Acetyl-L-Carnitine (ALCAR): Chemical Profile & Research Applications

Acetyl-L-Carnitine (ALCAR, also written as acetylcarnitine or ALC) is the acetyl ester of the naturally occurring quaternary ammonium compound L-carnitine. The molecule features a trimethylammonium group, a hydroxyl-bearing carbon chain, and a carboxylate group — with the acetyl moiety esterified at the C3 hydroxyl position. ALCAR exists as a zwitterion at physiological pH.

• IUPAC Name: (R)-3-(Acetyloxy)-4-(trimethylammonio)butanoate
• Molecular Formula: C9H17NO4
• Molecular Weight: 203.24 Da (free base, inner salt)
• CAS Number: 3040-38-8
• CAS (Hydrochloride salt): 5080-50-2 (MW 239.70 Da)
• Exact Mass: 203.1158 Da (monoisotopic)
• Chirality: (R)-configuration at C3 — only the L-isomer is biologically relevant
• Appearance: White crystalline powder (hygroscopic)

The defining structural feature of ALCAR is the acetyl group (CH3CO-) attached via an ester bond to the 3-hydroxyl position of the L-carnitine backbone. This ester bond is hydrolytically labile under acidic or basic conditions, meaning ALCAR can degrade to free L-carnitine and acetic acid. The acetyl group adds 42.04 Da to the molecular weight relative to L-carnitine (203.24 - 161.20 = 42.04 Da, corresponding to C2H2O). In aqueous solution, the ester bond stability is pH-dependent — optimal stability occurs between pH 3 and pH 5.

• Acid hydrolysis: Accelerated below pH 2, producing L-carnitine + acetic acid
• Base hydrolysis: Rapid above pH 8, saponification of the ester bond
• Optimal stability range: pH 3.0–5.0 in aqueous solution
• Thermal sensitivity: Hydrolysis rate doubles approximately every 10°C
• Enzymatic hydrolysis: Carnitine acetyltransferases catalyze reversible acetyl transfer

The following table compares chemical properties only. No biological activity or health claims are made.

ALCAR is freely soluble in water (>500 mg/mL at 25°C) and shows good solubility in methanol and ethanol. It is practically insoluble in non-polar organic solvents such as chloroform and diethyl ether. The compound is hygroscopic and should be stored in tightly sealed containers. In aqueous solution, ALCAR behaves as an inner salt (zwitterion) with a melting point of approximately 195–198°C (with decomposition for the hydrochloride salt).

• Water solubility: >500 mg/mL (inner salt); >1000 mg/mL (HCl salt)
• Methanol solubility: ~200 mg/mL
• LogP: -3.24 (highly hydrophilic)
• pKa values: ~3.8 (carboxyl), ~14+ (quaternary amine — permanently charged)
• Melting point: 195–198°C (HCl salt, with decomposition)
• Optical rotation: [α]D20 = -27° to -31° (c = 1, H2O, HCl salt)
• UV absorption: Weak — no significant chromophore above 220 nm

Quantitative determination of ALCAR requires methods capable of distinguishing it from free L-carnitine (the primary hydrolysis product) and other acylcarnitine species. The lack of a strong UV chromophore above 220 nm makes direct UV detection challenging, favoring mass spectrometric or derivatization-based approaches.

• LC-MS/MS: HILIC or C18 column with ESI+ detection — m/z 204.1 → 85.0 (quantifier), 204.1 → 145.1 (qualifier). Gold standard for specificity and sensitivity
• RP-HPLC with UV: Requires ion-pair reagent (e.g., heptanesulfonic acid) or pre-column derivatization for adequate retention and detection at 210 nm
• HILIC-ELSD: Hydrophilic interaction chromatography with evaporative light scattering detection — suitable for purity assessment without derivatization
• Karl Fischer Titration: Water content determination for solid samples (specification typically ≤1.0%)
• Chiral HPLC: Crown ether or cyclodextrin columns to verify L-configuration and exclude D-isomer contamination
• Potentiometric Titration: Assay value determination (≥98.0% on dried basis for research grade)

Solid ALCAR (both inner salt and HCl salt) is relatively stable when stored under appropriate conditions. The primary degradation pathway is ester hydrolysis to L-carnitine and acetic acid, accelerated by moisture, elevated temperature, and extreme pH values. The hydrochloride salt form is generally preferred for long-term storage due to lower hygroscopicity compared to the inner salt.

• Solid form: Store at 2–8°C, desiccated, protected from light — stable ≥36 months
• Room temperature storage: Acceptable for HCl salt (15–25°C) if sealed and desiccated — stable ≥24 months
• Aqueous solutions: Prepare fresh or store at 2–8°C, pH 3–5, use within 7 days
• Avoid: pH >7 in solution (rapid ester hydrolysis), high humidity, temperatures >30°C
• ICH stability: Follows ICH Q1A(R2) guidelines — accelerated stability at 40°C/75% RH for 6 months
• Degradation marker: Free L-carnitine content by LC-MS (should be <2% at release)

ALCAR has an acetyl group (CH3CO-) esterified at the 3-hydroxyl position of L-carnitine, adding 42.04 Da to the molecular weight (203.24 vs 161.20 Da). This ester bond changes the compound's chromatographic behavior, lipophilicity (slightly higher logP), and susceptibility to enzymatic processing. Analytically, they are readily distinguished by LC-MS: ALCAR appears at m/z 204.1 and L-carnitine at m/z 162.1.

Acetyl-L-Carnitine Arginate (also known as ALCAR Arginate dihydrochloride) is a salt form combining ALCAR with L-arginine, resulting in an estimated MW of ~357 Da. It retains the acetylcarnitine core structure but includes the guanidinium-containing amino acid arginine as a counter-ion or conjugate. This form was developed as a proprietary derivative and differs from standard ALCAR in its salt composition and molecular weight.

The hydrochloride salt (CAS 5080-50-2, MW 239.70 Da) is generally preferred for research applications because it is less hygroscopic, more stable in solid form, and easier to weigh accurately. The inner salt (zwitterion, CAS 3040-38-8, MW 203.24 Da) is more hygroscopic but avoids introducing chloride ions. For molar concentration calculations, remember to use the correct MW for the form you are using.

The primary degradation product is free L-carnitine from ester hydrolysis. LC-MS analysis showing increasing L-carnitine (m/z 162.1) relative to ALCAR (m/z 204.1) indicates degradation. A vinegar-like odor may also indicate acetic acid release from hydrolysis. For quantitative assessment, the ratio of free L-carnitine to total carnitine species should be <2% for fresh, high-quality material.

ALCAR lacks a significant UV chromophore above 220 nm because it contains no aromatic rings, conjugated double bonds, or other strongly absorbing functional groups. Detection at 210 nm is possible but suffers from poor selectivity and sensitivity, as many solvents and buffer components also absorb at this wavelength. LC-MS/MS with electrospray ionization is the preferred detection method, offering both high sensitivity (low ng/mL) and specificity through characteristic fragmentation patterns.

• Read more: Acetate vs Arginate Salt Forms in Peptides: Which Is Better? → https://www.chemverify.com/learn/acetate-vs-arginate-salt-peptides-comparison
• Read more: Peptide Aggregation: Why Peptides Clump and How to Prevent It → https://www.chemverify.com/learn/peptide-aggregation-clumping-prevention
• Read more: Peptide Degradation: Deamidation, Oxidation, and How to Prevent It → https://www.chemverify.com/learn/peptide-degradation-deamidation-oxidation-prevention
• Read more: Peptide Modifications: PEGylation, Lipidation, Cyclization, and D-Amino Acids → https://www.chemverify.com/learn/peptide-modifications-pegylation-lipidation-cyclization