Microwave-assisted chemical ligation of S-acyl peptides containing non-terminal cysteine residues

Developments and recent advancements in the field of endogenous amino acid selective bond forming reactions for bioconjugation

Bioconjugation methodologies have proven to play a central enabling role in the recent development of biotherapeutics and chemical biology approaches. Recent endeavours in these fields shed light on unprecedented chemical challenges to attain bioselectivity, biocompatibility, and biostability required by modern applications. In this review the current developments in various techniques of selective bond forming reactions of proteins and peptides were highlighted. The utility of each endogenous amino acid-selective conjugation methodology in the fields of biology and protein science has been surveyed with emphasis on the most relevant among reported transformations; selectivity and practical use have been discussed.

A benzotriazole-mediated route to protected marine-derived hetero-2,5-diketopiperazines containing proline

A procedure for the cyclization of dipeptidoyl benzotriazolides containing proline derivatives promoted by triethylamine under MW activation is introduced. The reaction is general for a variety of dipeptidoyl benzotriazolides and represents a very practical and convenient method for the preparation of Pro- or Hyp-derived 2,5-diketopiperazines (2,5-DKPs) and bis-DKPs with a disulfide linker. This method can be used for the construction of 2,5-DKP compound libraries and for the synthesis of natural products with diketopiperazine cores.

Traceless chemical ligation from S-, O-, and N-acyl isopeptides

Peptides are ubiquitous in nature where they play crucial roles as catalysts (enzymes), cell membrane ion transporters, and structural elements (proteins) within biological systems. In addition, both linear and cyclic peptides have found use as pharmaceuticals and components of various conjugate molecular systems. Small wonder then that chemists throughout the ages have sought to mimic nature by synthesis of the amide polymers known as peptides and proteins. The fundamental reaction in the formation of a peptide bond is condensation of an amine of one amino acid with the activated carbonyl group of another. This "fragment condensation" has been achieved in many ways both in solution and by solid-phase peptide synthesis (SPSS) on resin. The most successful method for in-solution coupling is known as native chemical ligation (NCL), and the technique dates back to the pioneering work of Wieland (1953) and subsequently Kent (1994) among many others. This Account builds on the established principles of NCL as applied specifically to S-, O-, and N-isopeptides, molecules that are generally more soluble and less prone to aggregation than native peptides. This Account also covers NCL of isopeptides containing terminal and nonterminal S-acylated cysteine units, reactions that enable the synthesis of native peptides from S-acyl peptides without the use of auxiliaries. With C-terminal S-acyl isopeptides, NCL was carried out under microwave irradiation in phosphate buffer (pH 7.3) at 50 °C. Intramolecular acyl migration was observed through 5-19-membered transition states with relative rates, as assessed by product analysis, in the order, 5 > 10 > 11 > 14, 16, or 17 > 12 > 13, 15, or 19 > 18 ≫ 9 > 8. The rate/pH profile for the 15-membered TS showed a maximum for ligated product versus transacylation at pH 7.0-7.3 presumably associated with the pKa of the N-nucleophile in the hydrogen-bonded TS. Cysteine occurs at low abundance (1.7%) in natural peptides and is rarely available in a terminal position thus limiting the utility of the method. This Account reports, however, NCL at nonterminal acyl cysteine through 5-, 8-, 11-, and 14-membered TSs with relative rates of ligation in the order, 5 ≫ 14 > 11 ≫ 8, thus paralleling the results with acylated terminal cysteine residues. In an obvious sequel to the work with acylated cysteine, we discuss intramolecular O- to N-acyl shift in O-acyl serine and O-acyl tyrosine isopeptides where the story becomes more complex in terms of viable conditions and optimum size of the cyclic TS. N- to N-acyl migration in acyl tryptophan isopeptides is described, and finally, chemical ligation is applied to the synthesis of cyclic peptides. Conformational analysis and quantum chemical calculations are used to rationalize ligation through a range of cyclic transition states. This Account highlights the fact that NCL of acyl isopeptides is an extremely useful strategy for the synthesis of a wide variety of native peptides in good yields and under mild conditions. Mechanistic aspects of the ligations are not fully resolved, but theoretical studies indicate that hydrogen bonding within the various cyclic transition states plays a major role.

Sequence-specific peptide synthesis by an artificial small-molecule machine

The ribosome builds proteins by joining together amino acids in an order determined by messenger RNA. Here, we report on the design, synthesis, and operation of an artificial small-molecule machine that travels along a molecular strand, picking up amino acids that block its path, to synthesize a peptide in a sequence-specific manner. The chemical structure is based on a rotaxane, a molecular ring threaded onto a molecular axle. The ring carries a thiolate group that iteratively removes amino acids in order from the strand and transfers them to a peptide-elongation site through native chemical ligation. The synthesis is demonstrated with ~10(18) molecular machines acting in parallel; this process generates milligram quantities of a peptide with a single sequence confirmed by tandem mass spectrometry.