Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers (high molecular weight compounds) and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. Charged molecular chains, commonly present in soft matter systems, play a fundamental role in determining structure, stability and the interactions of various molecular assemblies. Theoretical approaches to describe their statistical properties differ profoundly from those of their electrically neutral counterparts, while technological and industrial fields exploit their unique properties. Many biological molecules are polyelectrolytes. For instance, polypeptides, glycosaminoglycans, and DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
Polyanions in the context of Metachromasia
Metachromasia (var. metachromasy) is a characteristic change in the color of staining carried out in biological tissues, exhibited by certain dyes when they bind to particular substances present in these tissues, called chromotropes. For example, toluidine blue becomes dark blue (with a colour range from blue-red dependent on glycosaminoglycan content) when bound to cartilage. Other widely used metachromatic stains include the family of Romanowsky stains that also contain thiazine dyes: the white cell nucleus stains purple, basophil granules intense magenta, whilst the cytoplasm (of mononuclear cells) stains blue, which is called the Romanowsky effect. The absence of color change in staining is named orthochromasia.
The underlying mechanism for metachromasia requires the presence of polyanions within the tissue. When these tissues are stained with a concentrated basic dye solution, such as toluidine blue, the bound dye molecules are close enough to form dimeric and polymeric aggregates. The light absorption spectra of these stacked dye aggregates differ from those of the individual monomeric dye molecules. Cell and tissue structures that have high concentrations of ionized sulfate and phosphate groups—such as the ground substance of cartilage, heparin-containing granules of mast cells, and rough endoplasmic reticulum of plasma cells—exhibit metachromasia. This depends on the charge density of the negative sulfate and carboxylate anions in the glycosaminoglycan (GAG). The GAG polyanion stabilizes the stacked, positively charged dye molecules, resulting in a spectral shift as the conjugated double bond π-orbitals of adjacent dye molecules overlap. The greater the degree of stacking, the greater the metachromatic shift. Thus, hyaluronic acid, lacking sulphate groups and with only moderate charge density, causes slight metachromasia; chondroitin sulfate, with an additional sulfate residue per GAG saccharide dimer, is an effective metachromatic substrate, whilst heparin, with further N-sulfation, is strongly metachromatic. Therefore, toluidine blue will appear purple to red when it stains these components.
View the full Wikipedia page for Metachromasia