Insights into the mechanism and catalysis of the native chemical ligation reaction

Efficient substitution reaction from cysteine to the serine residue of glycosylated polypeptide: repetitive peptide segment ligation strategy and the synthesis of glycosylated tetracontapeptide having acid labile sialyl-T(N) antigens

This paper reports the synthesis of a 40-residue glycopeptide having two antigenic sialyl-T(N) (NeuAc-alpha-(2,6)-GalNAc-Thr) residues in the MUC1 sequence. This target glycopeptide is a tandem repeat form of 20-residue glycopeptides. For the synthesis of this large molecule, native chemical ligation (NCL) at the serine site was used ((Cys)NCL(Ser)). The concept of (Cys)NCL(Ser) relies on the following: (1) conventional NCL between peptide-alpha-thioester and the cysteine residue of another peptide segment; (2) methylation of the thiol that was used for NCL; (3) acidic CNBr conversion of the cysteine residue to the serine residue forming an O-ester linkage; and (4) an O- to N-acyl shift to couple the two glycopeptides through a native amide bond. To synthesize glycopeptide having an acid-labile sugar moiety, a 20-residue glycopeptide-alpha-thioester and 20-residue glycopeptide having a cysteine residue at the N-terminal were synthesized by solid phase glycopeptide synthesis, and then coupled by (Cys)NCL(Ser). As the result of extensive investigation, CNBr activation with an additional acid (trifluoroacetic acid) was found to be essential to obtain good reactivity and yield, and this condition afforded a tandem repeat form of 40-residue sialylglycopeptide having two sialyl-T(N) residues. In addition to this, it was demonstrated that the cysteine thiol protected by the acetoamidomethyl (Acm) group did not react with the CNBr reagent, and therefore (Cys)NCL(Ser) can be used for repetitive native chemical ligation in the presence of a protecting N-terminal cysteine residue with an Acm group.

3-Mercaptopropionic acid-mediated synthesis of peptide and protein thioesters

Peptides and proteins fragment sequence-specifically in the presence of 3-mercaptopropionic acid to afford thioesters which can be used in native chemical ligation reactions.

Rapid oxime and hydrazone ligations with aromatic aldehydes for biomolecular labeling

A high-yielding and rapid chemoselective ligation approach is presented that uses aniline catalysis to activate aromatic aldehydes toward two amine nucleophiles, namely, 6-hydrazinopyridyl and aminooxyacetyl groups. The rates of these ligations are resolved for model reactions with unprotected peptides. The resulting hydrazone and oxime conjugates are attained under ambient conditions with rate constants of 10(1)-10(3) M(-1) s(-1). These rate constants exceed those of current chemoselective ligation chemistries and enable efficient labeling of peptides and proteins at low muM concentrations, at neutral pH, without using a large excess of one of the components. The utility of the approach is demonstrated by the p-fluorobenzylation of human serum albumin and by the fluorescent labeling of an unprotected peptide with Alexa Fluor 488.

Protein phosphorylation by semisynthesis: from paper to practice

Deconvolution of specific phosphorylation events can be complicated by the reversibility of modification. Protein semisynthesis with phosphonate analogues offers an attractive approach to functional analysis of signaling pathways. In this technique, N- and C-terminal synthetic peptides containing nonhydrolyzable phosphonates at target residues can be ligated to recombinant proteins of interest. The resultant semisynthetic proteins contain site specific, stoichiometric phosphonate modifications and are completely resistant to phosphatases. Control of stoichiometry, specificity, and reversibility allows for complex signaling systems to be broken down into individual events and discretely examined. This chapter outlines the general methods and considerations for designing and carrying out phosphoprotein semisynthetic projects.

Segmental isotopic labeling of proteins for nuclear magnetic resonance

Nuclear magnetic resonance (NMR) spectroscopy has emerged as one of the principle techniques of structural biology. It is not only a powerful method for elucidating the three-dimensional structures under near physiological conditions but also a convenient method for studying protein-ligand interactions and protein dynamics. A major drawback of macromolecular NMR is its size limitation, caused by slower tumbling rates and greater complexity of the spectra as size increases. Segmental isotopic labeling allows for specific segment(s) within a protein to be selectively examined by NMR, thus significantly reducing the spectral complexity for large proteins and allowing for the application of a variety of solution-based NMR strategies. Two related approaches are generally used in the segmental isotopic labeling of proteins: expressed protein ligation and protein trans-splicing. Here, we describe the methodology and recent application of expressed protein ligation and protein trans-splicing for NMR structural studies of proteins and protein complexes. We also describe the protocol used in our lab for the segmental isotopic labeling of a 50-kDa protein Csk (C-terminal Src kinase) using expressed protein ligation methods.

Synthesis of full length PB1-F2 influenza A virus proteins from 'Spanish flu' and 'bird flu'

The proapoptotic influenza A virus PB1-F2 protein contributes to viral pathogenicity and is present in most human and avian isolates. Previous synthetic protocols have been improved to provide a synthetic full length H1N1 type PB1-F2 protein that is encoded by the 'Spanish flu' isolate and an equivalent protein from an avian host that is representative of a highly pathogenic H5N1 'bird flu' isolate, termed SF2 and BF2, respectively. Full length SF2, different mutants of BF2 and a number of fragments of these peptides have been synthesized by either the standard solid-phase peptide synthesis method or by native chemical ligation of unprotected N- and C-terminal peptide fragments. For SF2 chemical ligation made use of the histidine and the cysteine residues located in positions 41 and 42 of the native sequence, respectively, to afford a highly efficient synthesis of SF2 compared to the standard SPPS elongation method. By-product formation at the aspartic acid residue in position 23 was prevented by specific modifications of the SPPS protocol. As the native sequence of BF2 does not contain a cysteine residue two different mutants of BF2 (Y42C) and BF2 (S47C) with appropriate cysteine exchanges were produced. In addition to the full length molecules, fragments of the native sequences were synthesized for comparison of their physical characteristics with those from the H1N1 human isolate A/Puerto Rico/8/34 (H1N1). All peptides were analyzed by mass spectrometry, (1)H NMR spectroscopy, and SDS-PAGE. The protocols allow the synthesis of significant amounts of PB1-F2 and its related peptides.

Semisynthesis and segmental isotope labeling of the apoE3 N-terminal domain using expressed protein ligation

Apolipoprotein E (apoE) is an exchangeable apolipoprotein that functions as a ligand for members of the LDL receptor family, promoting lipoprotein clearance from the circulation. Productive receptor binding requires that apoE adopt an LDL receptor-active conformation through lipid association, and studies have shown that the 22 kDa N-terminal (NT) domain (residues 1-183) of apoE is both necessary and sufficient for receptor interaction. Using intein-mediated expressed protein ligation (EPL), a semisynthetic apoE3 NT has been generated for use in structure-function studies designed to probe the nature of the lipid-associated conformation of the protein. Circular dichroism spectroscopy of EPL-generated apoE3 NT revealed a secondary structure content similar to wild-type apoE3 NT. Likewise, lipid and LDL receptor binding studies revealed that EPL-generated apoE3 NT is functional. Subsequently, EPL was used to construct an apoE3 NT enriched with 15N solely and specifically in residues 112-183. 1H-15N heteronuclear single quantum correlation spectroscopy experiments revealed that the ligation product is correctly folded in solution, adopting a conformation similar to wild-type apoE3-NT. The results indicate that segmental isotope labeling can be used to define the lipid bound conformation of the receptor binding element of apoE as well as molecular details of its interaction with the LDL receptor.

Surfaces for immobilization of N-terminal cysteine derivatives via native chemical ligation

This paper presents an improved method for fast, reliable, and quantitative native chemical ligation (NCL--the reaction between an N-terminal cysteine and a thioester to yield an amide bond) at thiophenyl ester-functionalized glass surfaces. For the first time, the degree of surface functionalization has been measured and can be readily controlled by varying the concentration of thiophenyl ester groups on the surface. This methodology facilitates the preparation of tailor-made functionalized glass surfaces for diverse applications. S-Phenyl 11-(chlorodimethylsilyl)-undecanethiolate, and the benzyl analogue, are readily prepared from 10-undecanoic acid via thioester formation and platinum-catalyzed hydrosilylation of the terminal alkene functionality. Thioesters covalently bound to glass surfaces were then formed by submerging clean glass in toluene-solutions of the relevant thioester silylchloride (1%) containing the non-nucleophilic base, ethyldiisopropylamine (1%). NCL was explored with a cysteine-lissamine conjugate, and the degree of surface functionalization was quantified by UV/vis absorption spectroscopy, using the lissamine chromophore. NCL at thiophenyl ester surfaces proved fast (half-life less than 10 min) and yielded levels of surface functionalization consistent with a dense monolayer of dimethyloctylsilane (ca. 2.0 molecules/nm2). Water contact angles on thiophenyl ester surfaces were found to decrease after NCL reaction with the cysteine-lissamine conjugate, whereas surfaces treated with the same buffer solution containing an unreactive alanine-lissamine conjugate showed no significant changes in contact angle, indicating that thioester hydrolysis is not significant during the course of the reaction. NCL with the cysteine-lissamine conjugate at mixed surfaces containing both thiophenyl esters and inert octyl chains showed lower levels of surface functionalization as expected. Plotting the proportion of thiophenyl esters used in the preparation of the substrates against integrated absorption from the surface yielded a linear relationship, demonstrating that NCL on these surfaces occurs in a controllable manner.

Expanding the scope of chemoselective peptide ligations in chemical biology

Chemoselective ligation methods have increased the efficiency of bioconjugation, enabling complex macromolecules to be assembled. In particular, these methods have been utilized for the ligation and modification of peptides and proteins. The chemical synthesis of proteins from unprotected peptide fragments has enabled the introduction of unnatural amino acids, site-specific isotopic labeling, and the site-specific attachment of affinity tags or labels for imaging. A greater insight into current ligation methods has led to higher reaction rates, higher reaction yields, and greater biocompatibility, thereby increasing the impact of chemoselective ligation reactions in chemical biology.

Oxo-ester mediated native chemical ligation: concept and applications

A direct oxo-ester peptide ligation method has been developed. Through the use of an activated C-terminal para nitrophenyl ester (1), it is possible to achieve direct cysteine ligations (1 + 2 --> 4). Peptide substrates incorporating bulky C-terminal amino acids (1) can be accommodated with high reaction efficiency.

Optimisation of chemical protein cleavage for erythropoietin semi-synthesis using native chemical ligation

Selective protein cleavage at methionine residues is a useful method for the production of bacterially derived protein fragments containing an N-terminal cysteine residue required for native chemical ligation. Here we describe an optimised procedure for cyanogen bromide-mediated protein cleavage, and ligation of the resulting fragments to afford biologically active proteins.

Total chemical synthesis of proteins

This tutorial review outlines the modern ligation methods that enable the efficient total chemical synthesis of enzymes and other protein molecules. Key to this success is the chemoselective reaction of unprotected synthetic peptides ('chemical ligation'). Notably, native chemical ligation enables the reaction of two unprotected peptides in aqueous solution at neutral pH to form a single product in near quantitative yield. Full-length synthetic polypeptides are folded to form the defined tertiary structure of the target protein molecule, which is characterized by mass spectrometry, NMR, and X-ray crystallography, in addition to biochemical and/or biological activity.

Mirror image forms of snow flea antifreeze protein prepared by total chemical synthesis have identical antifreeze activities

The recently discovered glycine-rich snow flea antifreeze protein (sfAFP) has no sequence homology with any known proteins. No experimental structure has been reported for this interesting protein molecule. Here we report the total chemical synthesis of the mirror image forms of sfAFP (i.e., L-sfAFP, the native protein, and D-sfAFP, the native protein's enantiomer). The predicted 81 amino acid residue polypeptide chain of sfAFP contains Cys residues at positions 1, 13, 28, and 43 and was prepared from four synthetic peptide segments by sequential native chemical ligation. After purification, the full-length synthetic polypeptide was folded at 4 degrees C to form the sfAFP protein containing two disulfides. Chemically synthesized sfAFP had the expected antifreeze activity in an ice recrystallization inhibition assay. Mirror image D-sfAFP protein was prepared by the same synthetic strategy, using peptide segments made from d-amino acids, and had an identical but opposite-sign CD spectrum. As expected, D-sfAFP displays the same antifreeze properties as L-sfAFP, because ice presents an achiral surface for sfAFP binding. Facile synthetic access to sfAFP will enable determination of its molecular structure and systematic elucidation of the molecular basis of the antifreeze properties of this unique protein.

Semisynthesis of H-Ras with a glutamic acid methylester at position 61

The mechanism of the guanosine triphosphate (GTP) hydrolysis reaction of small G-proteins such as Ras is generally understood; however, some important molecular details are still missing. One example concerns the role of Gln61 in the catalysis of the GTP hydrolysis reaction. This amino acid is frequently mutated in oncogenic Ras leading to constitutively active variants of the protein. To elucidate the role of Gln61, subtle structural changes were introduced at this position by exchanging the natural occurring glutamine against a glutamic acid methyl ester (GluOme). Thereby the H-bond donor properties of this residue are changed and analysis of the GTP hydrolysis reaction can provide information on the function of the native carboxamide moiety. Using a semisynthetic approach, Ras(1-166)Gln61GluOMe was synthesized by sequential native chemical ligation of three unprotected peptide segments. Peptides Ras(1-50) and Ras(51-79)Gln61GluOMe were synthesized using Boc chemistry. The C-terminal peptide Ras(80-166) was expressed in E. coli. Initial tests of this semisynthetic strategy were performed by synthesis of the N- and C-terminally truncated protein variant Ras(39-101)Gln61GluOMe. The identified optimal reaction conditions were then applied to the synthesis of Ras(1-166)Gln61GluOMe. Refolding of the semisynthetic product in the presence of GTP was successful and revealed intrinsic GTPase activity of Ras(1-166)Gln61GluOMe.

X-ray structure of snow flea antifreeze protein determined by racemic crystallization of synthetic protein enantiomers

Chemical protein synthesis and racemic protein crystallization were used to determine the X-ray structure of the snow flea antifreeze protein (sfAFP). Crystal formation from a racemic solution containing equal amounts of the chemically synthesized proteins d-sfAFP and l-sfAFP occurred much more readily than for l-sfAFP alone. More facile crystal formation also occurred from a quasi-racemic mixture of d-sfAFP and l-Se-sfAFP, a chemical protein analogue that contains an additional -SeCH2- moiety at one residue and thus differs slightly from the true enantiomer. Multiple wavelength anomalous dispersion (MAD) phasing from quasi-racemate crystals was then used to determine the X-ray structure of the sfAFP protein molecule. The resulting model was used to solve by molecular replacement the X-ray structure of l-sfAFP to a resolution of 0.98 A. The l-sfAFP molecule is made up of six antiparallel left-handed PPII helixes, stacked in two sets of three, to form a compact brick-like structure with one hydrophilic face and one hydrophobic face. This is a novel experimental protein structure and closely resembles a structural model proposed for sfAFP. These results illustrate the utility of total chemical synthesis combined with racemic crystallization and X-ray crystallography for determining the unknown structure of a protein.

Ultra-stable peptide scaffolds for protein engineering-synthesis and folding of the circular cystine knotted cyclotide cycloviolacin O2

The cyclic cystine knot motif, as defined by the cyclotide peptide family, is an attractive scaffold for protein engineering. To date, however, the utilisation of this scaffold has been limited by the inability to synthesise members of the most diverse and biologically active subfamily, the bracelet cyclotides. This study describes the synthesis and first direct oxidative folding of a bracelet cyclotide-cycloviolacin O2-and thus provides an efficient method for exploring the most potent cyclic cystine knot peptides. The linear chain of cycloviolacin O2 was assembled by solid-phase Fmoc peptide synthesis and cyclised by thioester-mediated native chemical ligation, and the inherent difficulties of folding bracelet cyclotides were successfully overcome in a single-step reaction. The folding pathway was characterised and was found to include predominating fully oxidised intermediates that slowly converted to the native peptide structure.

Nitrite-assisted peptide iodination and conjugation

In this study, a simple method for selective iodination of peptides and proteins is established. Using angiotensin II as the model system, we demonstrate that nitrite catalyzed the selective iodination of the peptide at the N-terminus in an acidic solution. The N-terminal-iodinated peptides alkylated thiol-containing molecules such as N-acetylcysteine and glutathione to form peptide conjugates in a basic solution. Reactive species formed by increasing the pH of the reaction mixture of sodium nitrite and sodium iodide from 4 to 8 selectively iodinated peptides and proteins at tyrosine and histidine residues. These results show that nitrite is a useful catalyst for peptide and protein ligation.

Water-soluble phosphinothiols for traceless staudinger ligation and integration with expressed protein ligation

The traceless Staudinger ligation is an effective means to synthesize an amide bond between two groups of otherwise orthogonal reactivity: a phosphinothioester and an azide. An important application of the Staudinger ligation is in the ligation of peptides at a variety of residues. Here, we demonstrate that the traceless Staudinger ligation can be achieved in water with a water-soluble reagent. Those reagents that provide a high yield of amide product discourage protonation of the nitrogen in the key iminophosphorane intermediate. The most efficacious reagent, bis(p-dimethylaminoethyl)phosphinomethanethiol, mediates the rapid ligation of equimolar substrates in water. This reagent is also able to perform a transthioesterification reaction with the thioester intermediate formed during intein-mediated protein splicing. Hence, the traceless Staudinger ligation can be integrated with expressed protein ligation, extending the reach of modern protein chemistry.

Dechalcogenative allylic selenosulfide and disulfide rearrangements: complementary methods for the formation of allylic sulfides in the absence of electrophiles. Scope, limitations, and application to the functionalization of unprotected peptides in aqueous media

Primary allylic selenosulfates (seleno Bunte salts) and selenocyanates transfer the allylic selenide moiety to thiols giving primary allylic selenosulfides, which undergo rearrangement in the presence of PPh3 with the loss of selenium to give allylically rearranged allyl alkyl sulfides. This rearrangement may be conducted with prenyl-type selenosulfides to give isoprenyl alkyl sulfides. Alkyl secondary and tertiary allylic disulfides, formed by sulfide transfer from allylic heteroaryl disulfides to thiols, undergo desulfurative allylic rearrangement on treatment with PPh3 in methanolic acetonitrile at room temperature. With nerolidyl alkyl disulfides this rearrangement provides an electrophile-free method for the introduction of the farnesyl chain onto thiols. Both rearrangements are compatible with the full range of functionality found in the proteinogenic amino acids, and it is demonstrated that the desulfurative rearrangement functions in aqueous media, enabling the derivatization of unprotected peptides. It is also demonstrated that the allylic disulfide rearrangement can be induced in the absence of phosphine at room temperature by treatment with piperidine, or simply by refluxing in methanol. Under these latter conditions the reaction is also applicable to allyl aryl disulfides, providing allylically rearranged allyl aryl sulfides in good yields.

Insights from atomic-resolution X-ray structures of chemically synthesized HIV-1 protease in complex with inhibitors

The human immunodeficiency virus 1 (HIV-1) protease (PR) is an aspartyl protease essential for HIV-1 viral infectivity. HIV-1 PR has one catalytic site formed by the homodimeric enzyme. We chemically synthesized fully active HIV-1 PR using modern ligation methods. When complexed with the classic substrate-derived inhibitors JG-365 and MVT-101, the synthetic HIV-1 PR formed crystals that diffracted to 1.04- and 1.2-A resolution, respectively. These atomic-resolution structures revealed additional structural details of the HIV-1 PR's interactions with its active site ligands. Heptapeptide inhibitor JG-365, which has a hydroxyethylamine moiety in place of the scissile bond, binds in two equivalent antiparallel orientations within the catalytic groove, whereas the reduced isostere hexapeptide MVT-101 binds in a single orientation. When JG-365 was converted into the natural peptide substrate for molecular dynamic simulations, we found putative catalytically competent reactant states for both lytic water and direct nucleophilic attack mechanisms. Moreover, free energy perturbation calculations indicated that the insertion of catalytic water into the catalytic site is an energetically favorable process.

Expressed protein ligation using an N-terminal cysteine containing fragment generated in vivo from a pelB fusion protein

Advances in expressed protein ligation (EPL) methods that permit specific introduction of unique modifications into proteins have facilitated protein engineering, structure-function and protein interaction studies. An EPL-generated hybrid exchangeable apolipoprotein has been constructed from recombinant fragments of apolipoprotein E (apoE) and apolipophorin III (apoLp-III). A recombinant fusion protein comprised of human apoE N-terminal residues 1-111, a modified Saccharomyces cerevisiae intein and a chitin binding domain was subjected to 2-mercaptoethanesulfonic acid (MESNA) induced cleavage to generate apoE(1-111)-MESNA. A second fusion protein was comprised of a bacterial pelB leader peptide fused to a variant form of Galleria mellonella apoLp-III residues 1-91. The N-terminal pelB leader sequence directed the newly synthesized fusion protein to the Escherichia coli perisplamic space where endogenous leader peptidase cleavage generated the desired N-terminal cysteine-containing protein fragment. The resulting apoLp-III fragment, which contained no sequence tags or tails, escaped the bacteria and accumulated in the culture medium. When cultured in M9 minimal medium, Asp1Cys apoLp-III(1-91) was produced in high yield and was the sole major protein in the culture supernatant. Ligation reactions with apoE(1-111)-MESNA yielded an engineered hybrid apolipoprotein. The results document the utility of the pelB fusion protein system for generating active N-terminal cysteine containing proteins for EPL applications.

Total chemical synthesis and biophysical characterization of the minimal isoform of the KChIP2 potassium channel regulatory subunit

The potassium channel accessory subunit KChIP2 associates with Kv4.2 channels in the cardiac myocyte and is involved in the regulation of the transient outward current (I(to)) during the early phase of repolarization of the action potential. As a first step to biophysically probe the mechanism of KChIP2, we have chemically synthesized its minimal isoform, KChIP2d, using Boc chemistry solid phase peptide synthesis in conjunction with native chemical ligation. The synthetic KChIP2d protein is primarily alpha-helical as predicted and becomes more structured upon binding calcium as assessed by (1)H-NMR and CD spectroscopy. Synthetic KChIP2d is in a monomer-dimer equilibrium in solution, and there is evidence for two monomer binding sites on an N-terminal peptide of Kv4.2. Planned future studies include the incorporation of fluorescent and spin labeled probes in KChIP2d to yield structural information in parallel with electrophysiologic studies to elucidate KChIP2d's mechanism of action.

Total synthesis by modern chemical ligation methods and high resolution (1.1 Å) X-ray structure of ribonuclease A

The total chemical synthesis of RNase A using modern chemical ligation methods is described, illustrating the significant advances that have been made in chemical protein synthesis since Gutte and Merrifield's pioneering preparation of RNase A in 1969. The identity of the synthetic product was confirmed through rigorous characterization, including the determination of the X-ray crystal structure to 1.1 Angstrom resolution.

Single nucleotide specific detection of DNA by native chemical ligation of fluorescence labeled PNA-probes

DNA-directed chemical ligations provide the opportunity to diagnose DNA sequences with very high sequence specificity. Fluorescent labels have been attached to reactive probes to enable the homogeneous detection of DNA and RNA. However, it has frequently been found that the attachment of fluorescent labels results in decreases of ligation fidelity. Herein we describe the development of a fluorogenic ligation reaction that provides for 10(2)-fold to perfect sequence selectivity. The reaction is based on the isocysteine-mediated native chemical PNA ligation. It is shown that DNA-induced rate accelerations of approximately 43.000-fold can be obtained through subtle variations of the ligation conditions. PNA-thioesters and isocysteine-PNA conjugates were labeled with FAM and TMR fluorophores, respectively. For gaining rapid synthetic access, a convenient on-resin labeling approach was developed. A new PNA monomer featuring an Alloc-protected lysine side chain was synthesized and coupled in solid-phase PNA synthesis. In the event of a ligation reaction the two fluorophores are brought into proximity. It is shown that fluorescence resonance energy transfer provides a positive fluorescence signal which is specific for product formation rather than for loss of starting materials. Single base mutations can be detected within minutes and with very high sequence selectivity at optimized conditions.

Convergent chemical synthesis and high-resolution x-ray structure of human lysozyme

In this article, we report the total chemical synthesis of human lysozyme. Lysozyme serves as a widespread model system in various fields of biochemical research, including protein folding, enzyme catalysis, and amyloidogenesis. The 130-aa wild-type polypeptide chain of the human enzyme was assembled from four polypeptide segments by using native chemical ligation in a fully convergent fashion. Key to the assembly strategy is the application of the recently developed kinetically controlled ligation methodology, which provides efficient control over the ligation of two peptide (alpha)thioesters to yield a unique product. This result enables the facile preparation of a 64-residue peptide (alpha)thioester; this segment is joined by native chemical ligation to a 66-aa Cys peptide, to yield the target 130-aa polypeptide chain. The synthetic polypeptide chain was folded in vitro into a defined tertiary structure with concomitant formation of four disulfides, as shown by 2D TOCSY NMR spectroscopy. The structure of the synthetic human lysozyme was confirmed by high-resolution x-ray diffraction, giving the highest-resolution structure (1.04 A) observed to date for this enzyme. Synthetic lysozyme was obtained in good yield and excellent purity and had full enzymatic activity. This facile and efficient convergent synthesis scheme will enable preparation of unique chemical analogs of the lysozyme molecule and will prove useful in numerous areas of lysozyme research in the future.

Towards the total chemical synthesis of integral membrane proteins: a general method for the synthesis of hydrophobic peptide-thioester building blocks

Modification of a peptide-(α)thioester with a sequence of six arginines on the thioester leaving group can render soluble all peptides derived from a polytopic integral membrane protein. This strategy greatly simplifies the synthesis of peptide-(α)thioester building blocks for the total chemical synthesis of integral membrane proteins by native chemical ligation.