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Malachite Green (CAS: 569-64-2)
Melting point ℃
The crystallization of dark green with metallic luster
Malachite green is an organic compound that is used as a dyestuff and has emerged as a controversial agent inaquaculture. Malachite green is traditionally used as a dye for materials such as silk, leather, and paper. Although called malachite green, the compound is not related to the mineral malachite — the name just comes from the similarity of color.
Structures and properties
Malachite green is classified in the dyestuff industry as a triarylmethane dye and also using in pigment industry. Formally, Malachite green refers to the chloride salt [C6H5C(C6H4N(CH3)2)2]Cl, although the term Malachite green is used loosely and often just refers to the colored cation. The oxalate salt is also marketed. The chloride and oxalate anionshave no effect on the color. The intense green color of the cation results from a strong absorption band at 621 nm (extinction coefficient of 105 M−1cm−1).
Malachite green is prepared by the condensation ofbenzaldehyde and dimethylaniline to give leuco malachite green (LMG):
C6H5CHO + 2 C6H5N(CH3)2 → C6H5CH(C6H4N(CH3)2)2 + H2O
Second, this colorless leuco compound, a relative oftriphenylmethane, is oxidized to the cation that is MG:
C6H5CH(C6H4N(CH3)2)2 + HCl + 1/2 O2 → [C6H5C(C6H4N(CH3)2)2]Cl + H2O
A typical oxidizing agent is manganese dioxide.
On the left is leuco-Malachite Green (LMG) and on the right are the two equivalent resonance structures of the MG cation. The carbinol derivative of MG is derived from LMG by replacement of the unique C-H by C-OH.
Hydrolysis of MG gives the carbinol form:
[C6H5C(C6H4N(CH3)2)2]Cl + H2O → C6H5C(OH)(C6H4N(CH3)2)2 + HCl
This alcohol is important because it, not MG, traverses cell membranes. Once inside the cell, it is metabolized into LMG. Only the cation MG is deeply colored, whereas the LMG and carbinol derivatives are not. This difference arises because only the cationic form has extended pi-delocalization, which allows the molecule to absorb visible light.
Malachite green is traditionally used as a dye. Millions of kilograms of MG and related triarylmethane dyes are produced annually for this purpose.
MG is active against the oomycete Saprolegnia, which infects fish eggs in commercial aquaculture, and other fungi. Furthermore, MG is also used as a parasiticideand antibacterial. It is a very popular treatment against ichthyophthirius in freshwater aquaria. The principal metabolite, LMG, is found in fish treated with malachite green, and this finding is the basis of controversy and government regulation. See also Antimicrobials in aquaculture.
MG has frequently been used to catch thieves and pilferers. The bait, usually money, is sprinkled with the anhydrous powder. Anyone handling the contaminated bait will find that on washing the hands, the contact with water will provoke an indelible green stain on the skin lasting for several days.
A preparation of Bacillus subtilisshowing endospores stained with malachite green (vegetative cells are stained red)
Numerous niche applications exploit the intense color of MG. It is used as a biological stain for microscopic analysis of cell biology and tissue samples. In the Gimenez staining method, basic fuchsin stains bacteria red or magenta, and malachite green is used as a blue-green counterstain. Malachite green is also used in endospore staining since it can directly stain endospores within bacterial cells; here a safranin counterstain is often used. Malachite green can also be used as a saturable absorber in dye lasers, or as a pH indicator between pH 0.2–1.8. However this use is relatively rare. Leuco-malachite green (LMG) is used as a detection method for latent blood in forensic science. Hemoglobincatalyzes the reaction between LMG and hydrogen peroxide, converting the colorless LMG into malachite green. Therefore, the appearance of a green color indicates the presence of blood.
The LD50 (oral, mouse) is 80 mg/kg. Rats fed malachite green experience "a dose-related increase in liver DNA adducts" along with lung adenomas. Leuco-malachite green causes an "increase in the number and severity of changes". As leuco-malachite green is the primary metabolite of malachite green and is retained in fish muscle much longer, most intake of malachite green would be in the leuco form. During the experiment, rats were fed up to 543 ppm of leuco-malachite green, an extreme amount compared to the average 5 ppb discovered in fish. After a period of two years, an increase in lung adenomas in male rats was discovered but no incidences of liver tumors. Therefore it could be concluded that malachite green caused carcinogenic symptoms, but a direct link between malachite green andliver tumor was not established.
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