(2S)-1-[(2S)-2-({[(9H-Fluoren-9-Yl)Methoxy]Carbonyl}Amino)Propanoyl]Pyrrolidine-2-Carboxylicacid

What Drives Adoption of Fmoc-Ala-Pro-OH in High-Throughput Peptide Library Construction?

Combinatorial peptide libraries designed for epitope mapping or protease substrate profiling often incorporate an Ala-Pro dipeptide unit as a rigidified turn element that enhances resistance to non-specific proteolysis during screening assays. When a 96-well parallel synthesis format is employed on a MultiSynTech Syro I robotic platform, the need to reduce cycle times and minimise well-to-well variability makes manual stepwise coupling of Fmoc-Ala-OH to H-Pro-resin impractical due to extended DKP waiting periods. Fmoc-Ala-Pro-OH is employed at 1.5–2.0 equivalents relative to resin substitution (0.15–0.25 mmol/g on TentaGel S RAM resin) activated with 1.45 equiv of Oxyma Pure and 1.5 equiv of DIC in NMP; this equimolar slight excess is permissible because in-process HPLC monitoring (UV detection at 220 nm, YMC Triart C18 3 µm, 50×2.1 mm column) confirms residual starting material below 0.5% after 40 min recirculation. Compliance for such discovery-phase libraries follows institutional ISO 9001:2015 documentation standards rather than full cGMP, yet all solvents are controlled for peroxide content (Ph. Eur. 2.5.5) and trace metals (ICH Q3D guideline for elemental impurities, with Pd limit 10 µg/day for oral exposure extrapolated to handling limits). The downstream process consists of simultaneous resin cleavage and side-chain deprotection in each well with 200 µL of a TFA/EDT/water/thioanisole cocktail (92.5:2.5:2.5:2.5 v/v) for 3 h, followed by precipitation in cold methyl tert-butyl ether and centrifugation at 3000×g, 4°C. Crude peptides are analysed by MALDI-TOF MS (Bruker Autoflex Speed) and, for hits, repurified on a semi-preparative HPLC system to ≥90% purity. The terminal output is a lyophilised peptide library plate stored at -20°C under argon, ready for dissolution in assay buffer for fluorescence polarisation-based screening campaigns targeting SH2 domains or PDZ-binding motifs.

Diketopiperazine Mitigation Through Dipeptide Insertion: Process Validation Data

When a Pro residue occupies the C-terminal position attached to the resin, the Fmoc deprotection of the subsequent Ala generates a nucleophilic secondary amine in close proximity to the ester linkage of the resin, driving DKP release with rate constants that are temperature- and solvent-dependent. The use of Fmoc-Ala-Pro-OH shifts the point of vulnerability to the next coupling event: once the Fmoc group on alanine is removed, H-Ala-Pro-resin must be capped immediately with the incoming amino acid. Comparative kinetic data obtained on an EasyPure 800 automated synthesizer using inline UV monitoring allow quantification of the window of safety. On 2-chlorotrityl chloride resin loaded with Fmoc-Pro-OH at 0.28 mmol/g, after Fmoc cleavage and neutralisation, the resin-bound H-Ala-Pro-OH intermediate was held at 25°C in DMF; the released DKP (cyclo(Ala-Pro)) was sampled and quantified by UPLC (Waters Acquity H-Class, BEH C18 1.7 µm, 2.1×50 mm, gradient 5–95% acetonitrile in 0.1% TFA over 3 min) at intervals. The following table presents the percentage of resin-bound peptide remaining after specified delays.

The data demonstrate that a delay exceeding 5 min leads to a loss incompatible with acceptable crude purity for direct use in API synthesis. In full compliance with ICH Q2(R2) validation of analytical procedures, the UPLC method exhibited a limit of quantification for DKP of 0.05% relative to the parent peptide. The downstream process specification therefore mandates that after deprotection, the next Fmoc-amino acid (2.5 equiv) must be pre-activated and added within 2 min, and the coupling vessel must be thermostated at 18±1°C throughout. The terminal product arising from this optimized protocol is a protected peptide-resin intermediate that is directly subjected to global deprotection, producing a crude peptide with ≤1.5% D-Ala epimer as confirmed by Marfey’s derivative analysis (FDAA derivatisation, LC-MS, single-ion recording at m/z 340). The final finished form is a lyophilised research-grade peptide supplied with a certificate of analysis referencing USP 〈1041〉 for biologics characterization.

If the Target Conjugate Requires a Cleavage-Resistant Ala-Pro Spacer, What Conditions Govern Chemoselective Ligation?

Peptide–drug conjugates (PDCs) and PEGylated peptides frequently insert a short spacer containing the Ala-Pro dipeptide to confer resistance to exopeptidases and to distance a payload from the receptor-binding domain. Fmoc-Ala-Pro-OH is first assembled into a protected intermediate on a 2-CTC resin at 0.20 mmol/g loading using 1.8 equiv of dipeptide with 3.6 equiv DIPEA in DCM for 1.5 h, then capped with methanol. After Fmoc deprotection, an azidoacetic acid is coupled to the N-terminus to introduce a click-chemistry handle, and the protected peptide is cleaved with 20% HFIP in DCM to retain side-chain protection. The crude protected peptide is precipitated and dissolved in DMF for copper(I)-catalysed azide–alkyne cycloaddition with a maytansinoid derivative bearing a strained alkyne, using 0.2 equiv of Cu(I)Br and 0.4 equiv TBTA at 40°C for 16 h. The process is compliant with ICH M3(R2) guidance for nonclinical safety studies; residual copper is removed to ＜10 ppm (ICP–MS, USP 〈233〉). Side-chain deprotection is then completed with 95% TFA containing 2.5% TIS and 2.5% water, and the conjugate is purified on a Kromasil C8 10 µm, 250×20 mm semi-prep column with a 0.1% ammonium bicarbonate/acetonitrile gradient, yielding ＞97% purity. The terminal product is a lyophilised acetate salt stored at -80°C in amber vials, representing an intermediate for targeted oncology candidates entering IND-enabling toxicology.

Process-Scale Purification of Peptide APIs Containing the Ala-Pro Motif Resolves Epimerisation Impurities to Below 0.10%

During activation of the carboxylic acid of Fmoc-Ala-Pro-OH, partial racemisation at the α-carbon of alanine can occur via oxazolone formation, generating the Fmoc-D-Ala-Pro-OH epimer that is incorporated into the growing chain and yields a diastereomeric impurity. Although the steric bulk of the Pro residue slows oxazolone ring closure relative to other amino acids, activation with uranium salts in the presence of excess base has been shown, in a process characterisation study on a 50 mmol scale using a Büchi Sepacore Prep HPLC system, to produce 0.3–0.8% of the D-Ala-containing congener. To meet the ICH Q3A(R2) qualification threshold for individual unspecified impurities (≤0.10% for a 2 g daily dose), the purification train relies on a dynamic axial compression column (DAC 150 mm ID, packed with YMC Triart C18 15 µm, 250 mm bed height) operated at 40 bar with a linear gradient from 18–28% acetonitrile in 0.1 M triethylammonium phosphate pH 2.8 over 90 min at 350 mL/min. The diastereomer elutes approximately 0.7 min earlier than the desired peptide, and the fractionation window is set by real-time MS-directed diversion (Waters QDa detector, single-ion recording m/z target ± 0.5 Da). Pooled fractions exceeding 99.5% purity are concentrated on a wiped-film evaporator at 25°C under 15 mbar and lyophilised. A final polishing step on a Source 30RPC column (50×200 mm) equilibrated with 0.1% acetic acid removes column leachables. The finished substance is provided as a freeze-dried powder in USP Type I borosilicate glass vials, certified to contain ＜0.08% D-Ala epimer by a validated chiral CE method (USP 〈1053〉), suitable as a sterile injectable API after terminal gamma irradiation at 25 kGy.