Quinones are a group of compounds that are widely distributed in nature. They are found in a wide variety of plants, but also in fungi, bacteria and animals. Quinones share a basic structural pattern: an o- or para-substituted diketone conjugated to an aromatic nucleus (benzoquinone) or a thickened polycyclic aromatic system, such as benzoquinone, naphthoquinone, phenanthrenequinone and anthraquinone. Quinones have potential antioxidant, anti-inflammatory, antibacterial, antimicrobial and anticancer activities. Anthraquinone derivatives are an important subclass of quinones with a wide range of pharmacological activities, including laxative, anticancer, anti-inflammatory, anti-arthritic, antifungal, antibacterial, antiviral, anti-platelet aggregation and neuroprotective activities.
When there is no phenolic hydroxyl group in the molecule, quinones are almost colorless, but with the introduction of phenolic hydroxyl group and other auxiliary chromophores, they have a certain color, and the more auxiliary chromophores introduced, the darker the color is, such as yellow, orange, brownish red and purple-red. Anthracene derivatives are mostly yellow to orange-red. Natural quinones are mostly colored crystals, benzoquinone and naphthoquinone exist mostly in free state, while anthraquinones are often combined into glycosides and exist in traditional Chinese medicine, most of them are difficult to get good crystallization. The free anthraquinone and quinone derivatives are mostly sublimated. Small molecules of benzoquinone and naphthoquinone are volatile and can be distilled with water vapor. Free anthracene and quinone derivatives are soluble in methanol, ethanol, ethyl acetate, ether, benzene, chloroform and other organic solvents, slightly soluble or insoluble in water. After binding into glycosides, their polarity increases, and they are easily soluble in methanol and ethanol, and also soluble in hot water, but less soluble in cold water, and almost insoluble in benzene, ether, chloroform and other non-polar organic solvents.
Quinones are a class of compounds that include quinones or are easily transformed into compounds with quinone-like properties, as well as compounds that are closely related to quinones in biosynthesis, and often exist in nature as pigments in animals, plants and microorganisms. Many important traditional Chinese medicines such as rhubarb, cassia, senna, comfrey, thuja, he shou wu, aloe vera, etc. have quinones within their main active ingredients. According to different chemical structures, quinones can be divided into benzoquinones, naphthoquinones, anthraquinones and phenanthrenequinones. Some of these quinones play the role of electron transfer in the redox reaction in living organisms, while some have antibacterial or antitumor activities, such as carob quinone, lanxuequinone, lapachone, etc. Some are also the main active ingredients in Chinese herbal medicine, such as comfrey with hemostatic, anti-inflammatory, antiviral and antitumor activities of comfrey and iso-comfreyin, which are the main active ingredients of comfrey.
Quinone compounds have phenolic hydroxyl group, so they have certain acidity. In alkaline aqueous solution into a salt dissolved, acid acidification is free and can be precipitated again.
The acidity of quinone compounds varies significantly depending on the number and position of phenolic hydroxyl groups in the molecule. Generally, the quinones containing carboxyl groups are more acidic. The acidity of quinones without carboxyl groups increases with the number of phenolic hydroxyl groups, and when the number of phenolic hydroxyl groups is the same, the substitution position of phenolic hydroxyl groups has a greater influence on the acidity. The acidity of β-hydroxy quinones is stronger than that of α-hydroxy quinones because of the effect of electron absorption by carbonyl group, the electron cloud density of oxygen atom on β-hydroxy group is reduced and the dissociation degree of proton is increased. α-position of hydroxyl group is less acidic because it forms intramolecular hydrogen bond with adjacent carbonyl group, which reduces the dissociation degree of proton.