Malachite green

Malachite green
Names
IUPAC name
4-{[4-(dimethylamino)phenyl](phenyl)methylidene}-N,N-dimethylcyclohexa-2,5-dien-1-iminium chloride
Other names
Aniline green; Basic green 4; Diamond green B; Victoria green B
Identifiers
569-64-2 YesY
3D model (Jmol) Interactive image
ChEMBL ChEMBL186357 YesY
ChemSpider 10820 YesY
ECHA InfoCard 100.008.476
PubChem 11294
UNII 12058M7ORO YesY
Properties
C23H25ClN2 (chloride)
Molar mass 364.911 g/mol (chloride)
Pharmacology
QP53AX16 (WHO)
Hazards
Main hazards Moderately toxic, Extreme irritant
T Xi
Lethal dose or concentration (LD, LC):
80mg/kg (oral, mouse)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY verify (what is YesYN ?)
Infobox references

Malachite green is an organic compound that is used as a dyestuff and controversially as an antimicrobial in aquaculture. Malachite green is traditionally used as a dye for materials such as silk, leather, and paper. Although called malachite green, this dye is not prepared from 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 anions have 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−1 cm−1).

Malachite green (second transition) (pH indicator)
below pH 11.5 above pH 13.2
11.5 ⇌ 13.2
Malachite green (first transition) (pH indicator)
below pH 0.2 above pH 1.8
0.2 ⇌ 1.8

Malachite green is prepared by the condensation of benzaldehyde 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 of triphenylmethane, 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 alcohol derivative of MG is derived from LMG by replacement of the unique C–H by C–OH.

Hydrolysis of MG gives an alcohol:[1]

[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 leuco and alcohol derivatives are not. This difference arises because only the cationic form has extended pi-delocalization, which allows the molecule to absorb visible light.

Preparation

The leuco form of malachite green was first prepared by Hermann Fischer in 1877 by condensing benzaldehyde and dimethylaniline in the molecular ratio 1:2 in the presence of sulfuric acid.[2]

Uses

Malachite green is traditionally used as a dye. Millions of kilograms of MG and related triarylmethane dyes are produced annually for this purpose.[3]

MG is active against the oomycete Saprolegnia, which infects fish eggs in commercial aquaculture, MG has been used to treat Saprolegnia and is used as an antibacterial.[4] It is a very popular treatment against Ichthyophthirius multifiliis 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 money will find that on upon washing the hands, a green stain on the skin that lasts for several days will result.

Niche uses

A preparation of Bacillus subtilis showing 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. Hemoglobin catalyzes 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.[5]

Regulation

In 1992, Canadian authorities determined that eating fish contaminated with malachite green posed a significant health risk.[6] Malachite green was classified a Class II Health Hazard. Due to its low manufacturing cost, malachite green is still used in certain countries with less restrictive laws for nonaquaculture purposes. In 2005, analysts in Hong Kong found traces of malachite green in eels and fish imported from China and Taiwan. In 2006, the United States Food and Drug Administration (FDA) detected malachite green in seafood imported from China, among others, where the substance is also banned for use in aquaculture. In June 2007, the FDA blocked the importation of several varieties of seafood due to continued malachite green contamination.[7] The substance has been banned in the United States since 1983 in food-related applications. It is banned in the UK, also.[8] Animals metabolize malachite green to its leuco form. Being lipophillic (the leuco form has a log P of 5.70) The metabolite is retained in catfish muscle longer (HL = 10 days) than is the parent molecule (HL = 2.8 days).

Toxicity

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. Leucomalachite green causes an "increase in the number and severity of changes". As leucomalachite 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 leucomalachite 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 and liver tumor was not established.[9]

References

  1. ↑ Adina Raducan, Alexandra Olteanu, Mihaela Puiu, Dumitru Oancea "Influence of surfactants on the fading of malachite green" Central European Journal of Chemistry, 2008, Volume 6, pp. 1895–1066 (Print) 1644–3624 (Online). doi:10.2478/s11532-007-0066-0
  2. ↑ Dr. M Vishwanathan. Principles of organic chemistry. Jai Sai Publications. pp. 2/37.
  3. ↑ Thomas Gessner and Udo Mayer "Triarylmethane and Diarylmethane Dyes" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim.doi:10.1002/14356007.a27_179
  4. ↑ Srivastava, S; Sinha, R; Roy, D (2004). "Toxicological effects of malachite green". Aquatic Toxicology. 66 (3): 319–29. doi:10.1016/j.aquatox.2003.09.008. PMID 15129773.
  5. ↑ Protocol 2.18 Leucomalachite Green Presumptive Test for Blood, National Forensic Science Technology Center, July 8, 2010, Retrieved on July 8, 2010.
  6. ↑ Wendy C. Andersen, Sherri B. Turnipseed, and José E. Roybal "Quantitative and Confirmatory Analyses of Malachite Green and Leucomalachite Green Residues in Fish and Shrimp" J. Agric. Food Chem. 2006, volume 54, pp. 4517–4523.doi:10.1021/jf0532258 and references therein
  7. ↑ Chinese fish crisis shows seafood safety challenges, USA Today, 7/1/2007
  8. ↑ Veterinary Residues Committee. Annual Report on Surveillance for Veterinary Residues in Food in the UK for 2001, 2002, and 2003.
  9. ↑ Culp, S J; Beland, FA; Heflich, R H; et al. (2002). "Mutagenicity and carcinogenicity in relation to DNA adduct formation in rats fed leucomalachite green". Mutation Research. 506–507: 55–63. doi:10.1016/S0027-5107(02)00152-5. PMID 12351145.
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