useful radical thiol–ene reactions can be initiated by visible light irradiation in the presence of transition metal polypyridyl photocatalysts. The success of this method relies upon the use of p-toluidine as an essential additive. Using these conditions, high-yielding thiol–ene reactions of cysteine-containing biomolecules can be accomplished using biocompatibile wavelengths of visible light, under aqueous conditions, and with the thiol component as the limiting reagent. We present evidence that p-toluidine serves as a redox mediator that is capable of catalyzing the otherwise inefficient photooxidation of thiols to the key thiyl radical intermediate. Thus, we show that co-catalytic oxidants can be important in the design of synthetic reactions involving visible light photoredox catalysis.

An anti-Markovnikov hydrothiolation of olefins using a visible-light-absorbing transition metal photocatalyst is described with a variety of thiols and olefins.

We describe the anti-Markovnikov hydrothiolation of olefins using visible-light-absorbing transition metal photocatalysts. The key thiyl radical intermediates are generated upon quenching of photoexcited Ru*(bpz)3(2) with a variety of thiols. The adducts of a wide variety of olefins and thiols are formed in excellent yield (73-99%).

The use of this strategy enables the synthesis of a variety of cyclobutane carboxylic acid derivatives which are not accessible from direct cycloaddition of the corresponding unsaturated carbonyl compounds.

Domino reactions were designed to allow the byproduct of an upstream reaction to be internally recycled to catalyze a downstream reaction in a one-pot tandem sequence. Nitroarene reduction by In(0) generates an amine and In (III) byproducts. Addition of aldehyde followed by Danishefsky's diene or silyl ketene acetal provides access to dihydropyridin-4-ones or beta-amino esters, respectively, in yields that are comparable or superior to the reported stepwise reactions.