Peptide Stapling through Site-Directed Conjugation of Triazine Moieties to the Tyrosine Residues of a Peptide

Chemoselective, regioselective, and positionally selective fluorogenic stapling of unprotected peptides for cellular uptake and direct cell imaging

Peptide stapling reactions represent powerful methods for structuring native α-helices to improve their bioactivity in targeting protein-protein interactions (PPIs). In light of a growing need for regio- and positionally selective stapling methods involving natural amino acid residues in their unprotected states, we report a rapid, mild, and highly chemoselective three-component stapling reation using a class of molecular linchpins based on 2-arylketobenzaldehydes (ArKBCHOs) that create a fluorescent staple, hereafter referred to as a Fluorescent Isoindole Crosslink (FlICk). This methodology offers positional selectivity favouring i, i + 4 helical staples comprising a lysine and cysteine, in the presence of competing nucleophiles on unprotected peptides. In our efforts to further validate this chemistry, we have successfully shown in vitro cytotoxicity of a FlICk-ed peptide (IC50 = 5.10 ± 1.27 μM), equipotent to an olefin-stapled congener. In harnessing the innate fluorescence of the thiol-isoindole, we report new blue-green fluorophores, which arise as a consequence of stapling, with appreciable quantum yields that enable direct cellular imaging in the assessment of cell permeability, thus bridging therapeutic potential with cytological probe development.

Recent advances in peptide macrocyclization strategies

Recently, owing to their special spatial structures, peptide-based macrocycles have shown tremendous promise and aroused great interest in multidisciplinary research ranging from potent antibiotics against resistant strains to functional biomaterials with novel properties. Besides traditional monocyclic peptides, many fascinating polycyclic and remarkable higher-order cyclic, spherical and cylindric peptidic systems have come into the limelight owing to breakthroughs in various chemical (e.g., native chemical ligation and transition metal catalysis), biological (e.g., post-translational enzymatic modification and genetic code reprogramming), and supramolecular (e.g., mechanically interlocked, metal-directed folding and self-assembly via noncovalent interactions) macrocyclization strategies developed in recent decades. In this tutorial review, diverse state-of-the-art macrocyclization methodologies and techniques for peptides and peptidomimetics are surveyed and discussed, with insights into their practical advantages and intrinsic limitations. Finally, the synthetic-technical aspects, current unresolved challenges, and outlook of this field are discussed.

Comparison of Protocols to Test Peptide Stability in Blood Plasma and Cell Culture Supernatants

Due to their high specificity, peptides are promising candidates in drug development, but fast degradation often limits their biological activity. Thus, a short half-life is one of the major challenges in the development of new peptide therapeutics. Moreover, the enzymatic cleavage of peptides can be a reason for misleading results in biological assays. Peptide stability assays typically consist of incubation, precipitation, and detection steps. However, the current methods differ greatly regarding these three steps, thus limiting the compatibility. Here, we systematically evaluate different parameters of peptide stability assays. First, we quantified and compared the analyte loss during the precipitation of plasma proteins. Especially, broadly used precipitation by strong acids was found to be unsuitable, while mixtures of organic solvents preserved more peptides for further analysis. Next, the stability of four fluorescently labeled model peptides was analyzed in blood plasma and two different cell culture supernatants. Strong variation in the degradation dynamics and patterns was found. Finally, we evaluated the role of fluorescent labeling on peptide stability and compared results to peptides with isotopic labels, underlining the individual advantages of both methods. Altogether, the data provide important parameters for analyzing and comparing the peptide stability.

Peptide Stapling by Crosslinking Two Amines with α-Ketoaldehydes through Diverse Modified Glyoxal-Lysine Dimer Linkers

α-Ketoaldehydes play versatile roles in the ubiquitous natural processes of protein glycation. However, leveraging the reactivity of α-ketoaldehydes for biomedical applications has been challenging. Previously, the reactivity of α-ketoaldehydes with guanidine has been harnessed to design probes for labeling Arg residues on proteins in an aqueous medium. Herein, a highly effective, broadly applicable, and operationally simple protocol for stapling native peptides by crosslinking two amino groups through diverse imidazolium linkers with various α-ketoaldehyde reagents is described. The use of hexafluoroisopropanol as a solvent facilitates rapid and clean reactions under mild conditions and enables unique selectivity for Lys over Arg. The naturally occurring GOLD/MOLD linkers have been expanded to encompass a wide range of modified glyoxal-lysine dimer (OLD) linkers. In a proof-of-concept trial, these modular stapling reactions enabled a convenient two-round strategy to streamline the structure-activity relationship (SAR) study of the wasp venom peptide anoplin, leading to enhanced biological activities.

Research progress on chemical modifications of tyrosine residues in peptides and proteins

The chemical modifications (CMs) of protein is an important technique in chemical biology, protein-based therapy, and material science. In recent years, there has been rapid advances in the development of CMs of peptides and proteins, providing new approaches for peptide and protein functionalization, as well as drug discovery. In this review, we highlight the methods for chemically modifying tyrosine (Tyr) residues in different regions, offering a comprehensive exposition of the research content related to Tyr modification. This review summarizes and provides an outlook on Tyr residue modification, aiming to offer readers assistance in the site-selective modification of macromolecules and to facilitate application research in this field.