Salicylic acid
Introduction
Salicylic acid is a type of phenolic acid and a beta hydroxy acid (BHA) derived from willow bark. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is primarily known for its use in the treatment of various skin disorders, including acne, psoriasis, calluses, corns, keratosis pilaris, and warts.
History
The medicinal properties of salicylic acid have been known since ancient times, and it was originally derived from the bark of the willow tree. The ancient Egyptians used willow bark as an anti-inflammatory agent, and the Greek physician Hippocrates wrote about its medicinal properties in the 5th century BC.
Chemical Properties
Salicylic acid is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. It has the formula C7H6O3. This colorless crystalline organic acid is soluble in alcohol, ether, and slightly soluble in water. It can serve as an organic base in reactions due to the phenolic OH. It is also known to be involved in the process of photosynthesis.
Biosynthesis
In biosynthesis, salicylic acid is produced from phenylalanine. This process is initiated by the enzyme phenylalanine ammonia-lyase (PAL), which creates a pool of trans-cinnamic acid in the cytosol. The acid is then used by a cytochrome P450 monooxygenase to produce ortho-hydroxycinnamic acid, which is subsequently converted into salicylic acid by a BA2H enzyme.
Uses
Salicylic acid is used in many skincare products for its exfoliating properties. It works by softening keratin, a protein that forms part of the skin structure. This helps to loosen dry, scaly skin making it easier to remove. When applied to the skin it can penetrate oily skin surfaces, making it a popular ingredient in anti-acne products.
It is also used in the production of other pharmaceuticals including aspirin. This is achieved through acetylation of the salicylic acid. It is also used in the manufacture of some types of dye and as a laboratory reagent.
Safety and Toxicology
Salicylic acid can be toxic if ingested in large quantities, but in small amounts, it is safe to use on the skin. The LD50 (lethal dose) is 891 mg/kg for rats, and lethal blood concentrations in humans range from 700 to 1000 mg/L. Prolonged exposure to salicylic acid can lead to salicylate poisoning, an condition that affects the acid-base metabolism.