Question

In histology there are commonly used acidic (= negatively charged) dyes, like eosin, erythrosine, orange G, and basic (= positively charged) dyes, like haematoxylin, toluidine and methylene blue, azure II. Acidic dyes stain positively charged structures, like some cytoplasmic proteins, and basic dyes stain negatively charged molecules, like RNA and DNA. Structures stained with acidic dyes usually appear pink, with basic - blue.

That's what commonly offered in histology course. I could not find any information on what type of bonding these dyes use (is there a covalent bonding or non-covalent interaction?). Nor do I know why cell structures stain that way. Can you explain these properties of the dyes?

Answer 1

In histology, basic dyes stain negatively charged cellular components whereas acidic dyes repel from the cells, staining the background. These interactions are primarily based on non-covalent electrostatic attractions or repulsions, not covalent bonding. Staining techniques are essential for distinguishing cellular structures under a microscope.

Step-by-step explanation:

In histology, the bonding between dyes and cellular structures involves primarily non-covalent interactions. Cells have negatively charged cell walls due to the presence of molecules like phospholipids and proteins with negatively charged amino acid side chains. Basic dyes, which possess positively charged chromophores, are attracted to these negatively charged structures within the cells, leading to a phenomenon known as positive staining. Conversely, acidic dyes contain negatively charged chromophores that are repelled by the cell walls, resulting in negative staining where cells remain clear against a colored background.

For instance, the basic dye methylene blue will provide a positive stain to cellular components such as nuclei by binding to the negatively charged DNA. Eosin, an acidic dye, will stain positively charged components like some cytoplasmic proteins a pinkish color. In both cases, the primary interaction that facilitates staining is electrostatic attraction or repulsion, rather than covalent bonding.

Such staining techniques are crucial in allowing scientists to visualize and differentiate cellular structures under a microscope, enhancing details that would otherwise be invisible or indistinct. Dyes like haematoxylin and eosin are often used together in differential staining protocols, such as the widely used H&E stain, to provide contrast between different cellular components.