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| Product Name | Fmoc-Asp(OtBu)-OH |
|---|---|
| Synonyms | Fmoc-L-aspartic acid 4-tert-butyl ester; Nα-Fmoc-L-aspartic acid β-tert-butyl ester |
| Catalog No. | AS2486 |
| CAS Number | 71989-14-5 |
| Molecular Formula | C23H25NO6 |
| Molecular Weight | 411.45 g/mol |
| SMILES | CC(C)(C)OC(=O)CC@@HNC(=O)OCC1C2=CC=CC=C2C3=CC=CC=C13 |
| Storage Temperature | Cool, dry place (≤25°C) |
| Appearance | White powder |
| Melting Point | 145–160°C |
| Specific Rotation | -24° ± 2.5° (c=1 in DMF) |
| Functional Groups | Fmoc-protected α-amino group; tert-butyl-protected side-chain β-carboxyl group; free α-carboxylic acid |
| Primary Application | Fmoc solid-phase peptide synthesis (SPPS) |
Product Overview
Fmoc-Asp(OtBu)-OH is a protected L-aspartic acid derivative widely used as a building block in Fmoc-based solid-phase peptide synthesis (SPPS).
The α-amino group is protected by Fmoc, while the aspartic acid side-chain β-carboxyl group is protected as a tert-butyl ester (OtBu). This combination allows the amino acid to participate in repetitive Fmoc deprotection and peptide coupling cycles while limiting unwanted reactions involving the side-chain carboxyl group.
During final acidic cleavage and global deprotection, the OtBu protecting group is removed to regenerate the native aspartic acid side chain.
Fmoc-Asp(OtBu)-OH is therefore one of the standard protected amino-acid building blocks used for the synthesis of Asp-containing peptides, modified peptides, peptide libraries and complex research sequences.
Applications in Peptide Synthesis
| Application | Role of Fmoc-Asp(OtBu)-OH |
|---|---|
| Fmoc Solid-Phase Peptide Synthesis | Standard protected building block for incorporation of L-aspartic acid |
| Asp-Containing Peptides | Protects the side-chain β-carboxyl group during peptide elongation |
| Peptide Library Synthesis | Supports parallel preparation of sequence-diverse Asp-containing peptides |
| Modified Peptide Synthesis | Compatible with many terminal and side-chain modification strategies |
| Structure–Activity Relationship Studies | Enables systematic preparation of Asp-containing peptide analogs |
| Difficult Peptide Synthesis | Can be incorporated into optimized SPPS workflows where Asp-related side reactions require control |
| Custom Peptide Synthesis | Common building block for research-scale peptide production |
Fmoc-Asp(OtBu)-OH in Fmoc-SPPS
During Fmoc-SPPS, Fmoc-Asp(OtBu)-OH is activated through its free α-carboxyl group and coupled to the resin-bound peptide chain.
After coupling, standard base-mediated Fmoc deprotection exposes the α-amino group for the next elongation cycle. The side-chain carboxyl group remains protected as the tert-butyl ester during routine synthesis.
At the end of peptide assembly, acidic cleavage conditions remove the OtBu protecting group together with other acid-labile side-chain protecting groups.
This orthogonal protection strategy makes Fmoc-Asp(OtBu)-OH compatible with conventional Fmoc/tBu peptide synthesis chemistry.
Why Protect Aspartic Acid with OtBu?
Aspartic acid contains two carboxyl groups. Without selective side-chain protection, the β-carboxyl group can participate in unwanted chemistry during peptide assembly.
The OtBu group provides an acid-labile protecting strategy that remains compatible with the repeated basic Fmoc-removal cycles used during SPPS.
However, side-chain protection should not be interpreted as eliminating every Asp-related side reaction.
One of the most important challenges is aspartimide formation, a base-promoted intramolecular cyclization that can occur during Fmoc-SPPS, particularly in susceptible sequence environments.
A recent review continues to identify Asp(OtBu) as the conventional protecting group of choice for the Asp side chain, while also noting that OtBu alone does not completely suppress aspartimide in highly susceptible sequences.
Aspartimide Formation: A Practical Consideration
Aspartimide formation is particularly relevant when Asp is followed by residues such as Gly and in other sequence contexts that favor intramolecular cyclization during repeated base treatment.
This reaction can lead to several impurity pathways, including:
aspartimide intermediates
α- and β-linked peptide products
racemization-related impurities
difficult-to-separate chromatographic species
Classical comparative work showed that Asp(OtBu) remains useful, but additional backbone protection can be necessary for highly aspartimide-prone sequences. In one well-established model system, combining OtBu side-chain protection with backbone protection of the preceding Gly completely suppressed the measured aspartimide pathway.
More recent analyses have likewise shown that bulkier alternative Asp protecting groups can further reduce aspartimide formation in difficult sequences.
Practical view: Fmoc-Asp(OtBu)-OH should remain the routine starting building block for most Asp-containing peptides. Alternative protecting groups or backbone-protected dipeptide building blocks are more appropriately considered when the actual sequence indicates elevated aspartimide risk, rather than being used routinely for every Asp residue.
Product Documents
A Safety Data Sheet (SDS / MSDS) is available for this product to support laboratory handling, storage and safety assessment.
📎 MSDS_Fmoc-Asp(OtBu)-OH_AS2486.pdf
Related Technical Resources
Learn more about peptide chain assembly in Solid-Phase Peptide Synthesis (SPPS): A Practical Guide.
Explore additional Standard Fmoc-Amino Acids for peptide synthesis applications.
For guidance on selecting protected residues, visit Amino Acids & Peptide Building Blocks.
For difficult Asp-containing sequences, Pseudoproline Dipeptides and other specialized building blocks can be considered as part of an optimized synthesis strategy.
For complete peptide projects, Alan Scientific provides Custom Peptide Synthesis, including synthesis, purification, modification and analytical quality control.
Why Source Peptide Building Blocks from Alan Scientific?
Alan Scientific supplies protected amino acids and peptide building blocks for research and peptide synthesis applications, with competitive pricing, flexible ordering options and peptide synthesis technical support.
Researchers can source individual Fmoc amino acids or combine building-block procurement with Custom Peptide Synthesis according to project requirements.
Research Use Only