Peptide ligation assisted by an auxiliary attached to amidyl nitrogen

N–O linkage in carbohydrates and glycoconjugates

The synthesis and chemical and physicochemical properties as well as biological and medical applications of various hydroxylamine-functionalized carbohydrate derivatives are summarized.

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.

Development of new thioester equivalents for protein chemical synthesis

The chemical synthesis of proteins provides synthetic chemists with an interesting challenge and supports biological research through the generation of proteins that are not produced naturally. Although it offers advantages, studies of solid phase peptide synthesis have established limits for this technique: researchers can only prepare peptides up to 50 amino acids in length in sufficient yields and purity. Therefore, researchers have developed techniques to condense peptide segments to build longer polypeptide chains. The method of choice for chemical synthesis of these longer polypeptides is convergent condensation of unprotected protein fragments by the native chemical ligation reaction in aqueous buffer. As researchers apply this strategy to increasingly difficult protein targets, they have needed to overcome diverse problems such as the requirement for a thiol-containing amino acid residue at the ligation site, the difficulty in synthesizing thioester intermediates under mild conditions, and the challenge of condensing multiple peptide segments with higher efficiency. In this Account, we describe our research toward the development of new thioester equivalents for protein chemical synthesis. We have focused on a simple idea of finding new chemistry to selectively convert a relatively "low-energy" acyl group such as an ester or amide to a thioester under mild conditions. We have learned that this seemingly unfavorable acyl substitution process can occur by the coupling of the ester or amide with another energetically favorable reaction, such as the irreversible hydrolysis of an enamine or condensation of a hydrazide with nitrous acid. Using this strategy, we have developed several new thioester equivalents that we can use for the condensation of protein segments. These new thioester equivalents not only improve the efficiency for the preparation of the intermediates needed for protein chemical synthesis but also allow for the design of new convergent routes for the condensation of multiple protein fragments.