I. Physicochemical Properties
Appearance and Solubility
Appearance: Typically exists as a white or off-white crystalline powder with a regular crystal structure. It shows hygroscopicity due to polar groups (e.g., amino and carboxyl groups) in the molecule.
Solubility: Highly soluble in polar solvents (e.g., water, methanol, ethanol), with solubility in water increasing with temperature. Sparingly soluble in nonpolar solvents (e.g., diethyl ether, chloroform). This is because polar amide bonds, carboxyl, and amino groups form hydrogen bonds with water, while the hydrophobic benzene ring in the benzyl group restricts solubility in nonpolar media.
Melting and Boiling Points
Melting Point: Approximately 155–160°C (with decomposition). Heating to the melting point causes decomposition rather than simple melting, related to the thermal stability of the amide bond and benzyloxycarbonyl group in the molecule.
Boiling Point: No definite boiling point data (decomposes before reaching the boiling point). The thermal decomposition mainly involves cleavage of the benzyloxycarbonyl group and degradation of the amino acid backbone.
Acid-Base Property and Dissociation Characteristics
Amphoteric Property: Contains carboxyl group (–COOH, acidic) and amino group (–NH₂, basic), belonging to an amphoteric compound. Its isoelectric point (pI) can be estimated by the dissociation constants of functional groups. The amino group protected by the benzyloxycarbonyl group has weakened dissociation ability, so the pI value is slightly lower than that of L-lysine (the pI of free L-lysine is about 9.74).
Dissociation Behavior: Under acidic conditions (low pH), the amino group is protonated and positively charged; under alkaline conditions (high pH), the carboxyl group is deprotonated and negatively charged, while the amino group in the benzyloxycarbonyl group is protected, with significantly reduced dissociation ability.
Optical Activity
It has optical activity due to the presence of a chiral center (α-carbon of lysine) and belongs to the L-configuration. Its specific rotation [α]ₙ²⁰ (solvent: water) is about +10° to +15° (specific values are related to purity).
II. Stability Study
Thermal Stability
Room Temperature Stability: Relatively stable under dry conditions at room temperature (20–25°C) and can be stored for a long time. However, at high temperatures (e.g., exceeding 80°C), the benzyloxycarbonyl (–Cbz) group may gradually hydrolyze or be deprotected, releasing the benzyl (C₆H₅CH₂–) and free amino groups, leading to a decrease in product purity.
Decomposition Temperature: When heated above 150°C, the amide bond (formed by the benzyloxycarbonyl and amino group) and carboxyl group in the molecule may undergo thermal decomposition, generating CO₂, amine compounds, and benzyl derivatives. Therefore, high-temperature treatments (such as baking and strong heating) should be avoided.
Acid-Base Stability
Acidic Conditions: In dilute acids (such as hydrochloric acid and acetic acid), the benzyloxycarbonyl group can undergo hydrolysis, gradually removing the protecting group to produce L-lysine and benzyl benzoate. The reaction rate increases with the increase of acid concentration and temperature. For example, refluxing in 6 mol/L hydrochloric acid can rapidly remove the Cbz protecting group.
Alkaline Conditions: In strong alkali (such as NaOH and KOH) solutions, the carboxyl group in the molecule is easily neutralized, while the benzyloxycarbonyl group is relatively stable under strong alkaline conditions. However, prolonged high-temperature and strong alkali treatment may lead to hydrolysis of the amide bond, releasing the amino group.
Oxidative and Photostability
Antioxidant Property: The benzene ring structure in the molecule has certain antioxidant properties, but the amino and carboxyl groups may be oxidized by strong oxidants (such as potassium permanganate and hydrogen peroxide), leading to structural damage.
Photostability: The benzene ring in the benzyl group is sensitive to ultraviolet light. Long-term exposure to light (especially ultraviolet light) may cause photolysis, leading to cleavage of the benzyloxycarbonyl group and generation of free amino groups, benzaldehyde, and other products. Therefore, it should be stored in the dark (such as in a brown reagent bottle).
Hydrolytic and Enzymatic Stability
Hydrolytic Stability: In neutral aqueous solutions, the hydrolysis rate of the amide bond is slow, but slow hydrolysis may occur during long-term storage (especially in high-temperature and high-humidity environments), producing L-lysine and benzyloxycarboxylic acid. Acidic or alkaline conditions can accelerate the hydrolysis reaction.
Enzymatic Hydrolysis: In the presence of proteases (such as trypsin), the peptide bond (amide bond) in the molecule may be catalyzed and hydrolyzed by enzymes, but the protective effect of the benzyloxycarbonyl group will reduce the enzymatic hydrolysis efficiency, making it more difficult to be decomposed by enzymes than free L-lysine.
Influence of Storage Conditions on Stability
The optimal storage conditions are dark, dry, and cool (0–4°C or sealed at room temperature), avoiding contact with acids, bases, and oxidants. In a humid environment, hygroscopicity will cause molecular aggregation or accelerate hydrolysis, so it is often necessary to store with a desiccant.
The physicochemical properties of N6-Cbz-L-lysine are jointly determined by its amino acid backbone and benzyloxycarbonyl protecting group, combining the water solubility of polar groups and the hydrophobicity of the benzene ring. Its stability is good under normal temperature, neutral, and dark conditions, but structural changes are prone to occur under high temperature, strong acid, strong alkali, light, or oxidants, especially the deprotection reaction of the benzyloxycarbonyl group. In practical applications (such as organic synthesis and storage of pharmaceutical intermediates), reaction conditions and storage environments should be controlled according to its characteristics to maintain product purity and activity.
