L-Alanyl-L-Cystine is a dipeptide derivative formed by linking L-alanine and L-cystine through a peptide bond. Its structure incorporates an alanine residue, a cystine residue, and a disulfide bond, endowing it with distinctive chemical and biological properties. Below is a detailed introduction covering its properties, functional characteristics, and applications:

I. Basic Properties

Physical Properties

Appearance: Typically a white to off-white crystalline powder, odorless or with a faint characteristic smell.

Solubility: Moderately soluble in water (greater than free cystine but lower than small amino acids); readily soluble in dilute acids or bases, poorly soluble in ethanol, ether, and other organic solvents due to its polar peptide bond and hydrophilic amino acid residues.

Molecular Formula and Weight: C₈H₁₆N₂O₅S₂, with a molecular weight of 284.35 (formed by the condensation of L-alanine, C₃H₇NO₂, and L-cystine, C₆H₁₂N₂O₄S₂, with the loss of one molecule of water).

Melting Point: Decomposes at temperatures above \~200°C; the exact decomposition point may vary with purity. No defined melting point is observed.

Chemical Properties

Stability: Stable under dry, light-protected, neutral or mildly acidic conditions. The peptide bond is susceptible to hydrolysis under strong acidic or basic conditions, yielding L-alanine and L-cystine. The disulfide bond (-S-S-) can be reduced to thiol groups (-SH) by reducing agents (e.g., β-mercaptoethanol) or cleaved by oxidizing agents.

Optical Activity: Due to both residues being in the L-configuration, the molecule is optically active, typically with a positive specific rotation $[α]_D$, which varies depending on solvent and concentration.

Reactive Functional Groups:

Peptide bond: can undergo hydrolysis.

Amino (-NH₂) and carboxyl (-COOH) groups: retain typical amino acid reactivity (e.g., acylation, esterification).

Disulfide bond: can engage in redox reactions or cross-linking.

II. Functional Characteristics

Excellent Biocompatibility: Composed of naturally occurring L-amino acids, L-alanyl-L-cystine is highly compatible with biological metabolic systems. Once inside the body, it can be hydrolyzed by peptidases into alanine and cystine, both of which participate in protein synthesis and sulfur metabolism. It is considered safe under appropriate dosing.

Enhanced Stability Compared to Free Cystine: Unlike free L-cystine, which is prone to oxidation and has poor water solubility, this dipeptide form exhibits improved chemical stability and water solubility, making it better suited for liquid formulations.

Sulfur-Containing Functional Group: The disulfide bond within the molecule plays a vital role in redox balance regulation and serves as a sulfur source for the biosynthesis of glutathione (GSH) and other sulfur-containing bioactive molecules.

Sustained Release Characteristics: As a dipeptide, it must be hydrolyzed by peptidases before releasing the constituent amino acids, resulting in a slower absorption and metabolism compared to free amino acids—ideal for controlled nutritional delivery.

III. Applications

Medical and Nutritional Health

Nutritional Supplement: Used in special medical foods, post-operative nutritional formulas, or sports nutrition products to provide alanine (supports gluconeogenesis and blood glucose stability) and cystine (supplies sulfur and promotes protein synthesis). Especially suitable for populations requiring enhanced muscle metabolism or tissue repair.

Liver Protection Agent: Cystine metabolites participate in hepatic detoxification. The dipeptide form can offer sustained sulfur delivery, helping to maintain hepatic glutathione levels—used in hepatoprotective health products.

Antioxidant Supplements: Through its disulfide bond, it can neutralize free radicals and modulate cellular redox states. It is incorporated into antioxidant dietary supplements aimed at delaying aging or managing oxidative stress-related diseases (e.g., cardiovascular conditions).

Cosmetics and Skincare

Skin Repair Ingredient: By supplying a cystine precursor, it promotes keratin synthesis in the skin’s stratum corneum (rich in disulfide bonds), enhancing the skin’s barrier function. Its antioxidant properties also help mitigate UV or environmental oxidative damage—frequently included in repair creams and serums.

Hair Care Products: Cystine is a key component of hair, contributing to strength and elasticity. The dipeptide form is absorbable by hair follicles and aids in improving hair brittleness or breakage—commonly added to shampoos, conditioners, or hair growth serums.

Biochemistry and Research

Used as a model peptide compound to study dipeptide hydrolysis kinetics, membrane transport mechanisms, or enzyme-catalyzed reactions (e.g., peptidase specificity).

Serves as a culture medium additive providing complex nitrogen and sulfur sources to support microbial or cellular growth, particularly beneficial for strains with poor free cystine utilization efficiency.

IV. Precautions

Should be stored in a sealed, dry, light-protected environment, away from strong oxidizing or reducing agents to prevent disulfide cleavage or peptide hydrolysis.

When used in pharmaceutical or food applications, purity must be strictly controlled (to avoid racemization or contamination), and dosages must comply with relevant regulations.

L-Alanyl-L-Cystine, thanks to its excellent biocompatibility, stable physicochemical properties, and unique sulfur-containing structure, holds significant value in nutrition supplementation, medical support, and cosmetic applications. It is a peptide derivative that combines both nutritional and functional activity.