Salmon‑α‑Gal 

Salmon‑α‑Gal (CAS 198402‑61‑8), systematically 6‑chloro‑3‑indolyl α‑D‑galactopyranoside, is a synthetic α‑D‑galactosidase‑specific chromogenic substrate in which a 6‑chloro‑3‑indoxyl aglycone is linked via an α‑glycosidic bond to D‑galactose. When hydrolyzed by α‑D‑galactosidase (EC 3.2.1.22) or α‑galactoside‑cleaving enzymes, the indoxyl moiety is released and undergoes aerial oxidation to form an insoluble salmon‑colored indigo‑type precipitate, enabling clear visual detection of enzyme activity. This substrate is widely used in selective agar media to screen and identify α‑galactosidase‑positive microorganisms, and in enzyme‑kinetic assays for recombinant or purified α‑D‑galactosidases. The salmon‑pink dye provides good contrast against background pigmentation, especially in mixed‑culture or high‑density platings, improving sensitivity and ease of readout. The compound is compatible with standard microbiological media and biochemical buffers, and is supplied as a high‑purity, research‑use‑only reagent optimized for chromogenic solid‑medium and histochemical applications.

Appearance

• Off‑white to pale yellow crystalline or lyophilized powder.
• Fine, free‑flowing texture suitable for accurate weighing and dissolution in aqueous or mixed aqueous‑organic solvents (often used in DMF or methanol stock solutions for agar‑media incorporation).

Source / synthesis

• Synthetically produced by coupling 6‑chloro‑3‑indoxyl with α‑D‑galactose under glycosylation conditions to form the α‑D‑galactopyranoside derivative.
• Commercially manufactured by specialty enzyme‑substrate and carbohydrate‑chemistry suppliers (e.g., DC Fine Chemicals, Sigma‑Aldrich, DiscoFine Chemicals, GlpBio) as a high‑purity, research‑grade reagent.

Molecular weight and structure

• Molecular formula: C14H16ClNO6\text{C}_{14}\text{H}_{16}\text{ClNO}_{6}C14​H16​ClNO6​.
• Molecular weight: 329.73 g/mol.
• Structure: 6‑chloro‑1H‑indol‑3‑yl α‑D‑galactopyranoside, with an α‑glycosidic linkage between the indole C3‑O and the D‑galactose ring in the (2R,3R,4S,5R,6R) configuration.

Sugar specificity

• Specifically hydrolyzed by α‑D‑galactosidases (EC 3.2.1.22) and other α‑galactoside‑cleaving enzymes.
• Shows minimal cross‑reactivity with β‑glycosidases and other α‑glycosidases (e.g., α‑glucosidase, α‑fucosidase) under optimized assay conditions, providing high selectivity for α‑D‑galactosidases.

Biological activity

• Acts as a chromogenic reporter for α‑D‑galactosidase‑mediated hydrolysis of terminal α‑D‑galactosyl linkages in glycoproteins, glycolipids, and oligosaccharides.
• Enzymatic cleavage yields 6‑chloro‑3‑indoxyl, which oxidizes and dimerizes into an insoluble salmon‑colored dye, enabling sensitive detection in bacterial colonies, enzyme assays, and histochemical preparations.

Purity and microbial contamination

• Typically supplied at ≥97–98% purity as determined by HPLC or similar analytical methods.
• Certificates of analysis from major suppliers indicate acceptable levels of residual solvents, heavy metals, and non‑sterile, low‑microbial‑contamination status suitable for microbiological and biochemical use.

Identity and quality control

• Identity confirmed by molecular formula, molecular weight, NMR, HPLC, and spectral data, with reference to standard indolyl‑α‑D‑galactopyranoside profiles.
• Quality‑control procedures include checks for appearance, solubility, spectral characteristics, and performance in a defined α‑galactosidase activity assay system.

Shelf life and storage

• Shelf life is generally 12–24 months when stored as a dry powder at –20 °C in a tightly sealed container, protected from moisture and light.
• Aqueous or stock solutions should be prepared immediately before use or stored frozen for short periods to minimize degradation and nonspecific oxidation of the indoxyl moiety.

Application

• Screening and phenotypic differentiation of α‑D‑galactosidase‑producing microorganisms (e.g., yeasts and some bacteria) on selective agar media.
• Enzyme‑activity assays for recombinant or purified α‑D‑galactosidases in solution or immobilized formats.
• Histological or cytochemical localization of α‑D‑galactosidase activity in tissue sections and cell preparations.

Key characteristics

• High sensitivity and low background due to formation of an insoluble salmon‑colored dye upon α‑D‑galactosidase hydrolysis.
• Defined α‑D‑galactopyranoside linkage providing selectivity for α‑D‑galactosidases over other glycosidases.
• Crystalline, stable powder with a known molecular formula and weight, suitable for quantitative enzyme assays and standardized media formulations.

Citations

• DC Fine Chemicals – 6‑Chloro‑3‑indolyl‑α‑D‑galactopyranoside (Salmon‑α‑Gal) product page: https://www.dcfinechemicals.com/en/products/carbohydrates/6-chloro-3-indolyl–d-galactopyranoside-detail/
• ChemicalBook – 6‑Chloro‑3‑indolyl‑α‑D‑galactopyranoside entry (CAS 198402‑61‑8, molecular formula and MW): https://www.chemicalbook.com/ChemicalProductProperty_IN_CB4198505.htm
• GlpBio – 6‑Chloro‑3‑indolyl‑α‑D‑galactopyranoside product data sheet: https://file.glpbio.com/quotepdf/product.php?token=6_FQH81DUaT1HD6xGg5uFiObymbNi4zx3rDcscQFAXpkWKKdpyvhyW5R-7KPora6P3hTebhAuxqQ
• Sigma‑Aldrich – 6‑Chloro‑3‑indolyl α‑D‑galactopyranoside (CAS 198402‑61‑8, ≥98% HPLC) product page: https://www.sigmaaldrich.com/US/en/product/sigma/96317
• DiscoFine Chemicals – 6‑Chloro‑3‑Indolyl α‑D‑Galactopyranoside (Salmon‑α‑Gal) page: https://discofinechem.com/products/6-chloro-3-indolyl-%CE%B1-d-galactopyranoside-salmon-%CE%B1-gal/
• IND‑FINE Chemical Company – 6‑Chloro‑3‑indolyl‑α‑D‑galactopyranoside entry: https://indofinechemical.com/details.aspx?Sku=BIO-894
• Biorbyt – 6‑Chloro‑3‑indolyl‑α‑D‑galactopyranoside product overview (glycobiology‑research‑applications context): https://www.biorbyt.com/6-chloro-3-indolyl-d-galacto-orb2941907.html
• α‑Gal‑epitope and α‑galactosidase biology review (background on α‑galactosidase‑related systems, e.g., α‑Gal Syndrome): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5704288/
• General enzyme‑substrate catalog (chromogenic α‑galactosidase substrates overview): https://www.medchemexpress.com/ (search for “α‑galactosidase substrate” or “Salmon‑α‑Gal”)
• Standard α‑D‑glucosidase/α‑galactosidase‑substrate datasets (for enzyme‑specificity and assay‑format reference): https://pubs.acs.org/doi/10.1021/cb200267w (alkaloid‑biosynthetic‑glucosidase‑substrate‑specificity paper, to illustrate α‑glycosidase substrate concepts)