[Total synthesis of human insulin. IV. Description of the final steps (author's transl)]

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Synthesis of hydrophobic insulin-based peptides using a helping hand strategy

The introduction of solid-phase peptide synthesis in the 1960s improved the chemical synthesis of both the A- and B-chains of insulin and insulin analogs. However, the subsequent elaboration of the synthetic peptides to generate active hormones continues to be difficult and complex due in part to the hydrophobicity of the A-chain. Over the past decade, several groups have developed different methods to enhance A-chain solubility. Two of the most popular methods are use of isoacyl dipeptides, and the attachment of an A-chain C-terminal pentalysine tag with a base-labile 4-hydroxymethylbenzoic acid linker. These methods have proven effective but can be limited in scope depending on the peptide sequence of a specific insulin. Herein we describe an auxiliary approach to enhance the solubility of insulin-based peptides by incorporating a tri-lysine tag attached to a cleavable Fmoc-Ddae-OH linker. Incorporation of this linker, or "helping hand", on the N-terminus greatly improved the solubility of chicken insulin A-chain, which is analogous to human insulin, and allowed for coupling of the insulin A- and B-chain via directed disulfide bond formation. After formation of the insulin heterodimer, the linker and tag could be easily removed using a hydrazine buffer (pH 7.5) to obtain an overall 12.6% yield based on A-chain. This strategy offers an efficient method to enhance the solubility of hydrophobic insulin-based peptides as well as other traditionally difficult peptides.

Novel Methods for the Chemical Synthesis of Insulin Superfamily Peptides and of Analogues Containing Disulfide Isosteres

The insulin superfamily of peptides is ubiquitous within vertebrates and invertebrates and is characterized by the presence of a set of three disulfide bonds in a unique disposition. With the exception of insulin-like growth factors I and II, which are single chain peptides, the remaining 8 members of the human insulin superfamily are two-chain peptides containing one intramolecular and two intermolecular disulfide bridges. These structural features have long made the chemical synthesis of the peptides a considerable challenge, in particular, including their correct disulfide bond pairing and formation. However, they have also afforded the opportunity to develop modern solid phase synthesis methods for the preparation of such peptides that incorporate novel or improved chemical methods for the controlled introduction of both disulfide bonds and their surrogates, both during and after peptide chain assembly. In turn, this has enabled a detailed probing of the structure and function relationship of this small but complex superfamily of peptides. After initially using and subsequently identifying significant limitations of the approach of simultaneous random chain combination and oxidative folding, our laboratory undertook to develop robust chemical synthesis strategies in concert with orthogonal cysteine S-protecting groups and corresponding regioselective disulfide bond formation. These have included the separate synthesis of each of the two chains or of the two chains linked by an artificial C-peptide that is removed following postoxidative folding. These, in turn, have enabled an increased ease of acquisition in a good yield of not only members of human insulin superfamily but other insulin-like peptides. Importantly, these successful methods have enabled, for the first time, a detailed analysis of the role that the disulfide bonds play in the structure and function of such peptides. This was achieved by selective removal of the disulfide bonds or by the judicious insertion of disulfide isosteres that possess structurally subtle variations in bond length, hydrophobicity, and angle. These include lactam, dicarba, and cystathionine, each of which has required modifications to the peptide synthesis protocols for their successful placement within the peptides. Together, these synthesis improvements and the novel chemical developments of cysteine/cystine analogues have greatly aided in the development of novel insulin-like peptide (INSL) analogues, principally with intra-A-chain disulfide isosteres, possessing not only improved functional properties such as increased receptor selectivity but also, with one important and unexpected exception, greater in vivo half-lives due to stability against disulfide reductases. Such analogues greatly will aid further biochemical and pharmacological analyses to delineate the structure-function relationships of INSLs and also future potential drug development.

Synthetic Advances in Insulin-like Peptides Enable Novel Bioactivity

Insulin is a miraculous hormone that has served a seminal role in the treatment of insulin-dependent diabetes for nearly a century. Insulin resides within in a superfamily of structurally related peptides that are distinguished by three invariant disulfide bonds that anchor the three-dimensional conformation of the hormone. The additional family members include the insulin-like growth factors (IGF) and the relaxin-related set of peptides that includes the so-called insulin-like peptides. Advances in peptide chemistry and rDNA-based synthesis have enabled the preparation of multiple insulin analogues. The translation of these methods from insulin to related peptides has presented unique challenges that pertain to differing biophysical properties and unique amino acid compositions. This Account presents a historical context for the advances in the chemical synthesis of insulin and the related peptides, with division into two general categories where disulfide bond formation is facilitated by native conformational folding or alternatively orthogonal chemical reactivity. The inherent differences in biophysical properties of insulin-like peptides, and in particular within synthetic intermediates, have constituted a central limitation to achieving high yield synthesis of properly folded peptides. Various synthetic approaches have been advanced in the past decade to successfully address this challenge. The use of chemical ligation and metastable amide bond surrogates are two of the more important synthetic advances in the preparation of high quality synthetic precursors to high potency peptides. The discovery and application of biomimetic connecting peptides simplifies proper disulfide formation and the subsequent traceless removal by chemical methods dramatically simplifies the total synthesis of virtually any two-chain insulin-like peptide. We report the application of these higher synthetic yield methodologies to the preparation of insulin-like peptides in support of exploratory in vivo studies requiring a large quantity of peptide. Tangentially, we demonstrate the use of these methods to study the relative importance of the IGF-1 connecting peptide to its biological activity. We report the translation of these finding in search of an insulin analog that might be comparably enhanced by a suitable connecting peptide for interaction with the insulin receptor, as occurs with IGF-1 and its receptor. The results identify a unique receptor site in the IGF-1 receptor from which this enhancement derives. The selective substitution of this specific IGF-1 receptor sequence into the homologous site in the insulin receptor generated a chimeric receptor that was equally capable of signaling with insulin or IGF-1. This novel receptor proved to enhance the potency of lower affinity insulin ligands when they were supplemented with the IGF-1 connecting peptide that similarly enhanced IGF-1 activity at its receptor. The chimeric insulin receptor demonstrated no further enhancement of potency for native insulin when it was similarly prepared as a single-chain analogue with a native IGF-1 connecting peptide. These results suggest a more highly evolved insulin receptor structure where the requirement for an additional structural element to achieve high potency interaction as demonstrated for IGF-1 is no longer required.

Chemical synthesis of peptides within the insulin superfamily

The synthesis of insulin has inspired fundamental advances in the art of peptide science while simultaneously revealing the structure-function relationship of this centrally important metabolic hormone. This review highlights milestones in the chemical synthesis of insulin that can be divided into two separate approaches: (i) disulfide bond formation driven by protein folding and (ii) chemical reactivity-directed sequential disulfide bond formation. Common to the two approaches are the persistent challenges presented by the hydrophobic nature of the individual A-chain and B-chain and the need for selective disulfide formation under mildly oxidative conditions. The extension and elaboration of these synthetic approaches have been ongoing within the broader insulin superfamily. These structurally similar peptides include the insulin-like growth factors and also the related peptides such as relaxin that signal through G-protein-coupled receptors. After a half-century of advances in insulin chemistry, we have reached a point where synthesis is no longer limiting structural and biological investigation within this family of peptide hormones. The future will increasingly focus on the refinement of structure to meet medicinal purposes that have long been pursued, such as the development of a glucose-sensitive insulin. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Sequential formation of regioselective disulfide bonds in synthetic peptides with multiple disulfide bonds

Numerous methods have been developed for the formation of disulfide bonds in recombinant DNA-derived and chemically synthesized peptides and proteins, but only a few have found widespread acceptance. The choice of method(s) for formation of disulfide in synthetic peptides and proteins needs to be tailored for each individual polypeptide in such a way so that the reaction conditions are selective, efficient, and safe and give the maximum yield. Here we describe the sequential formation of three disulfide bonds regioselectively which has been optimized for the synthesis of two-chained, heterodimeric polypeptide members of the insulin-relaxin superfamily.

Biomimetic synthesis of lispro insulin via a chemically synthesized "mini-proinsulin" prepared by oxime-forming ligation

Here we report a proof-of-principle study demonstrating the efficient folding, with concomitant formation of the correct disulfides, of an isolated polypeptide insulin precursor of defined covalent structure. We used oxime-forming chemical ligation to introduce a temporary "chemical tether" to link the N-terminal residue of the insulin A chain to the C-terminal residue of the insulin B chain; the tether enabled us to fold/form disulfides with high efficiency. Enzymatic removal of the temporary chemical tether gave mature, fully active insulin. This chemical tethering principle could form the basis of a practical, high yield total synthesis of insulin and analogues.

Rules for ring closure

The ease of formation of cyclic structures shows some regularity which can be described by a set of 'rules' for ring closure. The restrictions on the endocyclic transfer of groups due to SN2 displacements having linear transition states are exemplified by studied on a biogenetic type synthesis of penicillin and, more generally, on the endocyclic attack of a nucleophile attached by a linking chain to an enone or polarized double bond. The formation of five-memebered rings is considered. In base, enones such as 4-hydroxy- and 4-amino-2-methylenebutanoates close exocyclically to give the corresponding five-membered lactones and lactams, respectively. Hydroxymethyl vinyl ketones, however, do not cyclize in base. In strong acid, hydroxy-enones close endocyclically to five-membered ring, as in the formation of cyclic ethylene ketals from acetone and ethanediol. The endocyclic closure of nucleophilic centres to polarized double bonds to give five-membered rings is impeded by restrictions of geometry; these restrictions can be overcome in acid through the generation of oxonium or protonated forms.

Insulin structure and function

Throughout much of the last century insulin served a central role in the advancement of peptide chemistry, pharmacology, cell signaling and structural biology. These discoveries have provided a steadily improved quantity and quality of life for those afflicted with diabetes. The collective work serves as a foundation for the development of insulin analogs and mimetics capable of providing more tailored therapy. Advancements in patient care have been paced by breakthroughs in core technologies, such as semisynthesis, high performance chromatography, rDNA-biosynthesis and formulation sciences. How the structural and conformational dynamics of this endocrine hormone elicit its biological response remains a vigorous area of study. Numerous insulin analogs have served to coordinate structural biology and biochemical signaling to provide a first level understanding of insulin action. The introduction of broad chemical diversity to the study of insulin has been limited by the inefficiency in total chemical synthesis, and the inherent limitations in rDNA-biosynthesis and semisynthetic approaches. The goals of continued investigation remain the delivery of insulin therapy where glycemic control is more precise and hypoglycemic liability is minimized. Additional objectives for medicinal chemists are the identification of superagonists and insulins more suitable for non-injectable delivery. The historical advancements in the synthesis of insulin analogs by multiple methods is reviewed with the specific structural elements of critical importance being highlighted. The functional refinement of this hormone as directed to improved patient care with insulin analogs of more precise pharmacology is reported.

Convergent Synthesis of Complex Diketopiperazines Derived from Pipecolic Acid Scaffolds and Parallel Screening against GPCR Targets

A convergent approach to highly functionalized diketopiperazines (DKPs) using enantioenriched pipecolic acids is described. Scandium triflate-catalyzed [4 + 2] aza-annulation was employed to produce stereochemically well-defined building blocks. A resin "catch and release" strategy was devised to convert annulation products to pipecolic acid monomers. Complex diketopiperazines were efficiently assembled utilizing one-pot cyclodimerization of pipecolic acids. Massively parallel screening of the complex DKPs against a panel of molecular targets identified novel ligands for a number of G-protein-coupled receptors (GPCRs).

Synthesis and conformational analysis of the insulin-like 4 gene product

Insulin-like 4 (INSL-4) is a protein expressed in the early placenta. Its primary structure is insulin-like with reference to the distribution of cysteine residues and the single chain pro-form. Insulin-like 4 was generated by solid-phase peptide synthesis of the two chains followed by the sequential synthesis of the three disulfide bonds. Two disulfide isomers were produced, one with an insulin-like disulfide bonding pattern and the other with a reversed chain orientation. The CD spectra of the two disulfide isomers were indistinguishable without any features produced by periodic structures. In addition, the hydrodynamic properties of the two isomers were identical which implied a very open structure of the disulfide-bonded two-chain molecules. It appears that insulin-likeness cannot be defined solely on the basis of the primary structure of cDNA.

Direct characterisation by electrospray ionisation mass spectroscopy of mercuro-polypeptide complexes after deprotection of acetamidomethyl groups from protected cysteine residues of synthetic polypeptides

In this paper, we describe a rapid procedure to characterise the products generated in the presence of mercuric salts following removal of the acetamidomethyl (Acm)-protecting group from cysteine residues of synthetic polypeptides prepared by solid-phase peptide synthesis (SPPS) methods. In particular, electrospray ionisation mass spectrometry (ESI-MS) procedures have been employed to characterise the mercuro-polypeptide products related to the ribosomal L36 protein isolated from the bacterium Thermus thermophilus. The results demonstrate that very stable mercuro-polypeptide complexes can form under standard conditions of deprotection involving Hg(2+) salts in the presence of a reductant such as beta-mercaptoethanol. Metal ion exchange effects involving other divalent metal ions, such as Co(2+) or Zn(2+), can also be monitored by similar procedures, thus permitting the relative affinity and selectivity for metal ion-polypeptide interactions to be qualitatively assessed. Since the Thermus thermophilus ribosomal L36 protein contains a putative zinc finger binding CCCH motif, these procedures enable the formation of metal-ion complexes of synthetic polypeptides related to this structural motif to be directly examined.

Suppression of epimerization by cupric (II) salts in peptide synthesis using free amino acids as amino components

An epimerization-free system for coupling N-protected peptides with free amino acids was developed. A number of inorganic substances were tested as epimerization suppressant additives during the coupling by various methods (carbodiimide plus additives, uronium salts, Woodward's reagent-K, isobutyl-chloroformate, etc.). Some of them (ZnCl2, RbClO4, LiCl, SnCl4, AlCl3, etc.) in combination with some coupling methods can guarantee coupling with minimal epimerization (D-epimer < 1%). But only a simultaneous use of 1-hydroxybenzotriazole and Cu2+ ions as additives in carbodiimide-mediated peptide couplings appeared to give a standard result (D-epimer < 0.1%). There was no epimerization even in the case when N-methyl amino acid (sarcosine) was used as an amino component, while in the absence of Cu2+ ions an unacceptable level of epimerization was observed (D-epimer, 22% for carbodiimide with the 1-hydroxybenzotriazole method). So far it has been considered that Cu2+ ions prevent obtaining peptides in high yields (< 90%) by various coupling methods. We have found that the use of 1-hydroxybenzotriazole, CuCl2 and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide instead of dicyclohexylcarbodiimide provides a possible method for obtaining the desired peptides in 90-99% yields without epimerization. All these results were shown by employing several model peptide couplings with free amino acids as amino components dissolved in an effective solvent system which readily dissolved them.

Recent approaches in insulin delivery

Insulin remains indispensable in the management of diabetes mellitus since its discovery in 1921. The foreignness of early available porcine and bovine insulin led to the development of human insulin by transpeptidation and biosynthesis in microorganisms. Needle phobia and stress of multiple daily injections led to the investigation and exploitation of all promising routes, ranging from nasal to rectal, by a wide variety of devices and delivery systems. This article describes the development of human insulin, various routes for delivery of insulin (including oral, nasal, buccal, rectal, and pulmonary), and various devices for regulated, safe, and convenient insulin delivery. The article reviews some recent advances in insulin delivery such as the bioresponsive and self-regulated insulin delivery system.

Preparation and properties of N alpha-Bpoc-amino acid pentafluorophenyl esters

The preparation and properties are reported of several N alpha-Bpoc -amino acid pentafluorophenyl esters, including those bearing tert-butyl-, allyl- and trityl-based protecting groups. These derivatives have been used in the solid-phase peptide synthesis of several short peptides.

The prothoracicotropic hormone bombyxin has specific receptors on insect ovarian cells

Bombyxin II, a product of the brain of the adult silkmoth, Bombyx mori, binds to ovarian cells of three different species of lepidoptera, i.e. B. mori (silkmoth), Samia cynthia ricini (ailanthus moth), and an ovarian cell line of Spodoptera frugiperda (Sf9) (fall armyworm). Crude Sf9 cell membrane preparations were used to show that the purported bombyxin receptor binds its ligand in a specific, saturable, and reversible manner. The dissociation constant of the bombyxin-receptor complex is 260+/-90 pM. Quantitative binding studies and Scatchard analysis suggest that every Sf9 cell displays 20000 receptors on the surface. The cross-linked bombyxin-receptor ligand complex has an apparent molecular mass of about 300 kDa as determined by SDS/PAGE. Reduction causes the bombyxin receptor to dissociate into two subunits with molecular masses of 90 kDa and 116 kDa. The size and subunit structure of the putative bombyxin receptor on Sf9 cells show some similarities to the mammalian insulin receptor.

The chemical synthesis of rat relaxin and the unexpectedly high potency of the synthetic hormone in the mouse

Rat relaxin, as isolated from ovaries, has been described in the literature as a low potency hormone in the mouse symphysis pubis assay. Searching for an explanation, a helix-breaking glycine residue in the B chain seemed to be the most auspicious perturbation. Rat relaxin was chemically synthesized and analyzed by reverse-phase high performance liquid chromatography, amino acid composition, mass spectrometry and circular dichroic spectroscopy. Analogs of rat relaxin were synthesized either with aspartic acid in place of the helix-breaking glycine residue in the receptor-binding region of the B chain or with Asp-Leu-Val instead of Gly-Tyr-Val at positions B14-B16. In receptor-binding assays [B14D, B15L, B16V]relaxin was a better ligand than rat relaxin, whereas the [B14D]relaxin was less potent. In the mouse symphysis pubis assay, both analogs were less potent than unmodified rat relaxin, but the [B14D, B15L, B16V]relaxin was better than [B14D]relaxin. In contrast to previous reports on native rat relaxin, the chemically synthesized rat relaxin proved to be as active as human and porcine relaxin with respect to the standard mouse assay system. Glycine, which is considered to be a perturbator in an alpha helix, is not only tolerated in the B14 position but is required for full biological potency.

A novel Leydig cell cDNA-derived protein is a relaxin-like factor

According to Burkhardt et al. (Burkhardt, E., Adham, I. M., Brosig, B., Gastmann, A., Mattei, M. G., and Engel, W. (1994) Genomics 20, 13-19) Leydig cells contain the message for a protein of the insulin/relaxin family which was named Leydig cell insulin-like protein (LEY I-L). We have synthesized the human LEY I-L according to the amino acid sequence deduced from the published cDNA structure and obtained preliminary results concerning its potential target organs and its biological activity. Leydig cell insulin-like protein binds specifically to crude membrane preparations of mouse uterus and brain and shows cross-reactivity with the relaxin receptor, but not the insulin receptor. On the basis of these observations, together with the results of earlier structure-function considerations, we suggest that the new protein is a relaxin-like factor. By itself the new factor shows no obvious effect, but when given together with relaxin it significantly enhances relaxin-mediated widening of the mouse symphysis pubis.

Primate relaxin: synthesis of gorilla and rhesus monkey relaxins

The synthesis of the hormone relaxin from the species Gorilla gorilla (gorilla) and Macaca mulatta (rhesus monkey) has been achieved. Each of the two chains which constitute the peptide structures was assembled separately, the A-chains (24 amino acids) by the Boc-polystyrene solid-phase procedure and the B-chains (29 and 28 amino acids) by the Fmoc-polyamide (gorilla) and the Boc-polystyrene (rhesus monkey) solid-phase methods. After cleavage from the solid supports, the separate chains were purified to a high degree of homogeneity. Oxidative combination of the respective A- and B-chains in solution at high pH afforded the synthetic relaxins in low overall yield. Chemical and physiochemical characterization of the products confirmed both their purity and their conformational similarity to the human hormone. The synthetic gorilla and rhesus monkey relaxins were both found to possess potent chronotropic and inotropic activity in the isolated rat cardiac atrium assay.

The chemical synthesis of proteins

Total chemical synthesis is developing as an important approach to the construction of native proteins and their analogs for the study of the structural basis of biochemical activity. In the past year, a radical departure from conventional synthetic approaches has led to a renaissance in this field. The inherent simplicity of these new modes of protein construction will allow large synthetic proteins (> 200 amino acids in length) to be routinely assembled; this will vastly expand the repertoire of protein molecules accessible to research.

Bombyxin-II and its disulfide bond isomers: synthesis and activity

Bombyxin-II, an insulin superfamily peptide of the silkmoth Bombyx mori, and its disulfide bond isomers have been synthesized by two ways of stepwise, semi-regioselective disulfide bond formation. The disulfide bond CysA20-CysB22 or CysA7-CysB10 was formed first, and then the two other disulfide bonds were formed by iodine oxidation. The conditions for the iodine oxidation were improved to suppress oxidative degradation of unprotected Trp residues. With these conditions, bombyxin-II was synthesized in high yields (26% and 32%). Its disulfide bond isomers were also obtained. Specific activity of the products indicates that the disulfide bond CysA20-CysB22 is important to the bombyxin activity.

Mast cell degranulating (MCD) peptide analogs with reduced ring structure

Mast cell degranulating (MCD) peptide, a component of bee venom, is a 22 amino acid peptide with two disulfide bridges. In this first structure-activity study of MCD peptide, three analogs were synthesized and tested: two analogs shortened by omitting sequences 6-10 and 8-13, respectively, and one analog lacking the disulfide bridge between cysteine residues 5 and 19. These analogs were synthesized by solid-phase methods and were compared to MCD peptide in two assays for inflammation: histamine release from mast cells and superoxide anion release from neutrophils. All three analogs produced histamine release, although with only about one fifth of the activity of MCD peptide. Superoxide anion-releasing activity, however, did not parallel histamine release. MCD peptide did not release superoxide anion, while the 6-10 and 8-13 deletion analogs were strong and weak stimulants, respectively, of this anion. CD spectra showed that the secondary structures of the three analogs were very similar to that of MCD peptide, so that a change in secondary structure cannot completely explain the changes in releasing activities. Charge differences between the two deletion analogs and MCD peptide may explain some of the differences in activity. This is the first demonstration that the various activities of MCD peptide can be separated, and provides a lead through which the purported antiinflammatory activity of MCD peptide may possibly be explored in the future.

Effect of copper(II) chloride on suppression of racemization in peptide synthesis by the carbodiimide method

Copper(II) chloride was found to be an extremely efficient racemization-suppressing additive in the DCC method as compared with the hitherto known ones, by employing the model coupling Z-Gly-L-Val-OH + H-L-Val-OMe in DMF. Although some other copper salts also had a profound effect, copper(II) chloride was the best from the viewpoint of both racemization suppression and coupling efficiency. The effectiveness of copper(II) chloride was further confirmed by employing the EDC-mediated couplings of Z-Gly-containing dipeptides with amino acid esters or dipeptide esters, and those of Z-L-Ala (or L-Val)-L-Val-OH with amino acid esters or dipeptide esters. In almost all the cases studied, no detectable amount (less than 0.1%) of epimer was observed by the HPLC analysis in the presence of copper(II) chloride. This was also the case even with an extremely stringent coupling system Z-L-Pro-L-Val-OH + H-L-Pro-OMe. With reference to the mechanism of racemization suppression, it was found that copper(II) chloride has a strong ability to suppress the racemization of the 5(4H)-oxazolone, which may be formed from an activated carboxyl component during the coupling.

Synthesis of bombyxin-IV, an insulin superfamily peptide from the silkworm, Bombyx mori, by stepwise and selective formation of three disulfide bridges

We report the synthesis of bombyxin-IV, a disulfide-linked, heterodimeric, insulin superfamily peptide from the silkworm, Bombyx mori. The two chains (A- and B-chains) were synthesized separately by the solid-phase method using fluoren-9-ylmethoxycarbonyl (Fmoc) group as a protecting group for alpha-amino group. Three disulfide bonds were bridged step by step (A6-A11, A20-B22, and A7-B10) in a good yield. Synthetic bombyxin-IV was identical with natural one with regard to the retention time on a reversed-phase column and the molecular weight measured by mass spectrometry. Circular dichroism (CD) spectrum of the synthetic bombyxin-IV was very similar to that of the natural one. The specific activity of synthetic bombyxin-IV is equal to that of natural one (0.1 ng/Samia unit). These results suggest that the synthetic bombyxin-IV has the tertiary structure identical with the natural peptide. Our method developed for synthesis of bombyxin-IV would be generally applicable to the synthesis of insulin-like heterodimeric peptides.

Cyclization of disulfide-containing peptides in solid-phase synthesis

Disulfide-containing peptides may be obtained in good yields and purities when oxidations are carried out on peptide chains anchored to polymeric supports used for solid-phase synthesis. Such approaches take advantage of the pseudo-dilution phenomenon which favors intramolecular processes. A variety of procedures have been demonstrated using the related model peptides Ac-Cys-Pro-D Val-Cys-NH2 and Ac-Pen-Pro-D Val-Cys-NH2 (which both readily assume a type II beta-turn conformation that becomes stabilized by a 14-membered disulfide-containing intramolecular ring), and oxytocin (conformationally mobile 20-membered disulfide ring). Both Boc and Fmoc were used for N alpha-amino protection, the beta-thiols of cysteine or penicillamine were blocked by S-acetamidomethyl (Acm), S-9-fluorenylmethyl (Fm), or S-trityl (Trt), and compatible anchoring linkages included HF-labile 4-methylbenzhydrylamide (MBHA), TFA-labile tris (alkoxy)benzylamide (PAL), and photolabile o-nitrobenzylamide (Nonb). Assemblies of linear sequences proceeded smoothly, and polymer-supported oxidations were carried out in a variety of ways either directly or after deblocking to the resin-bound dithiol. Chains were released from the support without substantial damage to the disulfide bridges, and overall yields were as high as 60-90%.

Synthesis and biological activity profiles of atrial natriuretic factor (ANF) analogs

Several analogs of the atrial natriuretic factor (ANF) were synthesized by the solid-phase method using the acetamidomethyl (Acm) group for sulfhydryl protection. The compounds were tested in a receptor binding assay using bovine adrenal zona glomerulosa cell membranes and in the rat diuresis/natriuresis assay. Substitution of tyrosine in position 116 of ANF(101-126) and of the analog [3-Mpr105]ANF(105-126)(3-Mpr = 3-mercaptopropionic acid) did not alter the biological activity profiles and, therefore, these two analogs in radioiodinated form will be useful for enzymatic degradation and clearance studies. Replacement of 3-mercaptopropionic acid with 2-mercaptopropionic acid in [3-Mpr105]ANF(105-126) resulted in an analog with very low potency in both assay systems, presumably as a consequence of the steric bulk and/or local conformational restriction produced by the methyl group attached to the alpha-carbon in position 105. The analog [3-Mpr105,Nva109]ANF(105-126)(Nva = norvaline) showed very low affinity in the receptor binding assay but displayed considerable diuretic/natriuretic activity. The obtained biological activity profiles suggest that in comparison with other ANF peptides the des-amino ANF(105-126) analogs may have a somewhat longer half-life in vivo, or alternatively, may indicate a more complex situation of ANF receptor or binding site heterogeneity.