Macrolactonization of peptide thioesters catalyzed by imidazole and its application in the synthesis of kahalalide B and analogues

Identification and Pharmacological Characterization of a Low-Liability Antinociceptive Bifunctional MOR/DOR Cyclic Peptide

Peptide-based opioid ligands are important candidates for the development of novel, safer, and more effective analgesics to treat pain. To develop peptide-based safer analgesics, we synthesized a mixture-based cyclic pentapeptide library containing a total of 24,624 pentapeptides and screened the mixture-based library samples using a 55 °C warm water tail-withdrawal assay. Using this phenotypic screening approach, we deconvoluted the mixture-based samples to identify a novel cyclic peptide Tyr-[D-Lys-Dap(Ant)-Thr-Gly] (CycloAnt), which produced dose- and time-dependent antinociception with an ED50 (and 95% confidence interval) of 0.70 (0.52-0.97) mg/kg i.p. mediated by the mu-opioid receptor (MOR). Additionally, higher doses (≥3 mg/kg, i.p.) of CycloAnt antagonized delta-opioid receptors (DOR) for at least 3 h. Pharmacological characterization of CycloAnt showed the cyclic peptide did not reduce breathing rate in mice at doses up to 15 times the analgesic ED50 value, and produced dramatically less hyperlocomotion than the MOR agonist, morphine. While chronic administration of CycloAnt resulted in antinociceptive tolerance, it was without opioid-induced hyperalgesia and with significantly reduced signs of naloxone-precipitated withdrawal, which suggested reduced physical dependence compared to morphine. Collectively, the results suggest this dual MOR/DOR multifunctional ligand is an excellent lead for the development of peptide-based safer analgesics.

Biomimetic construction of phospholipid membranes by direct aminolysis ligations

Construction of artificial cells requires the development of straightforward methods for mimicking natural phospholipid membrane formation. Here we describe the use of direct aminolysis ligations to spontaneously generate biomimetic phospholipid membranes from water-soluble starting materials. Additionally, we explore the suitability of such biomimetic approaches for driving the in situ formation of native phospholipid membranes. Our studies suggest that non-enzymatic ligation reactions could have been important for the synthesis of phospholipid-like membranes during the origin of life, and might be harnessed as simplified methods to enable the generation of lipid compartments in artificial cells.

Metal-free synthesis of unsymmetric bis(thioesters)

The first metal-free protocol for efficient synthesis of unsymmetric bis(thioesters) via functionalization of dithiols with two different α,β-unsaturated aldehydes is presented. The methodology described leads to a novel class of dithiol-based building blocks.

One-Pot In Vitro Ribosomal Synthesis of Macrocyclic Depsipeptides

Here, we report a method for the one-pot ribosomal synthesis of macrocyclic depsipeptides. This method is based on a Ser-Pro-Cys-Gly (SPCG) motif discovered by in vitro selection of peptides for the function of self-acylation in the presence of a thioester acyl donor, which forms an O-acyl isopeptide bond via intramolecular S-to-O acyl transfer. Ribosomal synthesis of linear peptides containing the SPCG motif and a backbone "acyl donor" thioester at a downstream position results in spontaneous conversion to the corresponding cyclic depsipeptides (CDPs) in a nearly independent manner of ring size and sequence context. Mutational analysis of the SPCG motif revealed that the P and G residues are dispensable to some extent, but the arrangement of residues in SXCX is crucial for efficient acyl transfer, e.g., CPSG is much less efficient. Finally, one-pot ribosomal synthesis of macrocyclic depsipeptides with various ring sizes and sequences has been demonstrated. This synthetic method can facilitate the ribosomal construction of highly diverse CDP libraries for the discovery of de novo bioactive CDPs.

Depsipeptide synthesis using a late-stage Ag(i)-promoted macrolactonisation of peptide thioamides

Ag(i)-Triggered activation of the thioamide and attack by the C-terminal carboxylate generates an isoimide intermediate that undergoes an intramolecular acyl transfer to furnish the cyclic depsipeptide.

A gold mine for drug discovery: Strategies to develop cyclic peptides into therapies

As a versatile therapeutic modality, peptides attract much attention because of their great binding affinity, low toxicity, and the capability of targeting traditionally "undruggable" protein surfaces. However, the deficiency of cell permeability and metabolic stability always limits the success of in vitro bioactive peptides as drug candidates. Peptide macrocyclization is one of the most established strategies to overcome these limitations. Over the past decades, more than 40 cyclic peptide drugs have been clinically approved, the vast majority of which are derived from natural products. The de novo discovered cyclic peptides on the basis of rational design and in vitro evolution, have also enabled the binding with targets for which nature provides no solutions. The current review summarizes different classes of cyclic peptides with diverse biological activities, and presents an overview of various approaches to develop cyclic peptide-based drug candidates, drawing upon series of examples to illustrate each strategy.

Structure and Reactivity of Highly Twisted N-Acylimidazoles

A modular and efficient synthesis of highly twisted N-acylimidazoles is reported. These twist amides were characterized via X-ray crystallography, NMR spectroscopy, IR spectroscopy, and DFT calculations. Modification of the substituent proximal to the amide revealed a maximum torsional angle of 88.6° in the solid state, which may be the most twisted amide reported for a nonbicyclic system to date. Reactivity and stability studies indicate that these twisted N-acylimidazoles may be valuable, namely as acyl transfer reagents.

The role of imidazole in peptide cyclization by transesterification: parallels to the catalytic triads of serine proteases

The improved bioavailability, stability and selectivity of cyclic peptides over their linear counterparts make them attractive structures in the design and discovery of novel therapeutics. In our previous work, we developed an imidazole-promoted preparation of cyclic depsipeptides in which we observed that increasing the concentration of imidazole resulted in the concomitant increase in the yield of cyclic product and reduction in dimerization, but also resulted in the generation of an acyl-substituted side product. In this work, we used transition state analysis to explore the mechanism of the imidazole-catalyzed esterification of one such peptide, Ac-SAFYG-SCH2φ, and determined the acyl substitution product to be an intermediate in a competing reaction pathway involving acyl substitution of the thioester by imidazole. Our findings indicate that imidazole plays an essential role in this side-chain to C-terminal coupling, and by extension, in transesterifications in general, through a concerted mechanism wherein imidazole deprotonates the nucleophile as the nucleophile attacks the carbonyl. The system under study is identical to the histidine-serine portion of the catalytic triads in serine proteases and it is likely that these enzymes employ the same concerted mechanism in the first step of peptide cleavage. Additionally, relatively high concentrations of imidazole must be used to effectively catalyze reactions in aprotic solvents since the overall reaction involves imidazole acting both as an acid and as a base, existing in solution as an equilibrium distribution between the neutral form and its conjugate acid.

Contemporary strategies for peptide macrocyclization

Peptide macrocycles have found applications that range from drug discovery to nanomaterials. These ring-shaped molecules have shown remarkable capacity for functional fine-tuning. Such capacity is enabled by the possibility of adjusting the peptide conformation using the techniques of chemical synthesis. Cyclic peptides have been difficult, and often impossible, to prepare using traditional synthetic methods. For macrocyclization to occur, the activated peptide must adopt an entropically disfavoured pre-cyclization conformation before forming the desired product. Here, we review recent solutions to some of the major challenges in this important area of contemporary synthesis.