Side-chain Protection Research Articles

Synthesis of the carboxy-terminal octapeptide of cholecystokinin (CKK-8) based on incorporation of O4-sulfotyrosine by enzymatically catalyzed formation of peptide bonds

Papain-catalyzed condensation of sodium salt of tert-butyloxycarbonyl-β-tert-butyloxyaspartyl-O4-sulfotyrosine (fragment 1-2) with methionyl-glycyl-tryptophyl-methionyl-aspartyl-phenylalanine amide (fragment 3-8) has been elaborated. Deprotection of the thus-obtained octapeptide afforded CCK-8 which exhibited full biological activities. Benzyloxycarbonylaspartyl-phenylalanine amide (fragment 7-8) was prepared using thermolysin without protecting the aspartic acid side chain. Attempted condensation of tert-butyloxycarbonylmethionyl-glycyl-tryptophan (fragment 3-5) with methionyl-aspartyl-phenylalanine amide (fragment 6-8), catalyzed by α-chymotrypsin, subtilisin or proteinase K, afforded the product (fragment 3-8) in only low yields. Further use of proteolytic enzymes for preparing other peptide fragments of the CCK-8 molecule without side chain protection is investigated.

Enzymatic synthesis of cholecystokinin-octapeptide.

Cholecystokinin-octapeptide was synthesized by enzymatic condensations of three fragments, without side-chain protection, except for Tyr. Fmoc-Asp-Tyr (SO3Ba1/2)-OH, prepared by the concerted action of proteases, followed by pyridinium trifluoroacetylsulfate treatment, was used as the N-terminal fragment. In the final step, the Nα-Fmoc group employed as a sole protecting group was easily removed by base treatment without affecting the SO3 moiety.

Efficient routes to the Arnstein's tripeptide δ-(L-α-aminoadipyl)-L-cysteinyl-D-valine (LLD ACV) from the oxidation products of L-lysine derivatives

Two efficient syntheses of the linear tripeptide precursor of penam and cephem antibiotics are presented. The routes are characterised by the use of L-lysine derivatives, Z-L-Lys(Z)-OH and H-L-Lys(Z)-OH as starting materials. By the permanganate oxidation the protected side chain amino grouping in L-lysine derivatives is transformed into the benzyloxycarbonylcarbamoyl substituent with formation of Z-L-Aad(NHZ)-OH and H-L-Aad(NHZ)-OH compounds. Both oxidation products are easily transformed into [N, Cα]-diprotected derivatives. Subsequent condensation at the δ-carboxy group afforded protected LLD ACV tripeptide.

Effect of reductive alkylation of lysine in positions 6 and/or 8 on the histamine-releasing activity of luteinizing hormone-releasing hormone antagonists

The solid-phase reductive alkylation of the Lys side chain in positions 6 (D) and 8 (L) and position 8 alone of the LH-RH antagonist [N-Ac-D-Nal,D-Ph2,3,D-Arg6,Phe7,D-Ala10]LH-RH was investigated in an attempt to reduce the histamine-releasing activity inherent to most potent antagonists while retaining high antiovulatory activity. The protected parent analogues were prepared by conventional solid-phase peptide synthesis. After selective removal of the Lys Fmoc side chain protection, the resin-bound peptides were readily and conveniently alkylated at the epsilon amino groups with various aldehydes and ketones in the presence of NaBH3CN. The analogues were then cleaved from the resin with simultaneous deprotection by anhydrous hydrogen fluoride and purified to homogeneity in two stages: gel permeation followed by preparative reversed-phase liquid chromatography. The analogues were assayed in standard rat antiovulatory and in vitro histamine-release assays.

Effect of reductive alkylation of D-lysine in position 6 on the histamine-releasing activity of luteinizing hormone-releasing hormone antagonists

The reductive alkylation of the D-Lys side chain in position 6 of the LH-RH antagonist [N-Ac-D-Nal1,D-Phe2,3,D-Arg6,Phe7,D-Ala10]LH-RH was investigated in an attempt to reduce the histamine-releasing activity inherent to most potent antagonists while retaining high antiovulatory activity. The protected parent analogue was prepared by conventional solid-phase peptide synthesis. After selective removal of the Lys Fmoc side-chain protection, the resin-bound peptide was readily and conveniently alkylated at the epsilon amino group with various aldehydes and ketones in the presence of NaCNBH3. The analogues were then cleaved from the resin with simultaneous deprotection by anbydrous hydrogen fluoride and purified to homogeneity in two stages: gel permeation followed by preparative reversed-phase liquid chromatography. The analogues were assayed in standard rat antiovulatory and in vitro histamine-release assays. Simple alkyl groups such as ethyl, isopropyl, neopentyl, and cyclohexyl caused little reduction in histamine-releasing activity while exhibiting antiovulatory activity similar to that of the parent peptide. The presence of benzyl and substituted benzyl groups resulted in substantial losses of both histamine-releasing and antiovulatory activities. Thus, results showed that alterations in the hydrophobicity and size of the position-6 side chain have little effect on histamine-releasing activity or antiovulatory activity as long as a high degree of basicity is retained.

Synthesis of human [15-norleucine]little-gastrin-II and des-1-tryptophan-[12-norleucine]minigastrin-II.

By the use of a newly developed procedure for the synthesis of tyrosine-O-sulfate peptides based on the direct incorporation of the suitably N alpha-protected tyrosine-O-sulfate residue along the synthetic route, the synthesis of two human gastrin-II analogues was successfully accomplished. Thereby acid labile side chain protection was applied in combination with the N alpha-benzyloxycarbonyl group in the intermediate chain elongation steps. Despite the pronounced acid-lability of the sulfate ester moiety, its hydrolysis during the final acidolytic deprotection step was significantly reduced under optimized conditions. Subsequent chromatographic purification led to the two gastrin analogues in satisfactory yields as highly pure compounds as judged by various indicative analytical assays.

12] Solid-phase synthesis of type-α transforming growth factor

Type-α transforming growth factor (TGFα) is a mitogenic peptide factor produced extracellularly by tumor cells and by virally and chemically transformed cells in culture. TGFα is obtained from the natural source, and the synthesis of TGFsα provides sufficient material to enable more extensive studies on their mode of action and the role of their structures on their transforming activity. This chapter discusses the methods necessary to achieve the manual synthesis of rTGFα. The chapter also discusses several of the latest improvements of the solid-phase method, particularly the novel low-high HF to minimize strong acid-catalyzed side reactions. Some of the salient features and rationale for the improved version of the solid-phase method for the chemical synthesis of rTGFα are stable peptide–resin linkage, maximal and stable side chain protection, quantitative monitoring of the coupling step, and a new deprotection strategy: the low-high HF cleavage method.

Solid-phase synthesis of protected peptide fragments using a trialkoxy-diphenyl-methylester resin.

A trialkoxy-diphenyl-methylester and amide linkage for solid-phase peptide synthesis with Fmoc strategy is described. Protected peptide esters can be smoothly cleaved with weak acid, resulting in fragments with intact side-chain protection. Mild acidic cleavage of the corresponding peptide amide resins yields peptide amides.

Protection of histidine in peptide synthesis: A Reassessment of the trityl group

The trityl (Trt) group is ideally suited for the side-chain protection of His in peptide syntheses, in combination with 9-fluorenylmethyloxycarbonyl (Fmoc) in N α- and protecting groups cleavable by mild acidolysis in other positions of the peptide. 2,4,5-trichlorophenyl (Tcp)- and pentafluorophenyl (Pfp)-esters of Fmoc-His(Trt)-OH and Trt-His(Trt)-OH are strongly activated, but stable compounds. N α-Trt is selectively removable in the presence of N Im-Trt.

Solution synthesis of antigenic and inhibiting peptides of the human renin prosegment-1. [Tyr47, Nle53] preprorenin-(47-60) peptide methyl ester.

The [Tyr47, Nle53] preprorenin (47-60) peptide methyl ester, a tetradecapeptide related to the human renin prosegment, has been synthesized using a three-segment coupling strategy. Selective deprotection of the segments before coupling allowed an easy removal of the final tetradecapeptide side chain-protecting groups by acidolysis and an easy purification. Antibodies raised against this peptide bound the plasmatic inactive renin.

ChemInform Abstract: Synthesis of a Putative Antigenic Heptapeptide from Escherichia coli K88 ab Protein Fimbriae.

AbstractThe hexapeptide (III), spanning region 213‐219 of E. coli K88 ab protein fimbriae, was synthesized starting from the units (I) and (II) in a conventional stepwise fashion [using DCC and hydroxybenzotriazol (for preactivation in all condensation reactions), tert. butyloxy‐ and benzyloxycarbonyl groups (for side chain protection), and a mixture of TFA, phenol and p‐cresol (for repetitive depro‐ 150 tection)].

Preparation of tyrosine- O-[ 35S]sulfated cholecystokinin octapeptide from a nonsulfated precursor peptide

A rapid and simple one-pot method for O-sulfation of nonsulfated cholecystokinin octapeptide (CCK-8) was developed using sulfuric acid and dicyclohexylcarbodiimide (DCC) without protection of the amino acid side chains. The extent of sulfation was increased with increasing the amount of reactants, sulfuric acid, and DCC, and reached maximum (40%) with fourfold molar excess of sulfuric acid and 40-fold molar excess of DCC. The excess of nonsulfated peptide inhibited the sulfation. The sulfation product was purified by HPLC or TLC to give a pure sulfated substance which showed exactly the same behavior as that of an authentic O-sulfated CCK-8 on HPLC or TLC. The purified sulfated peptide was active in stimulating amylase secretion from rat pancreatic fragments, and amino acid analysis showed that the tyrosine residue in the peptide existed in O-sulfated form. Sulfation with [ 35S]sulfuric acid-DCC produced a radioactive substance, from which O-[ 35S]sulfated CCK-8 could be easily purified by two-dimensional TLC.

Synthesis of a putative antigenic heptapeptide from Escherichia coli K88 ab protein fimbriae.

The heptapeptide Tyr-Arg-Glu-Asp-Met-Glu-Tyr-OMe, spanning region 213-219 of Escherichia coli K88 ab protein fimbriae, was synthesized with an overall yield of 37% using dicyclohexylcarbodiimide (DCC) and 1-hydroxybenzotriazole (HOBt) preactivation in all condensation reactions. The C-terminal was protected as the methyl ester. The protection scheme of N alpha-tert-butyloxycarbonyl-(Boc) and benzyl-(Bzl) or benzyloxycarbonyl (Z) groups for side chain protection was found to be orthogonal when a mixture of trifluoroacetic acid (TFA), phenol (PhOH) and p-cresol (CrOH) was used for repetitive deprotection. The final deprotection of Boc-Tyr(Bzl)-Arg(Z2)-Glu(Bzl)-Asp(Bzl)-Met-Glu(Bzl+ ++)-Tyr(Bzl)-OMe (17) was accomplished in 80% yield by prolonged treatment with hydrogen fluoride, dimethyl sulfide, p-cresol and p-thiocresol. The BSA-linked synthetic peptide was used in immunisation experiments on rabbits.

An improved synthesis of crystalline mammalian glucagon.

Mammalian glucagon was synthesized by the stepwise solid-phase method using several improvements developed in recent years. Peptide was assembled on a 4-(oxymethyl)phenylacetamidomethyl-copoly(styrene-divinyl benzene) resin support with N alpha-t-butoxycarbonyl and benzyl-based side-chain protection for most of the trifunctional amino acids. Crude synthetic glucagon was obtained in 75% yield by deprotection and cleavage from the resin with a new modified HF procedure. Pure material was isolated in 48% overall yield by a one-step purification on preparative C18 reverse-phase chromatography. It was crystallized from aqueous solution at pH 9.2. The synthetic glucagon activated adenylate cyclase in rat liver membranes in the same manner as natural glucagon, with both achieving half-maximum activation at a concentration of 7 nM.

Semisynthetic studies on bovine pancreatic ribonuclease.

The S-peptide of the enzyme bovine pancreatic ribonuclease has been used as a model for covalent semisynthesis. Methods for side-chain protection, enzymatic cleavage of the peptide chain at the level of the single arginine-10 and for selective deprotection of the alpha-carboxyl function of this residue, have been examined. The partially protected [1-10] sequence has been coupled to a solid-phase generated [11-15] sequence attached to the polymer. After deblocking from the solid-support, the [1-15] semisynthetic peptide was complexed with native S-protein to give a complex with high biological activity.

Solid phase synthesis of tyrosine-containing histone fragments

The solid phase synthesis of Ac-Val-Val-Tyr-Ala-Leu-OH ( 1), Ac-Glu-Ala-Tyr-Leu-Val-OH ( 2) and Ac-Leu-Tyr-Gly-Phe-Gly-Gly-OH ( 3), segments 86–90 of histone H4, 97–101 of histone H3 and 97–102 of histone H4, respectively, is described using chloromethylresin and chloromethyl-Pab-resin as solid supports. In the synthesis of peptides 2 and 3 using 2,6-diclorobenzyl ether as tyrosine side chain protection, 7% and 8% of the 3-dichlorobenzyltyrosine-rearranged peptide could be isolated by Bio-Gel P-2 or diethylaminoethylcellulose chromatography. Alternative use of cyclohexyl ether as tyrosine protection has been explored with satisfactory results.

A total synthesis of nocardicins

Four-component-condensation of d-isoserine or l-isoserine, diphenylmethyl isocyanide and p-(benzyloxy)benzaldehyde was used to construct a functionalized β-lactam ring system which was transformed in 4 steps into 3-aminonocardicinic acid. A protected side chain of nocardicin D was synthesized from d-asparagine in 6 steps. Coupling of these units and further conversion to nocardicins A, B and D followed published procedures. Using l-asparagine for the synthesis of the side chain led to unnatural isonocardicin A. Biological activities of the products are compared.

Peptide synthesis. Part 5. Solid-phase synthesis of [15-leucine]little gastrin

A procedure is described for the solid-phase synthesis of gastrin peptides exemplified by [15-leucine]little gastrin. The method makes use of a polar polyamide resin support and stepwise addition of fluorenylmethoxycarbonylamino-acid derivatives. Treatment with acidic reagents is minimised, obviating the need for side-chain protection of tryptophan and eliminating acid-catalysed side reactions at the multiple glutamyl residues. Formation of the terminal pyroglutamyl residue was achieved through a glutaminyl intermediate.

Experiments on the protection of the thioether in methionine.

The decrease in the solubility of peptides when their methionine residues are replaced by methionine sulfoxides prompted the exploration of an alternative approach to the protection of the thioether in methionine side chains. Alkylation of tert.-butyloxycarbonyl methionine p-nitrophenyl ester with methyl p-toluenesulfonate yielded the crystalline derivative of methionine S-methyl p-toluenesulfonate which could be incorporated into peptide chains. Alternatively, methionine S-methyl p-toluenesulfonate (Mmt) residues could be generated by the action of methyl p-toulensulfonate on methionine containing peptides. The protecting group remained intact under the conditions of aminolysis and ammonolysis commonly used in peptide synthesis, and it was unchanged after the removal of other blocking groups with trifluoroacetic acid or diethylamine. On treatment with hydrobromic acid in acetic acid the toluenesulfonate anion was replaced by bromide ion, while hydrogenation resulted in the decomposition of the modified methionine side chain. In the process of deprotection Mmt residues could be smoothly converted to methionine residues by thiolysis. Thus, the protecting group functioned well in several respects but an increase in solubility (in dimethylformamide) on alkylation was observed only in a part of the peptide derivatives tested. Therefore, the value of the new approach for the protection of the methionine side chain in peptide synthesis remains to be established.

A new solid-phase synthesis of porcine vasoactive intestinal peptide using Nα-9-fluorenylmethyloxycarbonyl amino acids

The solid-phase synthesis of an octacosapeptide amide corresponding to the amino acid sequence of porcine vasoactive intestinal peptide (VIP) is described. No final treatment with strong, anhydrous acid was employed, since the use of base-labile 9-fluorenylmethyloxycarbonyl amino acids bearing tert-butyl based side chain protection enabled the peptide chain assembly to be performed on p-benzyloxybenzyl amine resin, which was cleaved from the whole peptide amide at the end of the synthesis by diluted trifluoroacetic acid.