Eosin

Eosin is a fluorescent acidic / negative compound that binds to and forms salts with basic, or eosinophilic, compounds containing positive charges (such as proteins that are basic / positive due to the presence of amino acid residues such as arginine and lysine) and stains them dark red or pink as a result of the actions of bromine on fluorescein. In addition to staining proteins in the cytoplasm, it can be used to stain collagen and muscle fibers for examination under the microscope. Structures that stain readily with eosin are termed eosinophilic.

Etymology

The name Eosin comes from Eos, the Ancient Greek word for 'dawn' and the name of the Ancient Greek goddess of the dawn.

Variants

There are actually two very closely related compounds commonly referred to as eosin. Most often used is Eosin Y (also known as eosin Y ws, eosin yellowish, Acid Red 87, C.I. 45380, bromoeosine, bromofluoresceic acid, D&C Red No. 22); it has a very slightly yellowish cast. The other eosin compound is eosin B (eosin bluish, Acid Red 91, C.I. 45400, Saffrosine, Eosin Scarlet, or imperial red); it has a very faint bluish cast. The two dyes are interchangeable, and the use of one or the other is a matter of preference and tradition.