Study of Chemical Ligation Via 17‐, 18‐ and 19‐Membered Cyclic Transition States

Abstract

Unprotected S-acylated cysteine isopeptides containing a-, b-or c-amino acid units have been synthesized, and their conversion to native hexapeptides by S-to the N-terminus ligations involving 17-, 18-and 19-membered cyclic transition states have been demonstrated both experimentally and computationally to be more favorable than intermolecular cross-ligations. Synthetic methods for peptides are of great interest: native chemical ligation (NCL) that chemically connects two unprotected peptides to yield a long-chain polypeptide has been intensively studied since first developed by Kent and co-workers (1).Alternative approaches to overcome the requirement in classical NCL of an N-terminal cysteine residue have included (i) traceless Staudinger ligation (2,3), (ii) NCL with a phenylalanine and valine analog bearing a sulfhydryl group at the b-position followed by removal of the sulfanyl group (4-6), (iii) NCL followed by the conversion of cysteine to serine (7), (iv) sugar-assisted ligation (8-10), and (v) cysteine-free 'direct aminolysis' methods (11). However, new ligation strategies are still in demand for the synthesis of underivatized and post-translational modified peptides and proteins.Native chemical ligation has been extensively studied in peptidic compounds bearing a cysteine residue at the N-terminus (12). Modified cysteine scaffolds have also been incorporated for the syntheses of novel peptides and proteins (13-16) and for surface immobilization (17). For instance, N-acylcysteines have found utility as photoactivatable analogs of glutathione (18) and for the synthesis of oxytocin-like peptides (19).Haase and Seitz showed that an internal cysteine with glycine or alanine as the N-terminal residue enhanced the rate of NCL. Ligation rate also depends on the ring size formed during the S fi N acyl transfer (20). Wong and coworkers (9,21) reported that N-terminal glycine favors second-generation sugar-assisted ligation in cysteine-containing and cysteine-free glycopeptides over sterically encumbered L-amino acids like valine, leucine, isoleucine, and proline. According to a generally accepted notion, the subsequent acyl shift should proceed via entropically favored 5-and 6-member ring intermediates to achieve useful rates. However, our group has tested the feasibility of chemical ligation of S-acylated cysteine peptides via 5-, 8-, 11-, and 14-membered cyclic transition states (22)(23)(24). We now report migration of the acyl group from S-acylated cysteine isopeptides containing a-, b-and c-amino acids via 17-, 18-and 19-membered cyclic transition states.

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

“…Internal Cys residues were reported to accelerate native chemical ligation (see Section 2.3.1), an especially selective approach for N-terminal cysteine conjugation, via cyclic transition states. [433][434][435][436][437][438]…”

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

“…The feasibility of intramolecular acyl migrations via 5- to 19-membered cyclic transition states was demonstrated. Long range S -to- N acyl transfers were found to depend significantly on the size of the macrocyclic TS, with relative rates, as judged from yields, following the TS ring-size trend, 5>10>11>14, 16, or 17>12>13, 15, or 19>18 ≫9>8 (refs 137 , 140 ). The ratio of the S -to- N ligation product versus formation of the corresponding intermolecular transacylation product for a 15-membered TS was studied under different pH conditions.…”

Chemical ligation from O-acyl isopeptides via 8- and 11-membered cyclic transition states

“…In previous publications 20,23 we have demonstrated this principle and shown that sterically hindered and thus poorly preorganized isopeptides gave lower yields.…”