Hematein (US spelling) or haematein is an oxidized derivative of
haematoxylin, used in staining. Haematein should not be confused with haematin, which is a brown to black iron-containing pigment formed by decomposition of
haemoglobin. In the Colour Index (but nowhere else), haematein is called haematine.
Properties
Hematein exhibits indicator-like properties, being blue and less soluble in aqueous
alkaline
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
conditions, and red and more soluble in alcoholic
acidic conditions. Dissolved haematein slowly reacts with atmospheric
oxygen, yielding products that have not found applications.
Applications
In acidic solutions, complexes of hematein with metals (usually
aluminium or
iron, but also
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
,
zirconium and several others) are used as biological stains. Aluminium-haematein (haemalum) is the "routine" stain for cell nuclei in sections of human and other animal tissues. Metal-haematein stains are available also for objects other than nuclei, including
myelin sheaths of nerve fibres and various cytoplasmic
organelle
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
s. The color of the stained objects depends on the salt used. Aluminium-haematein complexes are usually blue, whereas ferric complexes are very dark blue or black.
Aluminium-haematein complexes (haemalum) bind to the chromatin of the
nuclei of cells. Although haemalum staining methods have been in use since the 1860s, the chemical identity of the substance or substances that bind the dye-metal complex is still not known with certainty. Some histochemical investigations clearly indicate that a cationic aluminium-haematein complex is attracted to the
phosphate anions of
DNA. Others implicate the
arginine
Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) and both the am ...
residues of nuclear
histones as the substrate of nuclear staining by haemalum.
Structures that stain with aluminium-hematein (haemalum) are often said to be
basophilic, but the staining mechanism is not as simple as for basic (cationic) dyes with smaller molecules. Truly basophilic structures are ones containing
nucleic acid
Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cl ...
s or other
polyanions such as glycosaminoglycans of extracellular matrix or acidic
glycoprotein
Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...
s in many types of mucus. As usually used, aluminium-hematein stains only nuclear chromatin and a few other materials such as keratohyalin granules and calcified deposits. Very dilute solutions of aluminium-haematein, used at pH 3.2 (higher than is usual for staining), contain a cationic dye-metal complex and will slowly stain nucleic acids. Haemalum solutions used for routine staining are more concentrated and more acidic (pH 2-2.5) and are able to stain nuclei after chemical or enzymatic extraction of DNA and RNA from the tissue.
References
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Staining dyes
Phenol dyes
Catechols
Tertiary alcohols
Cyclopentenes