Selective Protection of Secondary Amines as the N-Phenyltriazenes. Application to Aminoglycoside Antibiotics

Synthesis of Methyl Aprabiosaminide and 2-Hydroxyapramycin from Apramycin

We describe a protocol for the selective cleavage of the 2-deoxystreptamine ring from the structurally unusual aminoglycoside antibiotic apramycin, enabling for the first time the preparation of aprabiosamine derivatives. We further describe reglycosylation of the aprabiosamine core with a selectively protected optically pure streptamine derivative, giving, after deprotection, 2-hydroxyapramycin, the first apramycin derivative functionalized at the 2 position.

A Chemoselective Enrichment Strategy for In-Depth Coverage of the Methyllysine Proteome

Proteomics is a powerful method to comprehensively understand cellular posttranslational modifications (PTMs). Owing to low abundance, tryptic peptides with PTMs are usually enriched for enhanced coverage by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). Affinity chromatography for phosphoproteomes by metal-oxide and pan-specific antibodies for lysine acetylome allow identification of tens of thousands of modification sites. Lysine methylation is a significant PTM; however, only hundreds of methylation sites were identified by available approaches. Herein we report an aryl diazonium based chemoselective strategy that enables enrichment of monomethyllysine (Kme1) peptides through covalent bonds with extraordinary sensitivity. We identified more than 10000 Kme1 peptides from diverse cell lines and mouse tissues, which implied a wide lysine methylation impact on cellular processes. Furthermore, we found a significant amount of methyl marks that were not S-adenosyl methionine (SAM)-dependent by isotope labeling experiments.

Site-Selective Modification of Secondary Amine Moieties on Native Peptides, Proteins, and Natural Products with Ynones

Site-selective modification of biologically relevant secondary amines in peptides, proteins, and natural products has been challenging due to the similar reactivity between primary and secondary amines. Even for the secondary amines, their reactivities are significantly influenced by their structures and environment. Herein, we report a ynone Michael bioconjugation method for selective modification of secondary amines in unprotected peptides and proteins and complex natural products. We show that fine tuning the electronic effect of the ynones enables controlling the Michael acceptor reactivity for the selective reaction with the structurally different secondary amines in densely functionalized complex structures and complicated biological environment.

Covalent Chemical Tools for Profiling Post-Translational Modifications

Nature increases the functional diversity of the proteome through posttranslational modifications (PTMs); a process that involves the proteolytic processing or catalytic attachment of diverse functional groups onto proteins. These modifications modulate a host of biological activities and responses. Consequently, anomalous PTMs often correlate to a host of diseases, hence there is a need to detect these transformations, both qualitatively and quantitatively. One technique that has gained traction is the use of robust chemical strategies to label different PTMs. By utilizing the intrinsic chemical reactivity of the different chemical groups on the target amino acid residues, this strategy can facilitate the delineation of the overarching and inclusionary roles of these different modifications. Herein, we will discuss the current state of the art in post-translational modification analysis, with a direct focus on covalent chemical methods used for detecting them.

Selective Triazenation Reaction (STaR) of Secondary Amines for Tagging Monomethyl Lysine Post-Translational Modifications

Lysine monomethylation (Kme) is an impactful post-translational modification (PTM) responsible for regulating biological processes and implicated in diseases, thus there is great interest in identifying these methylation marks globally. However, the progress in this area has been challenging because the addition of a small methyl group on lysine leads to negligible change in the bulk, charge, and hydrophobicity. Herein, we report an empowering chemical technology selective triazenation reaction, which we term "STaR", of secondary amines using arene diazonium salts to achieve highly selective, rapid, and robust tagging of Kme peptides from a complex mixture under biocompatible conditions. Although the resulting triazene-linkage with Kme is stable, we highlight the efficient decoupling of the triazene-conjugate to afford unmodified starting components under mild conditions when desired. Our work establishes a unique chemoselective, traceless bioconjugation strategy for the selective enrichment of Kme PTMs.

Application of Dimedone Enamines as Protecting Groups for Amines and Peptides

A simple protocol for the protection of amines was realized through a base-catalyzed one-pot reaction of dimedone, β-nitroalkene, and amine. Employing this strategy, a variety of amines/amino acids were protected in excellent yields. These acid/base stable protected amines can be deprotected by either ethylene diamine or hydrazine hydrate under mild conditions. The practical application of this orthogonal protecting group was demonstrated by the synthesis of cyclic peptide melanotan II via SPPS.

Synthesis, ribosomal selectivity, and antibacterial activity of netilmicin 4'-derivatives

Halogenation of a suitably protected netilmicin derivative enables preparation of 4'-chloro-, bromo-, and iodo derivatives of netilmicin after deprotection. Suzuki coupling of a protected 4'-bromo derivative with phenylboronic acid or butyltrifluoroborate affords the corresponding 4'-phenyl and 4'-butyl derivatives of netilmicin. Sulfenylation of suitably protected netilmicin derivative with ethanesulfenyl chloride followed by deprotection affords 4'-ethylsulfanylnetilmicin. All netilmicin 4'-derivatives displayed reduced levels of inhibition for prokaryotic ribosomes and reduced antibacterial activity against typical Gram-positive and Gram-negative strains. None of the derivatives displayed enhanced target selectivity.

Effects of the 1- N-(4-Amino-2 S-hydroxybutyryl) and 6'- N-(2-Hydroxyethyl) Substituents on Ribosomal Selectivity, Cochleotoxicity, and Antibacterial Activity in the Sisomicin Class of Aminoglycoside Antibiotics

Syntheses of the 6'- N-(2-hydroxyethyl) and 1- N-(4-amino-2 S-hydroxybutyryl) derivatives of the 4,6-aminoglycoside sisomicin and that of the doubly modified 1- N-(4-amino-2 S-hydroxybutyryl)-6'- N-(2-hydroxyethyl) derivative known as plazomicin are reported together with their antibacterial and antiribosomal activities and selectivities. The 6'- N-(2-hydroxyethyl) modification results in a moderate increase in prokaryotic/eukaryotic ribosomal selectivity, whereas the 1- N-(4-amino-2 S-hydroxybutyryl) modification has the opposite effect. When combined in plazomicin, the effects of the two groups on ribosomal selectivity cancel each other out, leading to the prediction that plazomicin will exhibit ototoxicity comparable to those of the parent and the current clinical aminoglycoside antibiotics gentamicin and tobramycin, as borne out by ex vivo studies with mouse cochlear explants. The 6'- N-(2-hydroxyethyl) modification restores antibacterial activity in the presence of the AAC(6') aminoglycoside-modifying enzymes, while the 1- N-(4-amino-2 S-hydroxybutyryl) modification overcomes resistance to the AAC(2') class but is still affected to some extent by the AAC(3) class. Neither modification is able to circumvent the ArmA ribosomal methyltransferase-induced aminoglycoside resistance. The use of phenyltriazenyl protection for the secondary amino group of sisomicin facilitates the synthesis of each derivative and their characterization through the provision of sharp NMR spectra for all intermediates.