Thionine, also known as Lauth's violet, is the salt of a

heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, proper ...

. It was firstly synthesised by Charles Lauth. A variety of salts are known including the chloride and acetate, called respectively thionine chloride and thionine acetate. The dye is structurally related to

methylene blue Methylthioninium chloride, commonly called methylene blue, is a salt used as a dye and as a medication. As a medication, it is mainly used to treat methemoglobinemia. It has previously been used for treating cyanide poisoning and urinary trac ...

, which also features a phenothiazine core. The dye's name is frequently misspelled with omission of the e, and is not to be confused with the plant protein thionin. The -ine ending indicates that the compound is an

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

Dye properties and use

Thionine is a strongly staining metachromatic dye, which is widely used for biological staining. Thionine can also be used in place of Schiff reagent in quantitative

Feulgen stain Feulgen stain is a staining technique discovered by Robert Feulgen and used in histology to identify chromosomal material or DNA in cell specimens. It is darkly stained. It depends on acid hydrolysis of DNA, therefore fixating agents using st ...

ing of DNA. It can also be used to mediate electron transfer in microbial fuel cells. Thionine is a pH-dependent

redox indicator A redox indicator (also called an oxidation-reduction indicator) is an indicator which undergoes a definite color change at a specific electrode potential. The requirement for fast and reversible color change means that the oxidation-reduction Ch ...

with E⁰ = 0.06 at pH 7.0. Its reduced form, leuco-thionine, is colorless. When both the amines are dimethylated, the product tetramethyl thionine is the famous

, and the intermediates are Azure C (Monomethyl thionine),

Azure A Azure A is an organic compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbo ...

(when one of the amines is dimethylated and the other remains a primary amine), and Azure B (Trimethyl thionine). When

is "polychromed" by ripening (oxidized in solution or metabolized by fungal contamination, as originally noted in the thesis of Dr D L Romanowski in 1890s), it forms thionine and all the Azure intermediates.Marshall, PN (1978) Romanowsky-type stains in haematology. Histochemical Journal 10: 1-29.

Notes and references

{{Reflist Staining Phenothiazines