Why is an N-terminal blocked phenylalanine residue required in solution-phase fragment condensation?

N-Acetyl-L-phenylalanine (Ac-Phe-OH, CAS 2018-61-3, C₁₁H₁₃NO₃, molecular weight 207.23 g/mol) is used as a pre-capped N-terminal fragment in solid-phase and solution-phase peptide manufacturing where the absence of a free α-amino group prevents further chain elongation at that terminus. In Fmoc/t-Bu solid-phase peptide synthesis, the compound is introduced after selective removal of the Fmoc group from the terminal residue, using HOBt/DIC activation in dry DMF. Production-scale solid-phase peptide synthesizers with reactor volumes between 0.5 L and 50 L typically operate at resin loadings of 0.5–1.0 mmol/g on 2-chlorotrityl chloride or Rink amide AM resin. The addition ratio is maintained at 1.1–1.2 molar equivalents relative to free amine sites for coupling, while solution-phase capping of peptide fragments uses 0.95–1.05 equivalents to limit overacylation; exceeding 1.5 equivalents in solution-phase conversions increases the formation of carboxylic acid-derived side products. Coupling is allowed to proceed for 45–120 min at 20–25 °C and is monitored by Kaiser test until a negative result is obtained, with confirmatory HPLC-MS using a C18 column and acetonitrile/water mobile phase. Cleavage from the resin is performed with TFA/H₂O/TIS 95:2.5:2.5 v/v, followed by preparative reversed-phase C18 HPLC and lyophilization. Terminal product types include N-acetylated peptide APIs, peptide fragments for conjugation, N-capped peptide immunogens, and peptide reference standards. For GMP manufacture, raw material acceptance is governed by ICH Q7 Section 7.1 and ICH Q3C residual solvent limits; the finished peptide is tested according to USP 467 for residual solvents, USP 232 for elemental impurities, and Karl Fischer titration with water content below 0.5%.

In leave-on facial emulsion manufacturing, the acetylated phenylalanine derivative is pre-dispersed in 1,3-butylene glycol before addition to the cooled aqueous phase at 40 °C to avoid acid-catalyzed destabilization of the pH-sensitive oil-in-water lamellar network. N-acetyl-L-phenylalanine is included as an INCI-listed skin-conditioning additive, not as a primary active, and the addition ratio in leave-on formulations is 0.05–0.20 wt% based on finished product weight. The oil phase, containing caprylic/capric triglyceride, cetearyl alcohol, and glyceryl stearate, is heated to 75 °C; the water phase with glycerin and xanthan gum is heated separately to 75 °C and then combined under a rotor-stator homogenizer at 5,000–10,000 rpm for 10–15 min. After cooling to 40 °C with anchor agitation, a 5 wt% solution of N-acetyl-L-phenylalanine in butylene glycol/water adjusted to pH 5.5–6.5 is metered into the batch. The pH is maintained above 5.0 to prevent protonation of the carboxylate group and below 7.0 to reduce microbiological pressure prior to preservative addition. Terminal product types include barrier-repair facial serums, night creams, eye contour emulsions, and non-comedogenic facial lotions. Compliance is determined under EC 1223/2009 Article 10 and Article 11, with manufacturing hygiene under ISO 22716:2007 and substance registration under REACH 1907/2006 Annex VII. The ingredient is not listed in Annex II or Annex III of EC 1223/2009; therefore, restriction concentration is not fixed by the regulation, but the safety assessment in the product information file must justify the margin of safety for the specific exposure scenario.

When a quaternary ammonium lamellar gel is selected as the deposition vehicle

When a quaternary ammonium lamellar gel is selected as the deposition vehicle, N-acetyl-L-phenylalanine is introduced after the network has formed at 35 °C to prevent disruption of the behenyltrimonium chloride–cetearyl alcohol packing. The disclosed addition ratio in rinse-off systems is 0.05–0.15 wt%; published quantitative formulations for this specific configuration are limited, but supplier technical dossiers place the upper limit below 0.2 wt% to maintain phase stability. A gel phase is prepared by combining behenyltrimonium chloride at 2.0–3.5 wt% with cetearyl alcohol at 4.0–6.0 wt% in water at 75–80 °C, followed by homogenization at 3,000–5,000 rpm for 5–8 min and cooling under anchor agitation. After the mixture reaches 35 °C, a water-phase solution of the acetylated amino acid adjusted to pH 4.0–5.0 with citric acid is metered in; addition at higher temperatures after gel formation is avoided because the lamellar network can shift from the Lα phase, with terminal viscosity falling below 20,000 mPa·s when measured on a Brookfield RV viscometer at 10 rpm with spindle 5. Terminal product types include rinse-off conditioners, hair masks, and split-end repair lotions. Compliance follows EC 1223/2009, ISO 22716:2007 for good manufacturing practices, and REACH 1907/2006 Annex VII.

Aminoacylase-mediated hydrolysis of N-acetyl-L-phenylalanine is monitored by LC-MS/MS with isotope-labeled phenylalanine as internal standard, and the assay is used to rank enantioselectivity of enzyme variants under defined catalytic conditions. The addition ratio in assay plates is 1.5–5.0 mM final concentration in 50 mM Tris-HCl buffer, pH 7.5, containing 0.1 mM CoCl₂ or ZnCl₂ as metal cofactor. Reactions are incubated at 25–37 °C for 15–60 min, quenched with 6% trichloroacetic acid, and analyzed by ninhydrin colorimetry or LC-MS/MS with isotope-labeled L-phenylalanine internal standard. Automated liquid handlers with dispensing precision of ±0.1 µL transfer substrate stocks from 100 mM DMSO or water reservoirs into 96-well or 384-well plates; stock solutions are stored at −20 °C under argon and shielded from repeated freeze–thaw because the acetyl group is susceptible to slow hydrolytic cleavage above pH 8.0. Terminal product types include aminoacylase activity assay kits, enantioselective enzyme screening panels, and chiral amino acid production screening modules for biocatalysis development. Standards include ISO 13485:2016 for assay component quality management, CLSI EP17-A2 for detection capability evaluation, and EU 2017/746 where the assay is supplied as a diagnosis-related component.

Chiral pool conversion to phenylalanine-derived oxazolidinones

N-Acetyl-L-phenylalanine functions as a chiral pool substrate in the preparation of phenylalanine-derived oxazolidinone auxiliaries and chiral amino alcohols for asymmetric small-molecule synthesis. The sequence begins with esterification in anhydrous methanol under thionyl chloride catalysis at 0.2–0.3 equivalents, followed by reduction with sodium borohydride–iodine or borane dimethyl sulfide at 1.2–1.5 equivalents relative to 1.0 molar equivalent of the acetylated amino acid. The reduction is exothermic and is controlled in jacketed Hastelloy or glass-lined vessels with internal temperature maintained at −5 °C to 5 °C during reagent addition; residual water must be below 0.1% to avoid hydrogen generation and incomplete ester formation. Cyclization to the oxazolidinone is carried out with triphosgene or phosgene in toluene at 0–10 °C and subsequent aqueous workup at 10–15 °C. Production-scale batches up to 500 L are transferred through in-line FTIR monitoring of the carbonyl region to terminate reduction when the ester C=O band at approximately 1,730 cm⁻¹ disappears. Terminal product types include (S)-4-benzyl-2-oxazolidinone, N-acetyl-L-phenylalaninol, and downstream chiral auxiliaries for asymmetric aldol and enolate alkylation. Compliance is governed by ISO 9001:2015 for quality management, REACH 1907/2006 Annex VII for substance registration, and ICH Q11 when the auxiliary is used earlier than the defined regulatory starting material in a drug substance synthesis.

Comparability studies for imported amino acid derivative batches rely on a certified reference material with a chromatographic purity specification of ≥ 98.0% and a defined chiral purity with L-enantiomer ≥ 99.5%. The compound is employed in quality control laboratories as a reference standard for quantifying N-acetylated L-phenylalanine in raw material receipts and in peptide synthesis process samples. Calibration standards are prepared in 20% acetonitrile/water with 0.1% formic acid across a range of 0.5–50 µg/mL; the addition ratio in this context is not a formulation level but an empirical calibration range established during method validation according to ICH Q2(R1). Injection volume is 5 µL onto a C18 column with UV detection at 257 nm, corresponding to the aromatic absorbance of the phenyl group; quantitation is performed against a five-point external calibration with correlation coefficient ≥ 0.999. Reference material characterization includes quantitative NMR, LC-MS/MS, and chiral HPLC on a zwitterionic chiral column; batch-specific certificates report water content by Karl Fischer titration, residual solvents by headspace GC, and elemental impurities by ICP-MS according to USP 232. Reference materials are stored at −20 °C under argon and reconstituted immediately before use; repeated solution freeze–thaw is avoided because degradation peaks may appear in the chromatogram. Terminal product types include certified reference materials, impurity markers for Ac-Phe-OH, and system suitability standards for chromatographic identity testing. Standards include ISO/IEC 17025:2017 for reference material characterization, USP 621 for chromatographic system suitability, USP 232 for elemental impurities, and ICH Q2(R1) for method validation.