Lability of N-alkylated peptides towards TFA cleavage

Trifluoroacetic acid (TFA) is a common reagent in both solid-phase and solution peptide synthesis. It is used for the deprotection and/or cleavage of the synthesized peptide from the resin. The use of TFA under these standardized conditions is thought to be sufficiently mild, thereby preventing degradation of the desired product. However, peptides of the general structure R¹-(N-alkyl X₁)-X₂-R² are hydrolyzed by standard TFA solid-phase peptide synthesis (SPPS) cleavage/deprotection conditions providing fragments R¹-(N-alkyl X₁)-OH and H-X₂-R². The fragmentation is observed during a TFA cleavage both from the resin and in solution. The hydrolysis is proposed to proceed via an oxazolone-like intermediate in which equilibration of the chiral center of the N-alkylated residue occurs. This mechanism is supported by H/D exchange as observed by MS and NMR in conjunction with HPLC. © Munksgaard 1996.     

Use of 10% sulfuric acid/dioxane for removal of N-α-tertiary-butyloxycarbonyl group during solid phase peptide synthesis

The hazards and high costs associated with the use of trifluoroacetic acid (TFA) in the removal of the N-α-tertiary-butyloxycarbonyl (Boc) group during solid phase peptide synthesis prompted an examination of alternative acidolytic reagents for α-amino group deprotection. N-α-Boc-glycine and N-α-Boc-isoleucine resins as well as an N-α-Boc-peptide resin were used to test the lability to various deprotection mixtures of both the N-α-Boc resin group as well as the amino acid or peptide-O benzyl ester resin linkage. Of the combinations tried, several were found, including 10% H₂SO₄/dioxane, which gave results roughly comparable to 50% TFA/CH₂Cl₂. Several peptides, 5–10 amino acid residues in length, have been successfully synthesized using the 10% H₂SO₄/dioxane mixture and were found to be comparable in purity to the same peptides prepared using the standard TFA/CH₂Cl₂ method of N-α-Boc removal. Thus, for the peptides examined, 10% H₂SO₄dioxane was found to be an inexpensive, safe, and practical alternative reagent to the more costly and hazardous 50% TFA/CH₂Cl₂ commonly used in the deprotection step of solid phase peptide synthesis.     

Acid-labile anchoring linkages for solid phase synthesis of C-terminal asparagine peptides using the Fmoc strategy

Two acid-labile substituted benzylamine type anchoring linkages, 4-benzoxy-2,6-dimethoxybenzylamine and 2-benzoxy-4,6-dimethoxybenzylamine, for solid phase synthesis of peptide amides were prepared. The N^a-9-fluorenylmethyloxycarbonyl (Fmoc) amino acids could be easily attached to the resins with DCC/HOBt (loading 0.5–0.6 mmol/g resin). After final removal of the N^a-protecting groups, treatment with TFA (50–95%) yielded amino acid and peptide amides in high purity. As we could show for the synthesis of thymulin (FTS, pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn), these two resins with anchoring linkages are well suited for the synthesis of C-terminal Asn peptides using protected aspartic acid derivative as starting material.     

Solid phase synthesis of bioadhesive analogue peptides with trifluoromethanesulfonic acid cleavage from PAM resin

Several decapeptides related to consensus peptide sequences found in the natural polyphenolic proteins from the sea mussels M. edulis or M. californianus have been synthesized in high yields and purities. The peptides were prepared by solid phase peptide synthesis on PAM support resins utilizing BOC protection strategies. The product peptides were then deprotected and cleaved from the PAM resins with TFMSA in a two-step procedure. The course of selective peptide deprotection with TFMSA was followed by high resolution ¹³C n.m.r. on aliquots of the peptide resin swollen in DMF and optimal deprotection times for batch processing of the peptide resins were chosen based on the n.m.r. results. Our two-step deprotection and cleavage procedure using TFMSA was shown to be at least as good as the usual HF procedure for the class of synthetic peptides described in this paper. N to O rearrangements of threonine and/or serine residues in the synthesized decapeptides were observed in both procedures but were reversible following treatment with ammonium bicarbonate. Coupled HPLC-u.v./vis spectroscopy, HPLC-MS and FAB MS analytical methods were used to characterize the product peptides and to provide substantial amino acid sequence information.     

Synthesis of peptide amides using Fmoc-based solid-phase procedures on 4-methylbenzhydrylamine resins

A two-step low-high protocol for the efficient synthesis of peptide amides is described. The protocol exploits the efficiency of Reagent K for side-chain deprotection with the capability of the hard acid trifluoromethane-sulfonic acid (TFSMA) for cleavage of the peptide from the benzhydrylamine resin. This procedure has proven to be an effective method for the synthesis of peptide amides. The formation of α-aminosuccinimide (Asu) derivatives were observed with aspartyl-containing peptides as a minor side reaction product of this procedure, but this Asp→Asu rearrangement could be successfully suppressed by employing low temperature conditions. The N- to O-acyl rearrangement of threonine and/or serine residues also only occurred to a minor extent under these synthetic conditions. © Munksgaard 1995.     

天然环肽auyuittuqamide A的全合成研究

目的用Fmoc固相直链合成和液相环合的方法合成天然环肽auyuittuqamide A。方法以2–氯三苯甲基氯(CTC)树脂为固相载体,1,3–二异丙基碳二亚胺(DIC)和1–羟基苯并三氮唑(HOBT)为缩合剂,9–芴基甲氧基羰基(Fmoc)保护的氨基酸,按照序列依次缩合,以三氟乙醇(TFE)作为切割试剂,获得全保护直链肽。以六氟磷酸苯并三唑–1–基–氧基三吡咯烷基磷(PyBOP)和1–羟基苯并三氮唑(HOBT)为环合试剂,全保护直链肽在二氯甲烷(DCM)溶液中环合,以三氟乙酸(TFA)为脱保护试剂,获得天然环肽auyuittuqamide A。用高效液相制备色谱进行纯化,采用HR-Q-TOF-MS,500MHz ¹HNMR进行表征分析。结果获得纯度大于95%的天然环肽auyuittuqamide A,总收率5.48%。结论此法合成步骤简单,产率较高,首次建立天然环肽auyuittuqamide A的全合成方法,为auyuittuqamide A的进一步研究奠定基础。     

Solid phase synthesis and biological activity of mast cell degranulating (MCD) peptide: a component of bee venom

Mast cell degranulating (MCD) peptide, a 22 amino acid residue basic peptide from bee venom, was synthesized by stepwise solid phase synthesis on a benzhydrylamine resin support. N^α-t-butyloxycarbonyl and benzyl type side chain protection was used. The two disulfide bridges were formed selectively by using S-acetamidomethyl protection for the cysteine residues in positions 5 and 19 and S-methylbenzyl protection for the cysteine residues in positions 3 and 15. Crude synthetic MCD peptide was obtained following deprotection and cleavage from the resin by the low/high HF method. The peptide was isolated in pure form by ion exchange chromatography and gel filtration. The final product has physical, chemical, and biological properties identical with those reported for the natural product. The synthetic strategy utilized for MCD peptide will facilitate the availability of structurally similar analogs for evaluating antihistaminic and anti-inflammatory activities.     

Efficient Fmoc/solid-phase synthesis of Abu(P)-containing peptides using Fmoc-Abu(PO3Me2)-OH

The synthesis of the two 4-phosphono-2-aminobutanoyl-containing peptides, Leu-Arg-Arg-Val-Abu(P)-Leu-Gly-OH.CF₃CO₂H and Ile-Val-Pro-Asn-Abu(P)-Val-Glu-Glu-OH.CF₃CO₂H was accomplished by the use of Fmoc-Abu(PO₃Me₂)-OH in Fmoc solid-phase peptide synthesis. The protected phosphoamino acid, Fmoc-Abu(PO₃Me₂)-OH, was prepared from Boc-Asp-O'Bu in seven steps, the formation of the C—P linkage being effected by the treatment of Boc-Asa-O'Bu with dimethyl trimethylsilyl phosphite. Peptide synthesis was performed using Wang Resin as the polymer support with both peptides assembled by the use of PyBOP® for the coupling of Fmoc amino acids and 20%, piperidine for cleavage of the Fmoc group from the Fmoc-peptide after each coupling cycle. Cleavage of the peptide from the resin and peptide deprotection was accomplished by the treatment of the peptide-resin with 5%, thioanisole/TFA followed by cleavage of the methyl phosphonate group by 1 M bromotrimethylsilane/l M thioanisole in TFA.     

3-NITRO-2-PYRIDINESULFENYL (Npys) GROUP

The novel 3-nitro-2-pyridinesulfenyl (Npys) group, which is useful for the protection and the activation of amino and hydroxyl groups for peptide synthesis, is reported. The Npys group is readily introduced by treatment of amino acids with 3-nitro-2-pyridinesulfenyl chloride. The Npys group is easily removed by treatment with very dilute HCl, e.g. 0.1-0.2 N HCl in dioxane, but it is resistant to trifluoroacetic acid and 88% formic acid. Npys is also selectively removed under neutral conditions using triphenylphosphine or 2-pyridinethiol 1-oxide without affecting benzyloxycarbonyl (Z), tert-butyloxycarbonyl (Boc), 2-(4-biphenylyl) propyl(2) oxycarbonyl (Bpoc), 9-fluorenylmethyloxycarbonyl (Fmoc), benzyl (Bzl) or tert-butyl (tBu) protecting groups. The N-Npys and O-Npys groups when activated in the presence of RCOOH by the addition of tertiary phosphine form peptide or ester bonds via oxidation-reduction condensation. The important features of the Npys group are demonstrated through the synthesis of peptides in solution and by solid phase methodology without a formal deprotection procedure. In solid phase synthesis, 4-(Npys-oxymethyl) phenylacetic acid is used as the key intermediate for the introduction of the trifluoroacetic acid resistant 4-(oxymethyl) phenylacetamido linking group to the resin.     

Synthesis of the very acid-sensitive Fmoc-Cys(Mmt)-OH and its application in solid-phase peptide synthesis

S-4-methoxytrityl cysteine was synthesized and converted into the corresponding Fmoc-Cys(Mmt)-OH by its reaction with Fmoc-OSu. As compared to the corresponding Fmoc-Cys(Trt)-OH, the S-Mmt-function was found to be considerably more acid labile. Quantitative S-Mmt-removal occurs selectively in the presence of groups of the tert butyl type and S-Trt by treatment with 0.5–1.0% TFA. The new derivative was successfully utilized in the SPPS of Tyr¹-somatostatin on 2-chlorotrityl resin. In this synthesis groups of the Trt-type were exclusively used for amino acid side-chain protection. Quantitative cleavage from the resin and complete deprotection was performed by treatment with 3% TFA in DCM–TES (95:5) for 30 min at RT. We observed no reduction of tryptophan under these conditions. © Munksgaard 1996.     

SOLID-PHASE PEPTIDE SYNTHESIS USING MILD BASE CLEAVAGE OF NαFLUORENYLMETHYLOXYCARBONYLAMINO ACIDS,EXEMPLIFIED BY A SYNTHESIS OF DIHYDROSOMATOSTATIN

N ^α-9-Fluorenylmethyloxycarbonyl (Fmoc) amino acids will be of advantage in solid phase peptide synthesis. The Fmoc-group is quantitatively cleaved by mild base (piperidine). This permits the use of tert-butyl-type side chain blocking and of peptide-to-resin linkage cleavable by mild acidolysis. Side reactions arising from repetitive acid deprotection and final HF cleavage in contemporary solid phase synthesis are avoided. Fully bioactive and homogeneous dihydrosomatostatin was obtained in 53% overall yield.     

Solid phase peptide synthesis of human endothelin precursor peptide using two-step hard acid deprotection/cleavage methods

Syntheses are described for the putative human and porcine biosynthetic precursors (hET-38 and pET-39) of endothelin, with the sequence previously deduced from human- and porcine-cDNA coding for prepro-endothelin. The Boc based solid phase synthetic method was applied, followed by weak hard acid, trimethylsilyl bromide, cleavage. The peptide removal from the resin was optimally accomplished with hydrogen fluoride. Disulfide bridges were formed by air-oxidation, and the linkage modes determined by enzymic (Endoproteinase Asp-N) digestion and HPLC. Five additional C-terminally elongated endothelin homologs were also synthesized. For alternative synthesis of pET-39, the use of trimethylsilyl tri-fluoromethanesulfonate for the removal of peptide from the resin generated a major side product, which was characterized. hET-38 was found to be less effective in vitro, when compared to endothelin. The vasoconstrictor activity in vitro of other related peptides was comparable to that of hET-38.     

Comparison of methods for the Fmoc solid-phase synthesis and cleavage of a peptide containing both tryptophan and arginine

A major side reaction which can occur during the synthesis of Trp-containing peptides is modification of the Trp indole by reactive carbonium ion species released during acidolytic cleavage. [Asn²,Trp⁴]Dynorphin A-(1–13), a sequence which is very susceptible to Trp modification, was chosen as a model peptide to compare the effectiveness of various methods proposed to minimize Trp modification during Fmoc solid-phase synthesis. The peptide was synthesized with the side chain of Trp unprotected and cleaved by Reagent K [82.5% trifluoroacetic acid (TFA)/5% phenol/5% water/5% thioanisole/2.5 % ethanedithiol (EDT)] [King, D.S. et al. (1990) Int. J. Peptide Protein Res. 36 , 255–2661, Reagent R [90% TFA/5 % thioanisole/3% EDT/2% anisole] [Albericio, F. et al. (1990) J. Org. Chem. 55 , 3730–3743], TFA containing 20% EDT and 4% water [Riniker, B. & Hartmann, A. (1990) in Peptides: Chemistry, Structure, and Biology (Rivier, J.E. & Marshall, G.R., eds.), pp. 950–952, Escom, Leiden], and TFA containing trialkylsilane, MeOH, and ethylmethyl sulfide [Chan, W.C. & Bycroft, B.W. (1992) in Peptides: Chemistry, Structure, and Biology, Op. cit., pp. 613–614]. Cleavage with Reagent K, Reagent R and TFA containing 20% EDT and 4% water yielded similar results; in addition to the desired peptide, the crude product contained 22–30% of a side product which appeared to result from Trp modification by a Pmc group. Cleavage with the triakylsilane-containing mixture gave the lowest recovery of the desired peptide and the highest levels of Pmc-containing peptides. In contrast, synthesis of the peptide by Fmoc solid-phase synthesis utilizing Fmoc-Trp(Boc) and subsequent cleavage with TFA containing 20% EDT and 5% water yielded the desired peptide in essentially pure form with < 5% of the Pmc-containing side product. Thus, in the Fmoc solid-phase synthesis of [Asn²,Trp⁴]dynorphin A-(1–13) protection of the indole nitrogen by Boc was the most effective method for suppressing the modification of Trp by Pmc. This demonstrates the potential for improving the yield and purity of peptides containing both Trp and Arg by utilizing Fmoc-Trp(Boc) during the Fmoc solid-phase synthesis of these peptides.     

Chemical mechanism to account for artifactual formation of shortened peptides with free alpha-amino groups in solid phase peptide synthesis

The formation of terminated peptides with free α-amino groups has often been observed in stepwise solid phase peptide synthesis. This has been attributed to variable accessibility in regions of the swollen crosslinked resin supports. It is now shown that impurities in the amino acid reagents are responsible for these by-products. Thus, sec. -butyloxycarbonylamino acids were isolated from tert. -butyloxycarbonylamino acids after treatment with trifluoroacetic acid under standard deprotection conditions for the removal of the tert. -butyloxycarbonyl (Boc) group. Direct reverse phase HPLC analysis of Boc-amino acids from commercial sources also showed the sec. -Boc-amino acids as impurities present at varying levels. The sec. -Boc group was stable to treatment at room temperature with trifluoroacetic acid in dichloromethane (1:1, v/v) (half-life 7 years), but was removed by HF-anisole under the standard conditions of cleavage and deprotection of assembled peptides. In model syntheses, the level of terminated free peptides corresponded to the level of preexisting sec. -Boc-amino acid impurities present in the Boc-amino acid reagents. Use of Boc-amino acids with no detectable sec. -Boc resulted in negligible levels (< 0.05%) of terminated peptides. The problem is thus readily overcome by the use of pure Boc-amino acid starting materials and is not a reflection of a shortcoming inherent to the polymer supported nature of solid phase syntheses as has been previously suggested.     

Synthesis of O-phosphonotyrosyl peptides

A general method for the synthesis of O-phosphonotyrosyl peptides using solid phase methodology is described. Protected O-phosphonotyrosine derivatives with the general structure Boc-Tyr(R₂PO₃)-OH (R = methyl, ethyl or benzyl) were prepared as potential synthons for the introduction of O-phosphonotyrosine residues into peptide sequences. Using ³¹P n.m.r. spectroscopy, the alkyl phosphate protecting groups (R = methyl or ethyl) were shown to be stable to the coupling, deprotection and neutralization cycles of the Merrifield method of solid phase peptide synthesis. Facile removal of the methyl phosphate protecting groups from the O-phosphonotyrosyl peptide analogue Ac-Tyr(Me₂PO₃)-NHMe was demonstrated using 45% HBr/acetic acid. The O-phosphonotyrosyl heptapeptide H-Leu-Arg-Arg-Ala-PTyr-Leu-Gly-OH was subsequently prepared using solid phase methodology via incorporation of N²-tert-butyloxycarbonyl-O-dimethylphosphonotyrosine.     

Comparison of 55% TFA/CH2Cl2 and 100% TFA for Boc group removal during solid-phase peptide synthesis

Two parallel syntheses of 40 C-terminal amide peptides, ranging in length from 4 to 20 residues, have been carried out using simultaneous multiple peptide synthesis. All synthetic steps, other than the removal of the Boc group, were performed simultaneously under identical experimental conditions. The two sets of peptides were deprotected with either 55% TFA/DCM for 30 min or 100% TFA for 5 min. The purity of the peptides obtained when deprotecting with 55% TFA/DCM was, on average, 9% higher than with 100% TFA. The major impurity obtained during synthesis when 100% TFA was used for Boc removal corresponded to the omission of the second amino-acid residue added. Volumetric measurements of the swelling of the resin in the different deprotection solvents were carried out. These showed that the omission analogs generated are probably due to insufficient swelling of the resin, resulting in limited solvent transfer of 100% TFA into the resin and, in turn, incomplete Boc removal.     

Solid-phase synthesis of a range of O-phosphorylated peptides by post-assembly phosphitylation and oxidation

A completely general method for the O-phosphorylation of peptides of any given composition using solid-phase methodology is described. Peptides were assembled using Fmoc amino acid active esters, with base used for Fmoc deprotection. Unprotected amino acid side chain hydroxyl groups were phosphitylated and oxidised at the end of the assembly using bis (benzyloxy)(diisopropylamino)phosphine and tert.-butylhydroperoxide respectively. TFA was used for final deprotection of the amino acid side chains and for simultaneous cleavage from the resin. The synthesis of O-phosphopeptides of up to 15 residues in length is described.     

Syntheses,characterization and application of cross‐linked polystyrene−ethyleneglycol acrylate resin (CLPSER) as a novel polymer support for polypeptide syntheses

Abstract: Cross‐linked polystyrene?ethyleneglycol acrylate resin (CLPSER) was developed for the solid‐phase synthesis of peptide by introducing a cross‐linker, O,O′‐bis(2‐acrylamidopropyl)polyethylene glycol₁₉₀₀ (Acr₂PEG), into polystyrene. The cross‐linker was prepared by treating acryloyl chloride with O,O′‐bis(2‐aminopropyl) polyethylene glycol₁₉₀₀[(NH₂)₂PEG] in the presence of diisopropylethylamine. The copolymer was prepared either by bulk or inverse suspension copolymerization of Acr₂PEG₁₉₀₀ and styrene using sorbitan monolaurate as the suspension stabilizer, and a mixture of ammonium peroxodisulfate and benzoyl peroxide as the radical initiators. The resin was characterized using gel‐phase ¹³C NMR, infrared (KBr) spectroscopic techniques and the morphological features of the resin were investigated using scanning electron microscopy photographs. CLPSER showed excellent swelling in a broad range of solvents and was found to be chemically inert to various reagents and solvents used in solid‐phase peptide synthesis. To demonstrate the usefulness of the new resin in polypeptide synthesis, the support was derivatized with an ‘internal reference’ amino acid (norleucine) and a handle 4‐(4‐hydroxymethyl‐3‐methoxy)butyric acid. The new resin was compared with commercial supports such as Merrifield and Sheppard resins by synthesizing an acyl carrier protein (65?74) fragment under the same experimental conditions. HPLC profiles revealed the high efficiency of the newly developed support. Resin capability in peptide synthesis was further demonstrated by the solid phase synthesis of a 25‐residue peptide from the E2/NS1 region hepatitis C viral polyprotein.     

STABLE ISOLATED SYMMETRICAL ANHYDRIDES OF Nα-9-FLUORENYLMETHYLOXYCARBONYLAMINO ACIDS IN SOLID-PHASE PEPTIDE SYNTHESIS:

The synthesis and isolation of symmetrical anhydrides of N^α-9-fluorenylmethyloxycarbonyl (Fmoc) amino acids using water soluble carbodiimide is described. These compounds were used in a solid phase peptide synthesis of methionine-enkephalin on a p-benzyloxybenzyl ester polystyrene 1% divinylbenzene resin support. Homogeneous free pentapeptide was obtained in 42% overall yield. The Fmoc amino acid symmetrical anhydrides were stable during prolonged storage (2 years at 0°) and offer advantages over present “Fmoc solid phase” methods which use anhydrides formed in situ.     

Unexpected lability of cysteine acetamidomethyl thiol protecting group Tyrosine ring alkylation and disulfide bond formation upon acidolysis

Cleavage and deprotection of the peptidyl resin H-Asn-Gly-Gly-Cys(Acm)-Glu(OBu^t)-Gln-Tyr(Bu^t)-Cys(Acm)-Ser(Bu^t)-Asp(OBu^t)-[(p-alkoxy)benzyloxy polystyrene resin] using standard conditions with various trifluoroacetic acid-containing mixtures were found to result in partial removal of ordinarily acid-stable S-Acm groups. Thus, apart from the desired peptide H-Asn-Gly-Gly-Cys(Acm)-Glu-Gln-Tyr-Cys(Acm)-Ser-Asp-OH, a disulfide-cyclic peptide derivative was also isolated. Furthermore, it was found that in another major by-product of the peptide resin cleavage the tyrosine side chain had been alkylated with an Acm group in a position ortho to the phenolic function. The formation of both by-products could be suppressed by carrying out the cleavage/deprotection reaction at higher dilution and by inclusion of scavengers such as phenol. An authentic sample of the disulfide-cyclic peptide was obtained by oxidation of H-Asn-Gly-Gly-Cys-Glu-Gln-Tyr-Cys-Ser-Asp-OH using Ellman's reagent. © Munksgaard 1997.