Optimized syntheses of Fmoc azido amino acids for the preparation of azidopeptides

Rapid diazotransfer for selective lysine labelling

Azide functionalization of protein and peptide lysine residues allows selective bioorthogonal labeling to introduce new, site selective functionaltiy into proteins. Optimised diazotransfer reactions under mild conditions allow aqueous diazotransfer to occur in just 20 min at pH 8.5 on amino acid, peptide and protein targets. In addition, conditons can be modified to selectively label a single lysine residue in both protein targets investigated. Finally, we demonstrate selective modification of proteins containing a single azidolysine using copper(I)-catalyzed triazole formation.

The final walk with preptin

Preptin, a 34-amino acid peptide derived from pro-IGF2, is believed to influence various physiological processes, including insulin secretion and the regulation of bone metabolism. Despite its recognized involvement, the precise physiological role of preptin remains enigmatic. To address this knowledge gap, we synthesized 16 analogs of preptin, spanning a spectrum from full-length forms to fragments, and conducted comprehensive comparative activity evaluations alongside native human, mouse and rat preptin. Our study aimed to elucidate the physiological role of preptin. Contrary to previous indications of broad biological activity, our thorough analyses across diverse cell types revealed no significant biological activity associated with preptin or its analogs. This suggests that the associations of preptin with various diseases or tissue-specific abundance fluctuations may be influenced by factors beyond preptin itself, such as higher levels of IGF2 or IGF2 proforms present in tissues. In conclusion, our findings challenge the conventional notion of preptin as an isolated biologically active molecule and underscore the complexity of its interactions within biological systems. Rather than acting independently, the observed effects of preptin may arise from experimental conditions, elevated preptin concentrations, or interactions with related molecules such as IGF2.

Combinatorial Libraries of Bipodal Binders of the Insulin Receptor

The binding process of insulin to its transmembrane receptor entails a sophisticated interplay between two proteins, each possessing two binding sites. Given the difficulties associated with the use of insulin in the treatment of diabetes, despite its remarkable efficacy, there is interest in smaller and more stable compounds than the native hormone that would effectively activate the receptor. Our study adopts a strategy focused on synthesizing extensive combinatorial libraries of bipodal compounds consisting of two distinct peptides linked to a molecular scaffold. These constructs, evaluated in a resin bead-bound format, were designed to assess their binding to the insulin receptor. Despite notable nonspecific binding, our approach successfully generated and tested millions of compounds. Rigorous evaluations via flow cytometry and specific antibodies revealed peptide sequences with specific interactions at either receptor binding Site 1 or 2. Notably, these sequences bear similarity to peptides discovered through phage display by other researchers. This convergence of chemical and biological methods underscores nature's beauty, revealing general principles in peptide binding to the insulin receptor. Overall, our study deepens the understanding of molecular interactions in ligand binding to the insulin receptor, highlighting the challenges of targeting large proteins with small synthetic peptides.

Modulation of the antagonistic properties of an insulin mimetic peptide by disulfide bridge modifications

Insulin is a peptide responsible for regulating the metabolic homeostasis of the organism; it elicits its effects through binding to the transmembrane insulin receptor (IR). Insulin mimetics with agonistic or antagonistic effects toward the receptor are an exciting field of research and could find applications in treating diabetes or malignant diseases. We prepared five variants of a previously reported 20-amino acid insulin-mimicking peptide. These peptides differ from each other by the structure of the covalent bridge connecting positions 11 and 18. In addition to the peptide with a disulfide bridge, a derivative with a dicarba bridge and three derivatives with a 1,2,3-triazole differing from each other by the presence of sulfur or oxygen in their staples were prepared. The strongest binding to IR was exhibited by the peptide with a disulfide bridge. All other derivatives only weakly bound to IR, and a relationship between increasing bridge length and lower binding affinity can be inferred. Despite their nanomolar affinities, none of the prepared peptide mimetics was able to activate the insulin receptor even at high concentrations, but all mimetics were able to inhibit insulin-induced receptor activation. However, the receptor remained approximately 30% active even at the highest concentration of the agents; thus, the agents behave as partial antagonists. An interesting observation is that these mimetic peptides do not antagonize insulin action in proportion to their binding affinities. The compounds characterized in this study show that it is possible to modulate the functional properties of insulin receptor peptide ligands using disulfide mimetics.

Synthesis of Novel Ciprofloxacin-Avibactam Conjugates for the Development of Second-Generation Non-β-Lactam-β-Lactamase Inhibitors

To overcome the antibiotic resistance challenge, we synthesized a novel class of conjugates based on ciprofloxacin and avibactam, covalently linked by diverse amino acids. In vitro studies of these conjugates have shown improved antibacterial efficacy of avibactam when used alone against some ESKAPE pathogens, i.e., S. aureus, E. coli, and A. baumannii. Further, ceftazidime was screened in combination with all conjugates and found to be less synergistically effective than avibactam-ceftazidime co-dosing against K. pneumoniae and E. coli bacterial strains. Subsequently, the top-ranked active conjugates were investigated against the commercially available β-lactamase-II (Penicillinase from Bacillus cereus) through in vitro studies. These studies elucidated two conjugates i.e, 9 (IC₅₀ = 1.69 ± 0.35 nM) and 24b (IC₅₀ = 57.37 ± 5.39 nM), which have higher inhibition profile than avibactam (IC₅₀ = 141.08 ± 12.20 nM). These outcomes allude to avibactam integration with ciprofloxacin is a novel and fruitful approach to discovering clinically valuable next-generation non-β-lactam-β-lactamase inhibitors.

Dipeptide-Derived Alkynes as Potent and Selective Irreversible Inhibitors of Cysteine Cathepsins

The potential of designing irreversible alkyne-based inhibitors of cysteine cathepsins by isoelectronic replacement in reversibly acting potent peptide nitriles was explored. The synthesis of the dipeptide alkynes was developed with special emphasis on stereochemically homogeneous products obtained in the Gilbert-Seyferth homologation for C≡C bond formation. Twenty-three dipeptide alkynes and 12 analogous nitriles were synthesized and investigated for their inhibition of cathepsins B, L, S, and K. Numerous combinations of residues at positions P1 and P2 as well as terminal acyl groups allowed for the derivation of extensive structure-activity relationships, which were rationalized by computational covalent docking for selected examples. The determined inactivation constants of the alkynes at the target enzymes span a range of >3 orders of magnitude (3-10 133 M^-1 s^-1). Notably, the selectivity profiles of alkynes do not necessarily reflect those of the nitriles. Inhibitory activity at the cellular level was demonstrated for selected compounds.

Hydrophilic Azide-Containing Amino Acid to Enhance the Solubility of Peptides for SPAAC Reactions

We report a new positively charged azidoamino acid for strain-promoted azide-alkyne cycloaddition (SPAAC) applications that overcomes possible solubility limitations of commonly used azidolysine, especially in systems with numerous ligation sites. The residue is easily synthesized, is compatible with Fmoc-based solid-phase peptide synthesis employing a range of coupling conditions, and offers efficient second-order rate constants in SPAAC ligations employing DBCO (0.34 M-1 s-1) and BCN (0.28 M-1 s-1).

Post-translational site-specific protein azidolation with an azido pyridoxal derivative

An azido pyridoxal derivative (PLAA) was developed as a protein azidolation reagent, with selectivity towards N-terminal glycine. Successful PLAA modification was achieved with small peptides, natural proteins and lab-expressed proteins under mild conditions, offering a unique method for post-synthesis site-specific azidolation of a peptide or protein.

Functional stapled fragments of human preptin of minimised length

Preptin is a 34-amino-acid-long peptide derived from the E-domain of a precursor of insulin-like growth factor 2 (pro-IGF2) with bone-anabolic and insulin secretion amplifying properties. Here, we describe the synthesis, structures, and biological activities of six shortened analogues of human preptin. Eight- and nine-amino-acid-long peptide amides corresponding to the C-terminal part of human preptin were stabilised by two types of staples to induce a higher proportion of helicity in their secondary structure. We monitored the secondary structure of the stapled peptides using circular dichroism. The biological effect of the structural changes was determined afterwards by the ability of peptides to stimulate the release of intracellular calcium ions. We confirmed the previous observation that the stabilisation of the disordered conformation of human preptin has a deleterious effect on biological potency. However, surprisingly, one of our preptin analogues, a nonapeptide stabilised by olefin metathesis between positions 3 and 7 of the amino acid chain, had a similar ability to stimulate calcium ions' release to the full-length human preptin. Our findings could open up new ways to design new preptin analogues, which may have potential as drugs for the treatment of diabetes and osteoporosis.

Design, Synthesis, and Utility of Defined Molecular Scaffolds

A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Reagent-Based Diversity-Oriented Synthesis of Triazolo[1,5-a][1,4]diazepine Derivatives from Polymer-Supported Homoazidoalanine

Herein, we report the synthesis of skeletally different triazolo[1,5-a][1,4]diazepines starting from immobilized homoazidoalanine. After sulfonylation with 2/4-nitrobenzenesulfonyl chlorides and Mitsunobu alkylation with various alkynols, the corresponding N-substituted nitrobenzenesulfonamides were obtained. Their catalyst-free Huisgen cycloaddition provided immobilized and functionalized triazolo[1,5-a][1,4]diazepines as the key intermediates for further modification. Using the concept of diversity-oriented, reagent-based synthesis, the key intermediates were subsequently converted to heterocycles bearing [5 + 7 + 5], [5 + 7 + 6], and [5 + 7 + 7] scaffolds. Furthermore, the synthesis of spirocyclic triazolodiazepines was developed.

Synthesis of Azido Acids and Their Application in the Preparation of Complex Peptides

Azido acids are important synthons for the synthesis of complex peptides. As a protecting group, the azide moiety is atom-efficient, easy to install and can be reduced in the presence of many other protecting groups, making it ideal for the synthesis of branched and/or cyclic peptides. α-Azido acids are less bulky than urethane-protected counterparts and react more effectively in coupling reactions of difficult-to-form peptide and ester bonds. Azido acids can also be used to form azoles on complex intermediates. This review covers the synthesis of azido acids and their application to the total synthesis of complex peptide natural products.

1 Introduction

2 Synthesis of α-Azido Acids

2.1 From α-Amino Acids or Esters

2.2 Via α-Substitution

2.3 Via Electrophilic Azidation

2.4 Via Condensation of N-2-Azidoacetyl-4-Phenylthiazolidin- 2-Thi one Enolates with Aldehydes and Acetals

2.5 Synthesis of α,β-Unsaturated α-Azido Acids and Esters

3 Synthesis of β-Azido Acids

3.1 Preparation of Azidoalanine and 3-Azido-2-aminobutanoic Acids

3.2 General Approaches to Preparing β-Azido Acids Other Than Azi doalanine­ and AABA

4 Azido Acids in Total Synthesis

4.1 α-Azido Acids

4.2 β-Azido Acids and Azido Acids Containing an Azide on the Side Chain

5 Conclusions

Total Synthesis of Analogs of A54145D and A54145A1 for Structure-Activity Relationship Studies

The total solid-phase synthesis and in vitro biological activity of a series of analogs of A54145 factor D (A5D) and A54145 factor A₁ (A5A₁), two cyclic lipodepsipeptide antibiotics, are reported. An on-resin cyclization strategy was employed to prepare A5A₁ analogs in which Thr4, the residue involved in the depsi (ester) bond, was replaced with either diaminopropionic acid (DAPA), (2S,3R)-diaminobutyric acid (DABA), or serine, effectively replacing the ring-closing ester bond with an amide linkage or with a primary ester. Antibacterial studies with these four analogs revealed that, contrary to a previous report, replacing the ester bond with an amide bond significantly reduces biological activity, and that both the ester bond and the methyl group at the γ-position of Thr4 are crucial for activity. Consistent with literature reports, we found that the single substitution of either 3-hydroxyasparagine (HOAsn) or 3-methoxyaspartate (MeOAsp) with Asn or Asp, respectively, in A5D is more detrimental to activity than the double substitution where both HOAsn and MeOAsp are replaced with Asn or Asp, respectively.

Optimized syntheses of Fmoc azido amino acids for the preparation of azidopeptides

The rise of CuI‐catalyzed click chemistry has initiated an increased demand for azido and alkyne derivatives of amino acid as precursors for the synthesis of clicked peptides. However, the use of azido and alkyne amino acids in peptide chemistry is complicated by their high cost. For this reason, we investigated the possibility of the in‐house preparation of a set of five Fmoc azido amino acids: β‐azido l‐alanine and d‐alanine, γ‐azido l‐homoalanine, δ‐azido l‐ornithine and ω‐azido l‐lysine. We investigated several reaction pathways described in the literature, suggested several improvements and proposed several alternative routes for the synthesis of these compounds in high purity. Here, we demonstrate that multigram quantities of these Fmoc azido amino acids can be prepared within a week or two and at user‐friendly costs. We also incorporated these azido amino acids into several model tripeptides, and we observed the formation of a new elimination product of the azido moiety upon conditions of prolonged couplings with 2‐(1H‐benzotriazol‐1‐yl)‐1,1,3,3‐tetramethyluronium hexafluorophosphate/DIPEA. We hope that our detailed synthetic protocols will inspire some peptide chemists to prepare these Fmoc azido acids in their laboratories and will assist them in avoiding the too extensive costs of azidopeptide syntheses.

Experimental procedures and/or analytical data for compounds 3–5, 20, 25, 26, 30 and 43–47 are provided in the supporting information. © 2017 The Authors Journal of Peptide Science published by European Peptide Society and John Wiley & Sons Ltd.