Evaluation of COMU as a coupling reagent for in situ neutralization Boc solid phase peptide synthesis

Benzotriazole‐based coupling reagents have dominated the last two decades of solid phase peptide synthesis. However, a growing interest in synthesizing complex peptides has stimulated the search for more efficient and low‐cost coupling reagents, such as COMU which has been introduced as a nonexplosive alternative to the classic benzotriazole coupling reagents. Here, we present a comparative study of the coupling efficiency of COMU with the benzotriazole‐based HBTU and HCTU for use in in situ neutralization Boc‐SPPS. Difficult sequences, such as ACP(65–74), Jung–Redeman 10‐mer, and HIV‐1 PR(81–99), were used as model target peptides on polystyrene‐based resins, as well as polyethylene glycol‐based resins. Coupling yields obtained using fast in situ Boc‐SPPS cycles were determined with the quantitative ninhydrin test as well as via LC‐MS analysis of the crude cleavage products. Our results demonstrate that COMU coupling efficiency was less effective compared to HBTU and HCTU with HCTU ≥ HBTU > COMU, when polystyrene‐based resins were employed. However, when the PEG resin was employed in combination with a safety catch amide (SCAL) linker, more comparable yields were observed for the three coupling reagents with the same ranking HCTU ≥ HBTU > COMU. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.     

COMU: A third generation of uronium‐type coupling reagents

COMU is a third generation of uronium‐type coupling reagent based on ethyl 2‐cyano‐2‐(hydroxyimino)acetate (Oxyma) as well as a morpholino carbon skeleton. The presence of the morpholino group has a marked influence on the solubility, stability and reactivity of the reagent. COMU performed extremely well in the presence of only 1 equiv. of base, thereby confirming the effect of the hydrogen bond acceptor in the reaction. The by‐products of COMU are water soluble and easily removed, making it an excellent choice of coupling reagent for solution‐phase peptide synthesis. Finally, COMU shows a less hazardous safety profile than benzotriazole‐based reagents, such as HATU and HBTU, which in addition exhibit unpredictable autocatalytic decompositions and therefore a higher risk of explosion. Furthermore, in contrast to benzotriazole‐based reagents, COMU is significantly less likely to cause allergic reaction. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Re‐evaluating the stability of COMU in different solvents

COMU is uronium‐type coupling reagent based on OxymaPure. It showed several advantages over classical benzotriazole‐based coupling reagents such as higher solubility, water‐soluble byproduct, and monitoring the reaction by changing of color. Although COMU is well known to perform excellent in solution, but its hydrolytic stability in DMF limits its use in automatic peptide synthesizer. Herein, we evaluated the hydrolytic stability of COMU in γ‐valerolactone (GVL), acetonitrile (ACN) and N‐formylmorpholine (NFM) and compared its stability against DMF. The stability of COMU after 24 h was found to be 88 and 89% in GVL and ACN, respectively, when compared in DMF (14%). Further, the demanding Aib‐ACP decapeptide and JR decapeptide were successfully synthesized using COMU dissolved in GVL or ACN while Fmoc amino acids were dissolved in DMF. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.     

Easy parallel screening of reagent stability,quality control,and metrology in solid phase peptide synthesis (SPPS) and peptide couplings for microarrays

Evaluating the stability of coupling reagents, quality control (QC), and surface functionalization metrology are all critical to the production of high quality peptide microarrays. We describe a broadly applicable screening technique for evaluating the fidelity of solid phase peptide synthesis (SPPS), the stability of activation/coupling reagents, and a microarray surface metrology tool. This technique was used to assess the stability of the activation reagent 1‐{[1‐(Cyano‐2‐ethoxy‐2‐oxo‐ethylidenaminooxy)dimethylamino‐morpholinomethylene]}methaneaminiumHexafluorophosphate (COMU) (Sigma‐Aldrich, St. Louis, MO, USA) by SPPS of Leu‐Enkephalin (YGGFL) or the coupling of commercially synthesized YGGFL peptides to (3‐aminopropyl)triethyoxysilane‐modified glass surfaces. Coupling efficiency was quantitated by fluorescence signaling based on immunoreactivity of the YGGFL motif. It was concluded that COMU solutions should be prepared fresh and used within 5 h when stored at ~23 °C and not beyond 24 h if stored refrigerated, both in closed containers. Caveats to gauging COMU stability by absorption spectroscopy are discussed. Commercial YGGFL peptides needed independent QC, due to immunoreactivity variations for the same sequence synthesized by different vendors. This technique is useful in evaluating the stability of other activation/coupling reagents besides COMU and as a metrology tool for SPPS and peptide microarrays. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Complete synthesis of the bicyclic ring of a mutacin analog with orthogonally protected lanthionine via solid‐phase intracyclization

Mutacin 1140 (MU1140) is a naturally occurring lantibiotic derived from posttranslational modifications of a ribosomally synthesized peptide during the fermentation of a bacterium called Streptococcus mutans, the etiological agent of dental cavities. A practical approach for chemically synthesizing lantibiotics would be a valuable tool to expand the MU1140 library with additional semisynthetic analogs. In turn, an expanded library may prove useful to explore additional therapeutic indications for this pipeline of novel compounds. In this work, orthogonally protected lanthionine analogs were synthesized via an aziridine ring opening strategy. This lanthionine was utilized to synthesize a cysteamine (Cya) instead of the (S)‐aminovinyl‐D‐cysteine (AviCys) that is naturally found in MU1140. The Cya containing bicyclic C/D ring of MU1140 was synthesized by Fmoc solid‐phase peptide synthesis (SPPS). The linear peptides were synthesized using OPfp ester derivatives and using various common coupling reagents such as COMU and TCTU. The linear peptide was intracyclized with DEPBT to construct the so‐called bicyclic ring C/D. This is the first report on the complete chemical synthesis of the bicyclic C/D ring of a MU1140 analog using orthogonally protected lanthionines using SPPS.     

Synthesis of linear and cyclic opioid‐based peptide analogs containing multiple N‐methylated amino acid residues

A series of six novel opioid peptide analogs containing one to three N‐methylamino acid residues, and six cyclic counterparts of these peptides were prepared by the solid‐phase method. Introduction of two consecutive N‐methylated amino acids, as well as cyclization of such conformationally constrained sequences, turned out to be challenging. The use of a recently reported triazine‐based coupling reagent, 4‐(4,6‐dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium toluene‐4‐sulfonate, enabled the synthesis and cyclization of the designed analogs in acceptable yields and with a lesser amount of by‐products than observed with the standard coupling reagents such as TBTU or HATU.Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.     

Evaluation of acid‐labile S‐protecting groups to prevent Cys racemization in Fmoc solid‐phase peptide synthesis

Phosphonium and uronium salt‐based reagents enable efficient and effective coupling reactions and are indispensable in peptide chemistry, especially in machine‐assisted SPPS. However, after the activating and coupling steps with these reagents in the presence of tertiary amines, Fmoc derivatives of Cys are known to be considerably racemized during their incorporation. To avoid this side reaction, a coupling method mediated by phosphonium/uronium reagents with a weaker base, such as 2,4,6‐trimethylpyridine, than the ordinarily used DIEA or that by carbodiimide has been recommended. However, these methods are appreciably inferior to the standard protocol applied for SPPS, that is, a 1 min preactivation procedure of coupling with phosphonium or uronium reagents/DIEA in DMF, in terms of coupling efficiency, and also the former method cannot reduce racemization of Cys(Trt) to an acceptable level (<1.0%) even when the preactivation procedure is omitted. Here, the 4,4′‐dimethoxydiphenylmethyl and 4‐methoxybenzyloxymethyl groups were demonstrated to be acid‐labile S‐protecting groups that can suppress racemization of Cys to an acceptable level (<1.0%) when the respective Fmoc derivatives are incorporated via the standard SPPS protocol of phosphonium or uronium reagents with the aid of DIEA in DMF. Furthermore, these protecting groups significantly reduced the rate of racemization compared to the Trt group even in the case of microwave‐assisted SPPS performed at a high temperature. © 2013 The Authors. European Peptide Society published by John Wiley & Sons, Ltd.     

Epimerization free synthesis of O-acyl isodipeptides employing COMU

O-Acyl isodipeptides are prepared by coupling Boc-Ser/Thr-OBzl with Fmoc-Xaa-OH employing COMU, well known third generation peptide coupling agent. The reaction proceeds with high yield and the chemical homogeneity of the synthesized molecules were established via chiral HPLC analyses. The O-acyl isodipeptide units play crucial role in the success of ' click peptide' protocol employed for assembling ' difficult sequence' peptides.     

Assessment of new 6-Cl-HOBt based coupling reagents for peptide synthesis. Part 2: Racemization studies

Summary Novel Cl-HOBt based coupling reagents have been evaluated for racemization extent in solid-phase peptide synthesis. The results show that all the coupling protocols based on the use of the novel reagents enable incorporation of the racemization prone residue serine with less than 2% racemization. Moreover, serine racemization obtained is less than 0.5% with protocols where a pre-activation step is avoided.     

Facile synthesis of peptide nucleic acids and peptide nucleic acid‐peptide conjugates on an automated peptide synthesizer

Peptide nucleic acids (PNAs) are DNA mimics with a neutral peptide backbone instead of the negatively charged sugar phosphates. PNAs exhibit several attractive features such as high chemical and thermal stability, resistance to enzymatic degradation, and stable binding to their RNA or DNA targets in a sequence‐specific manner. Therefore, they are widely used in molecular diagnosis of antisense‐targeted therapeutic drugs or probes and in pharmaceutical applications. However, the main hindrance to the effective use of PNAs is their poor uptake by cells as well as the difficult and laborious chemical synthesis. In order to achieve an efficient delivery of PNAs into cells, there are already many published reports of peptides being used for transport across the cell membrane. In this protocol, we describe the automated as well as cost‐effective semi‐automated synthesis of PNAs and PNA‐peptide constructs on an automated peptide synthesizer. The facile synthesis of PNAs will be helpful in generating PNA libraries usable, e.g. for high‐throughput screening in biomolecular studies. Efficient synthetic schemes, the automated procedure, the reduced consumption of costly reagents, and the high purity of the products are attractive features of the reported procedure. Copyright © 2010 European Peptide Society and John Wiley & Sons, Ltd.     

Facile Synthesis of N α -Protected Amino/Peptide Hydroxamic Acids Mediated by COMU

One-pot preparation of N ^α -protected amino/peptide hydroxamic acids from corresponding carboxylic acids is described using uronium-type coupling reagent COMU. The present protocol is simple and mild conditions are used. Thus the resulting hydroxamic acids are obtained in good yields without racemization.     

Assessment of new 6-Cl-HOBt based coupling reagents for peptide synthesis. Part 2: Racemization studies

Novel Cl-HOBt based coupling reagents have been evaluated forracemization extent in solid-phase peptide synthesis. The resultsshow that all the coupling protocols based on the use of the novel reagents enable incorporation of the racemization prone residue serine with less than 2% racemization. Moreover, serine racemization obtained is less than 0.5% with protocolswhere a pre-activation step is avoided.     

Preparation of tri(ethylene glycol) grafted core‐shell type polymer support for solid‐phase peptide synthesis

A core‐shell type polymer support for solid‐phase peptide synthesis has been developed for high coupling efficiency of peptides and versatile applications such as on‐bead bioassays. Although various kinds of polymer supports have been developed, they have their own drawbacks including poor accessibility of reagents and incompatibility in aqueous solution. In this paper, we prepared hydrophilic tri(ethylene glycol) (TEG) grafted core‐shell type polymer supports (TEG SURE) for efficient solid‐phase peptide synthesis and on‐bead bioassays. TEG SURE was prepared by grafting TEG derivative on the surface of AM PS resin via biphasic diffusion control method and subsequent acetylation of amine groups which are located at the core region of AM PS resin. The performance of TEG SURE was evaluated by synthesizing several peptides. Three points can be highlighted: (1) easy control of loading level of TEG, (2) improved efficiency of peptide synthesis compared with the conventional resins, and (3) applicability of on‐bead bioassays.     

Recent advances in solid-phase peptide synthesis and preparation of antibodies to synthetic peptides

Peptides prepared by the solid-phase peptide synthesis (SPPS) approach are used increasingly in biological research, for instance to elicit anti-peptide antibodies that will recognize the intact, cognate protein. Recent advances in SPPS are reviewed, including the use of new coupling reagents, new methods for evaluating peptide purity and new techniques of automated and multiple peptide synthesis. Methods for enhancing peptide immunogenicity are discussed such as the use of adjuvants and liposomes, and of synthetic branched polypeptides as carriers.     

Automated ‘X‐Y’ robot for peptide synthesis with microwave heating: application to difficult peptide sequences and protein domains

Precise microwave heating has emerged as a valuable method to aid solid‐phase peptide synthesis (SPPS). New methods and reliable protocols, as well as their embodiment in automated instruments, are required to fully use this potential. Here we describe a new automated robotic instrument for SPPS with microwave heating, report protocols for its reliable use and report the application to the synthesis of long sequences, including the β‐amyloid 1‐42 peptide. The instrument is built around a valve‐free robot originally developed for parallel peptide synthesis, where the robotic arm transports reagents instead of pumping reagents via valves. This is the first example of an ‘X‐Y’ robotic microwave‐assisted synthesizer developed for the assembly of long peptides. Although the instrument maintains its capability for parallel synthesis at room temperature, in this paper, we focus on sequential peptide synthesis with microwave heating. With this valve‐free instrument and the protocols developed for its use, fast and efficient syntheses of long and difficult peptide sequences were achieved. Copyright © 2010 European Peptide Society and John Wiley & Sons, Ltd.     

BOP‐OXy,BOP‐OBt,and BOP‐OAt: novel organophosphinic coupling reagents useful for solution and solid‐phase peptide synthesis

Stand‐alone coupling reagents derived from bis(2‐oxo‐3‐oxazolidinyl)phosphorodiamidic chloride show efficient performance in solution and SPPS. In particular, the Oxyma Pure (Luxembourg Biotech., Tel Aviv, Israel) derivative shows the additional advantage of being highly soluble in DMF and even fairly soluble in CH₃CN, which can extend its use for the synthesis of complex peptides. These new stand‐alone coupling reagents have the advantage of not bearing any counteranion such as PF₆ or BH₄, whose presence can jeopardize the purification of final peptides prepared in solution. Copyright © 2013 European Peptide Society and John Wiley & Sons, Ltd.     

Solid-phase peptide synthesis: from standard procedures to the synthesis of difficult sequences

This protocol for solid-phase peptide synthesis (SPPS) is based on the widely used Fmoc/tBu strategy, activation of the carboxyl groups by aminium-derived coupling reagents and use of PEG-modified polystyrene resins. A standard protocol is described, which was successfully applied in our lab for the synthesis of the corticotropin-releasing factor (CRF), >400 CRF analogs and a countless number of other peptides. The 41-mer peptide CRF is obtained within approximately 80 working hours. To achieve the so-called difficult sequences, special techniques have to be applied in order to reduce aggregation of the growing peptide chain, which is the main cause of failure for peptide chemosynthesis. Exemplary application of depsipeptide and pseudoproline units is shown for synthesizing an extremely difficult sequence, the Asn(15) analog of the WW domain FBP28, which is impossible to obtain using the standard protocol.     

Improved Fmoc‐based solid‐phase synthesis of homologous peptide fragments of human and mouse prion proteins

The synthesis of difficult peptide sequences has been a challenge since the very beginning of SPPS. The self‐assembly of the growing peptide chains has been proposed as one of the causes of this synthetic problem. However, there is an increasing need to obtain peptides and proteins that are prone to aggregate. These peptides and proteins are generally associated with diseases known as amyloidoses. We present an efficient SPPS of two homologous peptide fragments of HuPrP (106–126) and MoPrP105–125 based on the use of the PEGA resin combined with proper coupling approaches. These peptide fragments were also studied by CD and TEM to determine their ability to aggregate. On the basis of these results, we support PEG‐based resins as an efficient synthetic tool to prepare peptide sequences prone to aggregate on‐resin. Copyright © 2010 European Peptide Society and John Wiley & Sons, Ltd.     

Oxyma‐based phosphates for racemization‐free peptide segment couplings

Glyceroacetonide–Oxyma [(2,2‐dimethyl‐1,3‐dioxolan‐4‐yl)methyl 2‐cyano‐2‐(hydroxyimino)acetate ( 1 )] displayed remarkable physico‐chemical properties as an additive for peptide‐forming reactions. Although racemization‐free amide‐forming reactions have been established for N‐urethane‐protected α‐amino acids with EDCI, 1 , and NaHCO₃ in water or DMF‐water media, amide‐forming reactions of N‐acyl‐protected α‐amino acids and segment couplings of oligopeptides still require further development. Diethylphosphoryl–glyceroacetonide–oxyma (DPGOx 3 ) exhibits relative stability in aprotic solvents and is an effective coupling reagent for N‐acyl‐protected α‐amino acids and oligo peptide segments. The conditions reported here is also effective in lactam‐forming reactions. Unlike most of the reported coupling reagents, simple aqueous work‐up procedures can remove the reagents and by‐products generated in the reactions. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.     

Assessment of new 6-Cl-HOBt based coupling reagents for peptide synthesis. Part 1: Coupling efficiency study

The efficiency of a series of well-known coupling reagents (TBTU, HATU, and PyBOP) and of new in situ activating reagents (TCTU, HCTU, and DMTMM) was compared by synthesizing the 65–74 fragment of the Acyl Carrier Protein (H-Val-Gln-Ala-Ala-Ile-Asp-Tyr-Ile-Asn-Gly-NH₂₎, containing `a difficult sequence', as a test peptide, in a multiple peptide synthesizer. The longer sequence rMOG(35–55) was also synthesized. It was clear that the aminium salts are more efficient than the phosphonium salt (PyBOP) and that the new 6Cl-HOBt based reagents (HCTU and particularly TCTU) are very efficient, while DMTMM appeared to be not suitable for SPPS.