Fmoc-D-leucine (CAS 114360-54-2, C₂₁H₂₃NO₄, MW 353.4 g/mol) is metered as a protected D-amino acid building block in Fmoc/tBu solid-phase peptide synthesis where D-leucine is inserted at N-terminal, internal, or scissile-bond-protective positions. On automated peptide synthesizers with reactor capacities from 5 mmol to 500 mmol, the material is dissolved in anhydrous DMF or NMP at 0.2–0.4 mol/L immediately before coupling. Resin substitution is typically 0.3–0.5 mmol/g for Wang resin and Rink amide resin, and 0.5–0.8 mmol/g for 2-chlorotrityl chloride resin. The first coupling cycle uses 3–4 eq Fmoc-D-leucine relative to resin free amine, 3–4 eq HBTU or HATU, and 6–8 eq DIPEA. Coupling is maintained at 20–30 °C for 30–90 min. The Fmoc group is removed with 20% piperidine in DMF in two stages of 5–15 min each. The D-leucine side chain requires no acid-labile side-chain protection because it is an aliphatic isobutyl group, and it is stable under standard TFA cleavage. Incomplete couplings are identified by Kaiser or TNBS tests and are corrected by recoupling with 2 eq fresh activated Fmoc-D-leucine for 60–120 min.

Process deviations observed at production scale include resin bed collapse when DMF is replaced by NMP without a prior swelling check and reduced coupling completion after storage of stock solutions for more than 12 h at ambient temperature. Moisture uptake on cold containers opened under relative humidity above 60% increases Karl Fischer water content above 0.5% w/w and depresses active ester formation. The material should therefore be equilibrated to room temperature in a sealed desiccator before weighing and kept away from amine vapours. The use of DIC/Oxyma Pure instead of HBTU avoids phosphate-containing by-products that can precipitate in DMF. Terminal products span linear and cyclic D-Leu-containing peptide APIs intended for preclinical and clinical development, with final quality governed by ICH Q7 when the peptide is later manufactured as an API. The following table sets out coupling parameter ranges across resin types used with Fmoc-D-leucine.

Cleavage conditions differ by resin. Peptide acids are released from Wang resin with TFA/TIS/H₂O 95:2.5:2.5 (v/v/v) for 2–4 h. Protected fragments are obtained from 2-chlorotrityl chloride resin with 1–2% TFA in DCM or 0.1 mol/L HFIP in DCM. The C-terminal amide is obtained from Rink amide resin under the same TFA cleavage with 2.5% TIS and 2.5% water. Residual DMF in the lyophilized peptide is controlled by ICH Q3C Class 2 solvent limits. Peptide identity is confirmed by LC-MS and amino acid analysis after hydrochloric acid hydrolysis. D-leucine incorporation is verified by chiral HPLC after peptide hydrolysis or by synthesis of diastereomeric reference peptides. The enantiomeric purity of the raw material is treated as an in-process control rather than an optional release parameter, because chiral inversion at the monomer level is propagated into every subsequent peptide chain.

What Limits Optical Purity Retention in Fmoc-D-Leucine-Mediated Peptide Coupling?

The principal risk during activation of Fmoc-D-leucine is oxazolone-mediated epimerization. Carbodiimide-only activation of N-acyl amino acids generates a 5(4H)-oxazolone intermediate that deprotonates at the alpha carbon; in D-leucine this can interconvert with its L form. When coupling is performed with HOBt or Oxyma Pure as an additive, the active ester is formed more rapidly and the oxazolone concentration remains low. Process limits therefore set coupling temperature at 20–35 °C and total activation-plus-coupling time below 4 h. Preactivation of Fmoc-D-leucine with DIC in DMF for longer than 10 min before resin transfer is not recommended for sequences where optical purity is critical. The accepted procedure is to add coupling reagent, additive, and base to the amino acid solution, allow 3–5 min preactivation, and immediately deliver the mixture to the resin.

For GMP raw material release, Fmoc-D-leucine is specified with a chiral purity method using an amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase, a mobile phase of n-hexane/2-propanol with a trifluoroacetic acid modifier, and ultraviolet detection at 220 nm. Acceptance is typically not more than 0.5% area for the L enantiomer, in line with ICH Q6A decision tree 2 for chiral impurities. Batch-to-batch variation in residual piperidine, DMF, or methyl tert-butyl ether is controlled under ICH Q3C; elemental impurities are assessed under ICH Q3D Option 1. Water content is controlled by USP 921 Karl Fischer titration, with a limit of ≤0.5% w/w. A representative analytical control matrix is shown below.

Storage conditions also affect optical durability. The Fmoc group is quantitatively removed by primary and secondary amines, so storage must exclude amine vapours from adjacent containers. Long-term storage at −20 °C under argon is common; working quantities may be held at 2–8 °C for 30 days if sealed and desiccated. Unexpected peak splitting in chiral HPLC at 220 nm has been resolved by controlling column temperature at 25 °C and by filtering DMF stock solutions through 0.22 µm PTFE membranes. When a peptide contains several adjacent hydrophobic residues including D-leucine, incomplete deprotection after 10 min can occur on high-loading Wang resin; a double deprotection of 2 × 10 min with fresh piperidine is used rather than extending the single contact time, because prolonged base exposure increases racemization risk elsewhere in the chain.

In antimicrobial peptide optimization, D-amino acid scanning with Fmoc-D-leucine is performed on 0.05–0.25 mmol scale using automated microwave peptide synthesizers. The building block is coupled at 75 °C for 2 min with 5 eq Fmoc-D-leucine, 5 eq HATU, and 10 eq DIPEA relative to free amine. Rink amide AM resin is preferred for C-terminal amide peptides; its loading range is 0.3–0.5 mmol/g. The deprotection step uses 20% piperidine in DMF and is held at 75–90 °C for 1–2 min. This combination reduces cycle time but increases the risk of difficult couplings in sterically hindered positions when hydrophobic residues such as D-leucine are flanked by β-branched amino acids. The failure mode is observed as a delay in the next coupling and is handled by a second double coupling at 50 °C with 3 eq Fmoc-D-leucine and PyBOP/DIPEA.

D-Leucine substitution in cationic amphipathic peptides is used to test whether proteolytic stability can be raised without destroying membrane selectivity. Activity shifts are sequence-dependent and cannot be predicted from hydrophobicity alone. Published data for individual D-Leu-containing antimicrobial sequences vary widely; the only reliable method is a side-by-side D-scan followed by minimum inhibitory concentration testing against defined strains under CLSI broth microdilution conditions. Peptide crude batches are purified to ≥95% by RP-HPLC and characterized by LC-MS. Circular dichroism is used to compare the α-helical content of L- and D-Leu-substituted analogues in 50% trifluoroethanol. The terminal products are research-grade peptide panels used in serum stability assays, not pharmaceutical preparations. These peptides are not intended for human use and are released under a non-GMP research quality agreement.

When Fmoc-D-leucine Replaces L-Leucine in Diagnostic Protease Probe Synthesis

Within diagnostic protease substrate design, substitution of L-leucine with Fmoc-D-leucine is performed in P1 or P2 positions of peptide sequences carrying an N-terminal fluorophore such as 2-aminobenzoyl and a C-terminal quencher such as 2,4-dinitrophenyl. The D-configuration at leucine slows aminopeptidase and endopeptidase attack during extended incubation in diluted serum, allowing the probe to remain intact until the target protease cleaves the scissile bond. The peptide portion is assembled on 2-chlorotrityl chloride resin at 0.5–0.8 mmol/g. Fmoc-D-leucine is coupled with 3 eq HATU and 6 eq DIPEA in DMF at 25 °C for 60 min. N-terminal Fmoc is removed, and the fluorophore is coupled as a protected or free aromatic amine using HBTU. Cleavage and side-chain deprotection use TFA/TIS/H₂O 95:2.5:2.5 at room temperature for 2 h.

After cleavage, the crude quenched substrate is purified by preparative RP-HPLC on a C18 column with a 0.1% TFA acetonitrile gradient. Identity is confirmed by LC-MS and amino acid analysis. Kinetic validation is performed by Michaelis-Menten analysis in assay buffer at 37 °C using a fluorometric plate reader with excitation at 320 nm and emission at 420 nm. Substitution with D-leucine may reduce the target protease turnover number, so K_m and V_max must be re-determined for each new substrate. The terminal products are diagnostic research reagents and IVD components. When the peptide is incorporated into a regulated diagnostic kit, the substrate synthesis and change control fall under ISO 13485 design and development requirements. For research-use-only material, the supplier certificate of analysis is limited to HPLC purity, moisture, and MS identity.

Self-Assembled Hydrogel Precursors from Short Fmoc-D-leucine Peptides

Fmoc-protected dipeptides and tripeptides containing D-leucine are synthesized on 2-chlorotrityl chloride resin or by solution-phase active ester coupling and purified to ≥95% by preparative HPLC. The Fmoc-D-leucine residue supplies an aliphatic isobutyl side chain that contributes to hydrophobic association, but gelation also requires the presence of aromatic stacking residues such as phenylalanine or tyrosine in the same short sequence. For gel formation, the lyophilized peptide is dissolved in dilute sodium hydroxide at pH 8.0–8.5, filtered through 0.22 µm membranes, and the pH is lowered to 5.5–6.5 with glucono-δ-lactone or dilute hydrochloric acid. Gelation is confirmed by oscillatory rheology; the storage modulus is strongly concentration- and sequence-dependent, and published values for specific D-leucine-containing Fmoc-peptides are limited. Terminal products are self-supporting hydrogels used as 3D cell culture scaffolds and drug-release research matrices.

Process limitations are significant. A gel formed from an Fmoc-D-leucine peptide at 5 mg/mL may collapse upon pipetting if the aromatic residue is replaced by serine, indicating that D-leucine alone is not a sufficient gelator. Sterile filtration before pH reduction is used because the gel cannot pass through sterilizing-grade membranes after assembly. If the hydrogels are intended for later medical device testing, cytotoxicity is evaluated under ISO 10993-5. For routine cell culture research, endotoxin control below 0.1 EU/mg and residual solvent control under ICH Q3C are applied to the peptide powder. The incorporation of D-leucine instead of L-leucine alters supramolecular chirality, which may change fibre twist and ligand presentation; this must be verified by circular dichroism and scanning electron microscopy for each sequence.

Proteolytic Stabilization of Radiopharmaceutical Peptide Ligands by D-Leucine Incorporation

For peptide ligands intended for radiolabelling with 68Ga or 177Lu, D-leucine is incorporated to reduce rapid cleavage in serum and extend tumour uptake half-life. The peptide chain is assembled on 2-chlorotrityl chloride resin or TentaGel resin using Fmoc/tBu chemistry. Fmoc-D-leucine is coupled with 3–4 eq HATU and 6–8 eq DIPEA at 20–30 °C for 45–90 min. After full sequence assembly and N-terminal Fmoc removal, a bifunctional chelator such as DOTA-tris(tBu)ester or NOTA is coupled through an amide bond. Cleavage and deprotection with TFA/TIS/H₂O 95:2.5:2.5 yields the peptide chelator precursor, which is purified to ≥95% by RP-HPLC. The conjugate is lyophilized and stored at −20 °C under argon.

Radiolabelling of a 20–50 µg peptide precursor with 68GaCl₃ is carried out in 0.1 mol/L sodium acetate buffer at pH 4.0–4.5 and 80–90 °C for 10–15 min. For 177Lu, the pH is adjusted to 5.0–5.5 and heating lasts 20–30 min. Radiochemical purity is checked by radio-HPLC and thin-layer chromatography. When the peptide precursor is intended for clinical radiopharmaceutical preparation, the Fmoc-D-leucine raw material is controlled as a GMP starting material under ICH Q7 Section 7.3. Terminal products are investigational D-leucine-modified peptide receptor ligands for PET imaging or peptide radionuclide receptor therapy. Because the in vivo effect of D-leucine substitution is sequence- and target-receptor-dependent, published data for a specific radiolabelled peptide candidate remains limited and must be generated by biodistribution studies.