Exploring chemoselective S-to-N acyl transfer reactions in synthesis and chemical biology

Abstract

S-to-N acyl transfer is a high-yielding chemoselective process for amide bond formation. It is widely utilized by chemists for synthetic applications, including peptide and protein synthesis, chemical modification of proteins, protein-protein ligation and the development of probes and molecular machines. Recent advances in our understanding of S-to-N acyl transfer processes in biology and innovations in methodology for thioester formation and desulfurization, together with an extension of the size of cyclic transition states, have expanded the boundaries of this process well beyond peptide ligation. As the field develops, this chemistry will play a central role in our molecular understanding of Biology.

Figure 1. S-to-N acyl transfer reactions utilized in nature.

(a) Enzymatic ubiquitination where S-to-N acyl transfer forges an isopeptide bond between a terminal glycine and a lysine residue (b) protein splicing where S-to-N acyl transfer ligates the extein fragments (c) sortase mediated ligation where S-to-N acyl transfer introduces isopeptide bond between Thr residue of protein and Lys of lipid-II. (d) Transglutamination where S-to-N acyl transfer ligates proteins via an isopeptide bond.

Figure 2. Mechanisms of synthetic amide-forming reactions involving S-to-N acyl transfer.

(a) Native Chemical Ligation (NCL) (b) Ligation-desulfurization (c) Auxiliary-mediated ligation (AML) (d) Sequential NCL applied to chemical protein synthesis.

Figure 3. Native Chemical Ligation-Desulfurization.

(a) Structures of thiol containing amino acids investigated for NCL-desulfurization (b) Methods for desulfurization of thiol groups following NCL.

Figure 4. Auxiliary Mediated Ligation (AML).

Structures of common auxiliaries investigated for AML.

Figure 5. Expressed protein ligation (EPL) for the synthesis of proteins.

Initial N-to-S acyl transfer generates a thioester which subsequently undergoes trans-thioesterification with an exogenous thiol. The resulting recombinant protein thioester can participate in NCL with synthetic cysteinyl peptides.

Figure 6. Single glycoform of EPO.

Schematic representation of synthetic homogeneous EPO glycoform. N-glycosylation sites are shown at position 24, 38 and 83. O-glycosylation is shown at position 126.

Figure 7. Synthetic applications of S-to-N acyl transfer.

(a) S-to-N acyl transfer as a strategy for chemical ubiquitination of proteins (b) Intermolecular S-to-N acyl transfer mediated ligation for the preparation of glycopeptides (c) De novo synthesis of phospholipids membranes using NCL.

Figure 8. Applications of S-to-N acyl transfer in supramolecular chemistry and chemical probes.

(a) Molecular machine functioning through S-to-N acyl transfer involving increasing cyclic transition state of 11-, 14- and 17-membered rings (b) S-to-N acyl transfer enabled molecular probes.