Fmoc-[15N]Leu-OH

Fmoc-[15N]Leu-OH is an Fmoc-protected, isotopically labeled leucine derivative in which the amino acid nitrogen is enriched with 15N, retaining the (typically) L-chiral configuration at the α-carbon while presenting a carboxylic acid suitable for peptide coupling chemistry. The fluorene-9-ylmethoxycarbonyl (Fmoc) group masks the α-amino functionality during solid-phase or solution-phase assembly and can be removed under base to reveal the labeled amine for subsequent bond formation. The side chain isopropyl group characteristic of leucine provides hydrophobic recognition elements in peptide sequences and can influence conformational preferences in labeled analogs. The presence of a defined 15N handle enables isotope-resolved tracking in mass spectrometry and NMR workflows, while the free carboxyl group supports downstream derivatization, activation, and incorporation into peptide building blocks.

1. Isotope-Resolved Peptide Synthesis

Fmoc-[15N]Leu-OH supports isotope-resolved peptide construction for analytical and mechanistic studies where nitrogen labeling improves signal attribution in MS/MS and NMR. The Fmoc-protected α-amine enables controlled peptide coupling after deprotection, while the free carboxylic acid participates in standard amide bond formation to place the 15N label at a defined position within a peptide chain. The leucine side chain contributes hydrophobic microenvironments that can be important for folding, binding-site mimicry, or substrate recognition in labeled peptide sequences. Incorporation of this chiral, protected amino acid derivative into peptide building blocks facilitates generation of site-specific 15N-containing peptides used as reference standards and mapping tools in biochemical research and structure-function investigations.

2. Protein Engineering Labeling

Fmoc-[15N]Leu-OH can be applied to protein engineering and protein chemistry workflows that require site-specific nitrogen isotope incorporation for spectroscopic analysis. The labeled α-amino nitrogen becomes part of the backbone amide linkage upon peptide assembly, allowing researchers to monitor local dynamics or structural changes with isotope-sensitive readouts. The Fmoc protection strategy aligns with iterative sequence assembly, enabling placement of 15N at leucine positions that are relevant to helix propensity, hydrophobic packing, or domain-specific interactions. Downstream formation of labeled protein fragments or proteinogenic peptide segments can serve as inputs for larger construct assembly, enabling isotope-enabled characterization of engineered biomolecules.

3. Mass Spectrometry Standards

Fmoc-[15N]Leu-OH is suitable for analytical research and method development where isotopically labeled amino acids and peptides function as internal standards or calibration references. The 15N label provides predictable mass shifts for targeted detection, while the leucine identity supports incorporation into digestion-compatible peptide motifs during analytical workflows. Fmoc protection and subsequent peptide coupling compatibility allow preparation of defined labeled peptides that can be used to validate fragmentation patterns, retention behavior, and quantitation strategies for nitrogen-containing analytes. The resulting labeled derivatives can be employed in quantitative proteomics, peptide mapping, and workflow verification where position-specific isotope labeling improves confidence in peak assignment.

4. Solid-Phase Peptide Manufacturing

Fmoc-[15N]Leu-OH fits solid-phase peptide synthesis and process chemistry intermediate preparation where orthogonal protection and predictable deprotection behavior are required. The Fmoc group is designed for base-mediated removal, enabling stepwise chain elongation while maintaining the integrity of the 15N-labeled amide-forming nitrogen. The carboxylic acid functionality supports activation and coupling to growing peptide-resin intermediates, and the leucine side chain contributes to consistent hydrophobic interactions that can affect resin loading and peptide aggregation behavior. Manufacturing-oriented use can include preparation of labeled peptide intermediates for downstream fine chemical synthesis, analytical reference materials, and isotope-tagged building blocks used in applied peptide science.

5. Peptidomimetic Building Blocks

Fmoc-[15N]Leu-OH can be leveraged in peptidomimetic construction and chemical biology research where nitrogen isotope labeling is used to study amide-bond behavior and backbone recognition. The labeled amino nitrogen becomes embedded in amide linkages during peptide assembly, enabling isotope-resolved tracking of conformational changes, binding interactions, or degradation pathways in labeled analogs. The leucine side chain provides a hydrophobic handle that can be retained in peptide mimetics to preserve key steric and electronic features of leucine-containing motifs. The Fmoc-protected amino acid form supports incorporation into protected peptide intermediates that can be further transformed into non-natural scaffolds, labeled fragments, or substrate analogs for mechanistic studies in amino acid and peptide chemistry.

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