Polyanion

A polyanion is a molecule or polymer that carries multiple negative charges along its structure. These charges arise from deprotonated acidic groups such as carboxylates, phosphates, or sulfates, or from stable anionic moieties like sulfonates. Polyanions can be macromolecules, such as polyelectrolytes, or smaller inorganic/organic species with several negative sites. In water, the high charge density promotes electrostatic repulsion along the chain, influences conformation, and drives strong interactions with counterions and cations. The degree of ionization is typically dependent on pH and ionic strength.

Biological and chemical examples are widespread. DNA and RNA are quintessential polyanions due to their phosphate backbone. Other natural polyanions include heparin and alginate, which carry sulfate or carboxylate groups. Synthetic polyanions include poly(styrene sulfonate) and poly(acrylic acid) (in its deprotonated form), commonly used to impart negative charge to materials or to stabilize dispersions.

Applications and implications. Polyanions readily form complexes with polycations, enabling layer-by-layer assembly, multilayer films, and the creation of polyelectrolyte complexes used in drug and gene delivery. They play roles in water treatment and industrial processing as dispersants, scale inhibitors, or thickening agents. In biological systems, their negative charge influences interactions with positively charged proteins and metal ions and affects processes such as binding, condensation, and transport.

Terminology and properties. Polyanions contrast with polycations, which carry multiple positive charges. The properties of polyanions—charge density, molecular weight, and counterion atmosphere—strongly influence solubility, stability, and complexation behavior.