What stain demonstrates sulfated (only) acid mucopolysaccharides?

• A. Alcian Blue 1.0
• B. Alcian Blue 2.5
• C. Colloidal Iron
• D. PAS

Answer: (A)

Alcian Blue binds negatively charged acidic mucopolysaccharides because it is a cationic dye that sticks to anionic groups in mucus. The key is the pH. At very acidic conditions (pH around 1.0), the carboxyl groups of mucopolysaccharides become protonated and lose their negative charge, so they don’t bind the dye. Only sulfated groups remain negatively charged, so Alcian Blue at pH 1.0 stains sulfated acid mucopolysaccharides specifically. This allows you to distinguish sulfated mucopolysaccharides from those that are carboxylated but not sulfated.

If you used Alcian Blue at a higher pH (around 2.5), both sulfated and carboxylated mucopolysaccharides would carry negative charges and be stained, broadening what’s detected. Other stains like Colloidal Iron stain acidic mucopolysaccharides in general but don’t selectively target sulfated versus carboxylated, and PAS stains neutral mucopolysaccharides rather than acidic ones. So for sulfated mucopolysaccharides only, Alcian Blue at pH 1.0 is the correct choice.