L-Tyrosine ethyl ester is supplied in commerce primarily as the hydrochloride salt, registry number 4089-07-6, with the free base assigned 949-67-7. The free base has the molecular formula C11H15NO3 and a molecular weight of 209.24 g/mol; the hydrochloride salt adds one hydrochloride equivalent, giving C11H16ClNO3 and a molecular weight of 245.70 g/mol. No uniform international model number governs the product. Commercial lot designations typically encode the salt form, enantiomeric purity grade, and residual solvent profile, with a dried-basis HPLC assay of 98.0–101.0% and a chiral HPLC L-enantiomer area of at least 99.0%.

Chemically, the substance is ethyl (2S)-2-amino-3-(4-hydroxyphenyl)propanoate hydrochloride. The synthetic route generally proceeds by acid-catalysed esterification of L-tyrosine with ethanol, followed by crystallisation as the hydrochloride salt. The resulting material is a white to off-white crystalline powder. Because the C-terminus is blocked as an ethyl ester, the hydrochloride salt does not adopt the same zwitterionic lattice as L-tyrosine, and its handling characteristics in aqueous and solid-dosage manufacturing differ accordingly.

What Limits the Utility of the Free Base in Aqueous Formulation?

The free base lacks the free carboxylic acid that restricts the aqueous solubility of L-tyrosine, but it retains a primary amine that can deprotonate and reduce polarity as pH rises. In aqueous systems without a counterion, the free base can exhibit inconsistent wetting and dissolution. The hydrochloride salt avoids this by maintaining the amine in a protonated state. Supplier documentation frequently specifies that a 5% aqueous solution of the hydrochloride salt presents a pH of 3.0–4.5, although headspace carbon dioxide and water quality can shift the value. This acidic pH must be accounted for when the ester is preblended with acid-sensitive tablet coatings, enteric polymers, or alkaline buffers.

Published pKa values for the free base and hydrochloride salt in aqueous media are not consistently reported across supplier dossiers. The aromatic hydroxyl group and the α-amino group are expected to influence ionization, but formulation decisions should be based on measured pH-solubility profiles rather than values inferred from L-tyrosine. The free base is therefore reserved for non-aqueous or lipid-based matrices, while the hydrochloride salt remains the standard for aqueous or hydroalcoholic processing.

Hydrochloride Salt Release Specification and Compendial Method Alignment

Release testing for the hydrochloride salt ordinarily follows a supplier-validated HPLC method because no current USP-NF or Ph. Eur. monograph is assigned to the substance. Qualification is anchored to reference standards of known enantiomeric composition. The absence of a compendial monograph means that method transfer requires verification of column selectivity, mobile phase pH, and detector linearity. Typical acceptance criteria are provided in the following table.

These values are representative release criteria, not regulatory specifications. Because the substance can be used as an analytical reference material or as a building block, individual lots may require application-specific limits for endotoxin, microbial enumeration, or particle-size distribution.

In solid oral dosage manufacturing, the hydrochloride salt is usually screened through a 600 µm mesh before blending. As with other crystalline amino acid ester hydrochlorides, moisture content above 0.5% can promote agglomeration in low-shear tumble blending and adhesion in dosator-type capsule fillers. Where relative humidity exceeds 60%, pre-drying in a forced-air oven at 40 °C until Karl Fischer moisture returns below 0.5% is a conservative processing boundary. The powder's segregation tendency in bin blenders is controlled by ordered mixing with microcrystalline cellulose because the product's bulk density and particle-size distribution can differ from common excipients by more than 2:1. If direct compression is required, an instrumented single-station tablet press is used to generate a compaction profile; published data for this specific ester are limited, so scale-up should be verified on a rotary press with precompression capability.

When Ethyl Ester Hydrolysis Must Be Controlled in Aqueous Processing

The ethyl ester group is susceptible to both acid- and base-catalysed hydrolysis, generating ethanol and L-tyrosine. In aqueous solutions at pH 5.0–7.0, hydrolysis is slow enough for short-duration manufacturing hold times, but alkaline conditions accelerate the reaction. Aqueous granulation with alkaline buffering agents or prolonged exposure to hot water above 60 °C should be avoided unless solution stability is verified. The hydrochloride salt contributes acidic protons, which can lower local pH and reduce hydrolysis rate, but the same acidity can degrade acid-labile actives if the ester is preblended. Metal surfaces that catalyse ester hydrolysis in the presence of chloride should be limited to stainless steel or polymer-lined equipment. The generation of ethanol during hydrolysis is also a practical concern for drying and residual solvent limits under USP <467>.

At pH 3.0, the protonated amine reduces nucleophilic attack on the ester carbonyl, but the acid-catalysed pathway still operates at very low pH. At pH 8.0, hydroxide-catalysed hydrolysis dominates. The minimum hydrolysis rate is therefore not at the isoelectric point of L-tyrosine, because the ester is not zwitterionic. Stability screening should cover pH 2.0, 4.0, and 7.4 buffers with osmolarity adjusted by sodium chloride. Published data for this specific ester in these conditions is limited; therefore, the pH-stability profile should be generated internally before committing to a liquid formulation. In forced degradation studies, 0.1 M hydrochloric acid, 0.1 M sodium hydroxide, and 3% hydrogen peroxide are used to identify hydrolysis and oxidative degradation products. The appearance of L-tyrosine in HPLC chromatograms is the primary hydrolysis indicator. Oxidative degradation may produce quinone-like intermediates from the phenolic ring; these are usually not retained on C18 columns under acidic conditions and require mass detection for characterization.

Combining the hydrochloride salt with strong bases, anhydrous alkoxides, or strong oxidizing agents is incompatible. The primary amine can undergo Maillard reactions with reducing sugars during roller compaction or long blending at elevated temperature. Nucleophilic amines are not required to stabilize the ester; in solution, amine-based additives may accelerate ester aminolysis.

Structural and Physicochemical Differentiation from L-Tyrosine, Methyl Ester, and N-Acetyl Derivatives

Differentiation from other tyrosine derivatives is structural rather than merely particle-size based. L-Tyrosine base has a free carboxylic acid and a free amine, producing a zwitterionic lattice with low aqueous solubility. Esterification removes the carboxylate anion and changes the dissolution mechanism. The methyl ester has a smaller alkyl group and generally hydrolyses more rapidly under equivalent esterase exposure, whereas the ethyl ester provides a modest prolongation of the intact ester residence time in in vitro incubations. N-Acetyl-L-tyrosine is modified on the α-amino group rather than the carboxy terminus and therefore follows a different hydrolysis pathway. L-DOPA is not a tyrosine ester and contains a catechol group, which introduces oxidation and chelation reactions that are absent from the monophenolic tyrosine ethyl ester.

Additional ester analogues include the benzyl ester and tert-butyl ester, which are used almost exclusively in peptide synthesis. The benzyl ester is removed by hydrogenolysis, and the tert-butyl ester is removed by acidolysis. These derivatives are not appropriate for nutritional formulations because of protecting-group residues and non-food reagents. In contrast, the ethyl ester hydrochloride is available in dietary supplement grades only when the supplier certifies residual solvent and heavy metal profiles alongside assay and chiral purity.

In HPLC method qualification, the hydrochloride salt is commonly prepared as a stock solution in 0.1 M hydrochloric acid at 1.0 mg/mL. The detector wavelength is selected at 274 nm to exploit the phenolic chromophore. Mobile phase pH is maintained in the range of 2.5–4.0 to suppress silanol interactions on C18 columns. For chiral purity, a polysaccharide-based chiral stationary phase is used with a normal-phase or polar organic mobile phase; published data for this specific configuration is limited, so method transfer requires evaluating column lot-to-lot variability and modifier concentration.

The principal impurities observed in release testing include unreacted L-tyrosine, the free base, the methyl ester if methanol is used in the synthetic route, and oxidation products of the phenolic ring. HPLC methods generally resolve L-tyrosine at shorter retention time and the ethyl ester at longer retention time under acidic reversed-phase conditions. The L-enantiomer area is controlled to ensure that no D-tyrosine ethyl ester is present. A typical acceptance criterion of ≥99.0% L-enantiomer is consistent with chiral purity expectations for dietary supplement ingredients. Because the product is monophenolic rather than catecholic, it does not show the rapid aerial oxidation associated with L-DOPA under alkaline conditions.

Formulation use is concentrated in dietary supplement capsules, tablets, and research applications where a water-soluble tyrosine derivative with a blocked carboxylic acid is required. In peptide synthesis, the ethyl ester serves as a C-protected L-tyrosine building block; the amino group can be coupled with standard carbodiimide or mixed-anhydride methods, followed by saponification of the ethyl ester under mild alkaline conditions. The hydrochloride salt is not typically used directly in anhydrous coupling because the HCl must be neutralized first. In powder blends, the material is often added at a level of 0.5–5.0 wt% when used as a component of a formulated matrix, but the exact use level is application-specific and must be confirmed by stability testing.

Storage should be in tightly closed containers at controlled room temperature 20–25 °C, protected from direct light and moisture. Desiccated storage is advised once the container is opened. Batches should not be stored in aluminium packaging without a protective liner because the chloride salt can promote pitting corrosion under high humidity. The material is not classified as dangerous goods under IATA or IMDG for transport; however, the supplier's safety data sheet should be reviewed for local requirements.

Regulatory acceptance is jurisdiction-dependent. The substance is not assigned a compendial monograph in the USP-NF or Ph. Eur.. For dietary supplement finished products, verification under 21 CFR Part 111 and applicable new dietary ingredient requirements is the responsibility of the finished-product manufacturer. REACH registration status should be confirmed with the specific supplier for the 949-67-7 or 4089-07-6 inventory.