Structure-activity relationship of daptomycin analogues with substitution at (2S, 3R) 3-methyl glutamic acid position.

Daptomycin is a highly effective lipopeptide antibiotic against Gram-positive pathogens. The presence of (2S, 3R) 3-methyl glutamic acid (mGlu) in daptomycin has been found to be important to the antibacterial activity. However the role of (2S, 3R) mGlu is yet to be revealed. Herein, we reported the syntheses of three daptomycin analogues with (2S, 3R) mGlu substituted by (2S, 3R) methyl glutamine (mGln), dimethyl glutamic acid and (2S, 3R) ethyl glutamic acid (eGlu), respectively, and their antibacterial activities. The detailed synthesis of dimethyl glutamic acid was also reported.

Protein chemical synthesis by serine and threonine ligation.

An efficient method has been developed for the salicylaldehyde ester-mediated ligation of unprotected peptides at serine (Ser) or threonine (Thr) residues. The utility of this peptide ligation approach has been demonstrated through the convergent syntheses of two therapeutic peptides--ovine-corticoliberin and Forteo--and the human erythrocyte acylphosphatase protein (∼11 kDa). The requisite peptide salicylaldehyde ester precursor is prepared in an epimerization-free manner via Fmoc-solid-phase peptide synthesis.

Serine/threonine ligation for natural cyclic peptide syntheses.

Covering: 2000 to 2014Cyclic peptides are a class of abundant natural products, often exhibiting attractive biological activities. The challenges in the total synthesis of cyclic peptides lie in the preparation of unnatural amino acids if present and the peptide cyclization. Cyclization is an entropy-disfavoured process, with competition between intermolecular and intramolecular reactions. Biological methods can utilize the pre-organized conformation of the side chain unprotected peptide, which brings the reacting termini into proximity, while chemical synthesis requires protecting groups, often large-size and hydrophobic in nature. In this regard, performing peptide cyclization can be an arbitrary and trial-and-error practice. In this highlight, we discuss the application of chemoselective ligation-mediated peptide cyclization in the total synthesis of natural cyclic peptides.

A journey to the total synthesis of daptomycin.

Daptomycin, the first antibiotic of its class, provides a new structural motif for the development of new antibiotics. Recently, we have completed the total synthesis of daptomycin. The development of the successful synthetic strategy is described here, including the application of serine/threonine ligation mediated peptide cyclization to the daptomycin macrocyclization.

Total synthesis of daptomycin by cyclization via a chemoselective serine ligation.

A total synthesis of daptomycin, the first natural product antibiotic launched in a generation, was achieved. This convergent synthesis relies on an efficient macrocyclization via a serine ligation to assemble the 31-membered cyclic depsipeptide. The difficult esterification by the nonproteinogenic amino acid kynurenine was accomplished via the esterification of a threonine residue by a suitably protected Trp ester, followed by ozonolysis. This synthesis provides a foundation and framework to prepare varied analogues of daptomycin to establish its structure-activity profile.

Effective synthesis of kynurenine-containing peptides via on-resin ozonolysis of tryptophan residues: synthesis of cyclomontanin B.

The preparation of Kyn-containing peptides is difficult, owing to the low reactivity of Kyn in the coupling reaction. In this report, Kyn-containing peptides were efficiently obtained via on-resin ozonolysis of the corresponding Trp-containing peptide. In addition, a Kyn-containing cyclic peptide, cyclomontanin B, has been synthesized by this strategy in the combination with serine/threonine ligation (STL)-mediated cyclization.

A fluorogenic probe for recognizing 5-hydroxylysine inspired by serine/threonine ligation.

The 5-lysyl-hydroxylation has been found among protein post-translational modifications. In this report, we have devised a fluorescence turn-on probe which allows for selectively recognizing 5-hydroxylysine-containing peptides.

Realizing serine/threonine ligation: scope and limitations and mechanistic implication thereof.

Serine/Threonine ligation (STL) has emerged as an alternative tool for protein chemical synthesis, bioconjugations as well as macrocyclization of peptides of various sizes. Owning to the high abundance of Ser/Thr residues in natural peptides and proteins, STL is expected to find a wide range of applications in chemical biology research. Herein, we have fully investigated the compatibility of the STL strategy for X-Ser/Thr ligation sites, where X is any of the 20 naturally occurring amino acids. Our studies have shown that 17 amino acids are suitable for ligation, while Asp, Glu, and Lys are not compatible. Among the working 17 C-terminal amino acids, the retarded reaction resulted from the bulky ß-branched amino acid (Thr, Val, and Ile) is not seen under the current ligation condition. We have also investigated the chemoselectivity involving the amino group of the internal lysine which may compete with the N-terminal Ser/Thr for reaction with the C-terminal salicylaldehyde (SAL) ester aldehyde group. The result suggested that the free internal amino group does not adversely slow down the ligation rate.

Convergent synthesis of MUC1 glycopeptides via serine ligation.

Syntheses of MUC1 glycopeptides (40-mer and 80-mer) are described. The convergent synthesis was achieved by native serine ligation using side-chain unprotected glycopeptide segments.

Salicylaldehyde ester-induced chemoselective peptide ligations: enabling generation of natural peptidic linkages at the serine/threonine sites.

A serine/threonine-based chemoselective ligation method is described. It uses an O-salicylaldehyde ester at the C-terminus, reacting with N-terminal serine or threonine to realize peptide ligations. The utility of the O-salicylaldehyde ester enables the rapid coupling reaction and the production of an N,O-benzylidene acetal intermediate, which is readily converted into natural peptidic linkages (Xaa-Ser/Thr) at the ligation site.