Crystal violet or gentian violet (also known as methyl violet 10B or hexamethyl paraaminobenzoyl chloride) is a triarylmethane dye used for histological staining and classification of Gram bacteria. Crystal violet has antibacterial, antifungal, and insect repellent properties and was previously important as a topical preservative. Although the dye is still listed by the World Health Organization, its medical use has been largely replaced by more modern drugs. Gentian violet was originally named for a mixture of methyl-p-nitroaniline dye (methyl violet), but is now often considered a synonym for crystal violet. The name refers to its color, just like the petals of some gentian flowers. It is not made from gentian or violet.
Figure 1. skeletal formula of a crystal violet minor tautomer.
Gentian violet, whose 1 ~ 2% solution is commonly known as purple potion, is a familiar external medicine. Gentian violet is a basic cationic dye. Because its cation can bind to the carboxyl group of bacterial proteins and affect its metabolism, it has a bacteriostatic effect. It can inhibit Gram-positive bacteria, especially staphylococcus and diphtheria, and also has a good antibacterial effect on Candida albicans. It has a strong bactericidal effect, is non-irritating to tissues, and has no toxicity or side effects. Topical treatment can treat wound and infection of skin and mucous membranes, ulcers, minor burns, herpes labialis, ulcerative pharyngitis, thrush, fungal vaginitis, vulvitis, etc. Applying 1 ~ 2% aqueous solution or alcohol solution to the skin of the affected area can prevent bacterial infection and local tissue fluid leakage. It can also combine with necrotic tissue to form a protective film, which has an astringent effect and can be used to treat ascariasis. When gentian purple is applied topically, if the wound has become purulent, it should not be applied again. Because gentian violet can make a wound on the surface of the wound, it looks dry and clean, but the bacteria are still multiplying under the dry disease. Under the protection of the crusts, the bacteria may continue to spread, and deeper invasion may worsen the disease. Therefore, do not use gentian purple for purulent wounds. Instead, ask a doctor to clean the wound and take comprehensive treatment measures. When using gentian purple, it should be noted that a large amount of precipitate will be precipitated when it is stored for too long, and the purple solution should not be reused when it becomes light.
Figure 2. Bacteria stained with crystal violet.
When dissolved in water, the dye is blue-violet with maximum absorption at 590 nm and an extinction coefficient of 87,000 M-1 cm-1. The color of the dye depends on the acidity of the solution. At a pH value of +1.0, the dye is green with maximum absorption at 420 nm and 620 nm, while in a strongly acidic solution (pH -1.0), the dye is yellow and has maximum absorption at 420 nm. Different colors are the result of different charged states of the dye molecules. In the yellow form, all three nitrogen atoms are positively charged, two of them are protonated, and green corresponds to the dye form with two positively charged nitrogen atoms. At neutral pH, two additional protons are lost to the solution, leaving only one positively charged nitrogen atom. The pKas of the two proton losses are about 1.15 and 1.8, respectively. In alkaline solutions, nucleophilic hydroxyl ions attack the electrophilic central carbon, producing a colorless triphenylmethanol or methanol-based dye. Under very acidic conditions, when the positive charge on the nitrogen atom causes the enhancement of the dye, some triphenylmethanol will also be generated. The electrophilic nature of the central carbon allows water molecules to undergo nucleophilic attacks. This effect slightly discolors yellow.
Figure 3. Crystal violet in aqueous solution.