Fmoc-Asp(OBno)-OH Aspartimide formation remains a significant challenge in peptide synthesis, particularly within Fmoc-based solid-phase peptide synthesis (SPPS). This undesirable side reaction, often occurring with aspartic acid residues, can lead to low yields, complex purification processes, and ultimately, inaccessible peptide sequences. Understanding the mechanisms behind aspartimide formation and implementing effective prevention strategies are crucial for researchers and chemists aiming to produce high-purity peptidesAspartimide formation in peptide chemistry.
Aspartimide formation is an intramolecular cyclization reaction that arises from the side chain of aspartic acid. Under the basic conditions frequently employed in Fmoc SPPS, such as during deprotection steps with piperidine, the carboxyl group of an aspartic acid residue can attack the activated $\alpha$-amino group of the same residue.作者：J Bouchenna·2020·被引用次数：5—Aspartimide formation often complicates the solid phase synthesis of peptides. Much less discussed is the potential occurrence of this side ... This cyclization results in the formation of a five-membered imide ring, known as an aspartimide.
The primary issue with aspartimide formation is not just its occurrence, but the subsequent rearrangements it can undergo. Upon nucleophilic attack, the aspartimide can open to form either the desired $\alpha$-peptide bond or an undesired $\beta$-peptide bond, leading to isoaspartyl impurities.作者：K Sato·2024·被引用次数：8—Aspartimide formationis one of the major obstacles that impedes the solid phasesynthesisof largepeptidesand proteins. Until now, no cost- ... These impurities are structurally similar to the target peptide, making their separation difficult and significantly impacting the overall purity and yield of the synthesized peptide.
Several factors can influence the propensity for aspartimide formation, making it a complex challenge to overcome:
* Amino Acid Sequence: Certain sequences are more prone to aspartimide formation than others.How to prevent aspartimide formation during Microwave- ... Specifically, the Aspartic Acid, Glycine (DG) sequence is frequently cited as being highly susceptible. The presence of glycine immediately C-terminal to aspartic acid can facilitate the cyclization. Other sequences involving aspartic acid followed by small, flexible amino acids can also increase the risk.
* Reaction Conditions: The basic conditions used for Fmoc deprotection are a primary driver. The choice of base, its concentration, and the duration of exposure can all play a role.2023年2月6日—Aspartimide rearrangements are a particularly nasty side reaction thatcan occur during fmoc-based solid phase peptide synthesis. Elevated temperatures can also accelerate the reaction.
* Protecting Groups: The nature of the protecting groups used for the aspartic acid side chain can influence the reactivity and susceptibility to cyclization. While some protecting groups are designed to mitigate this, their effectiveness can vary.
* Peptide Length and Structure: Longer peptides or those that undergo aggregation during synthesis can present unique challenges, potentially influencing the accessibility of residues and the microenvironment around aspartic acid2022年12月18日—Aspartimide formationis an important side reaction in thesynthesisof N-protected beta-cyanoalanine (beta-cyanoalanine CAS number is 6232-19-5) ....
Given its persistent nature, numerous strategies have been developed to minimize or completely block aspartimide formation during peptide synthesis.Aspartimide formation during peptide synthesisis a prevalent side reaction that compromises yield and purity, especially in Asp-containing peptides. These approaches generally fall into a few key categories:
#### 1The aspartimide problem persists .... Modifying Protecting Group Strategies
One of the most effective methods involves the use of specific protecting groups for the aspartic acid side chain that sterically hinder or electronically disfavor the cyclization.
* $\beta$-Ester Protection: Employing $\beta$-ester protecting groups for aspartic acid is a common tactic. These groups occupy the space that would otherwise be available for intramolecular cyclization. Examples include benzyl (Bzl) or tert-butyl (tBu) esters. However, even with $\beta$-ester protection, careful consideration of reaction conditions is still necessary.
* Amide Protection: In some cases, $\beta$-amide protecting groups can also be employed.
* Blocking Groups on the $\alpha$-Nitrogen: As mentioned in some literature, incorporating a blocking group on the $\alpha$-nitrogen of the preceding amino acid in the peptide sequence can also help prevent aspartimide formation.
#### 2. Optimizing Reaction Conditions
Fine-tuning the conditions of the peptide synthesis process can significantly reduce the risk of aspartimide formation.
* Milder Deprotection Conditions: Using milder bases or reducing the concentration and exposure time of deprotection reagents can limit the opportunity for cyclization.
* Lower Temperatures: Conducting reactions at lower temperatures can slow down the kinetics of aspartimide formation作者：K Neumann·2020·被引用次数：80—Aspartimide formation leads to low yieldsin addition to costly purification or even inaccessible peptide sequences. Here, we report an ....
* Acidic Conditions: While basic conditions are the primary culprit, some literature suggests that certain acidic conditions can also play a role or be used in prevention.作者：D Flora·2005·被引用次数：31—In summary, we have demonstrated thataspartimide formation is predominant in the standard Fmoc peptide synthesisusing an allyl ester as the side chain ... For instance, acid-mediated prevention strategies are being explored.
* Microwave-Assisted Synthesis: While microwave irradiation can accelerate peptide synthesis, it can also, in some instances, exacerbate side reactions like aspartimide formation if not carefully controlled. Specific strategies are being developed for microwave-assisted protocols.EP2886531A1 - Aspartic acid derivatives - Google Patents
#### 3.作者：K Toney·1993·被引用次数：21—This cytosolic enzyme is known to preferentially methylate L-isoaspartyl residues within model substrates. Control experiments in which JP was incubated in the ... Alternative Synthesis Methodologies
For particularly challenging sequences, alternative approaches to SPPS or fragment coupling might be consideredAspartimide formation during peptide synthesisis a prevalent side reaction that compromises yield and purity, especially in Asp-containing peptides..
* Native Chemical Ligation (NCL): For synthesizing very large peptides and proteins, NCL can be a powerful tool.作者：D Flora·2005·被引用次数：31—In summary, we have demonstrated thataspartimide formation is predominant in the standard Fmoc peptide synthesisusing an allyl ester as the side chain ... Adopting "good NCL practices" can involve restricting ligation steps or employing specific strategies that minimize the conditions favoring aspartimide formation in the joining peptide sequences.
#### 4. Sequence Design and Analysis
Proactive peptide design can mitigate the risk from the outset.
* Sequence Analysis: Identifying sequences known to be prone to aspartimide formation, such as Asp-Gly (DG) dipeptides, allows for targeted intervention.Frustrated byaspartimide formation during peptide synthesis? Get ready to discover different strategies to minimize or avoid the formation of these undesi- red ...
* Minimizing Prone Sequences: If possible, redesigning the peptide to minimize the occurrence of highly susceptible sequences can simplify synthesisFrustrated byaspartimide formation during peptide synthesis? Get ready to discover different strategies to minimize or avoid the formation of these undesi- red ....
Detecting and quantifying aspartimide formation and its resulting impurities is crucial for quality control. Techniques such as High-Performance Liquid Chromatography (HPLC) coupled with Mass Spectrometry (MS) are indispensable for identifying these sequence impurities. Understanding the mass difference introduced by aspartimide formation can aid in their identification.
Aspartimide formation is a persistent and often frustrating obstacle in peptide synthesis, directly impacting the yield and purity of synthetic peptides. The reaction is driven by the inherent chemistry of aspartic acid residues and is exacerbated by the basic conditions common in Fmoc SPPS. However, by understanding the factors that promote its formation, researchers can implement a range of effective strategies.Aggregation, Racemization and Side Reactions These include judicious selection of protecting groups, careful optimization of reaction conditions, and in some cases, employing alternative synthesis methodologies or thoughtful peptide design. Continuous research into novel protecting groups and refined protocols aims to further mitigate this challenge, paving the way for more efficient and successful synthesis of complex peptides.
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