Psilocybin is classified as an alkaloid. It is more specifically classified as a tryptamine, meaning that features an indole ring linked to an aminoethyl substituent. Like other tryptamines, it is structurally and biosynthetically related to the amino acid tryptophan and the neurotransmitter serotonin. Psilocybin is a member of the general class of tryptophan-based compounds that originally functioned as antioxidants in earlier life forms before assuming more complex functions in multicellular organisms, including humans. Other related indole-containing psychedelic compounds include dimethyltryptamine, found in many plant species and in trace amounts in some mammals, and bufotenine, found in the skin of psychoactive toads. In terms of physical properties, psilocybin is a white solid that is soluble in water, methanol and aqueous ethanol but insoluble in nonpolar organic solvents such as chloroform and petroleum ether. Its pKa values are estimated to be 1.3 and 6.5 for the two successive phosphate OH groups and 10.4 for the dimethylamine nitrogen, so in general it exists as a zwitterionic structure. Exposure to light is detrimental to the stability of aqueous solutions of psilocybin, and will cause it to rapidly oxidize—an important consideration when using it as an analytical standard. Osamu Shirota and colleagues reported a method for the large-scale synthesis of psilocybin without chromatographic purification in 2003. Starting with 4-hydroxyindole, they generated psilocybin from psilocin in 85% yield, a marked improvement over yields reported from previous syntheses. Purified psilocybin is a white, crystalline powder. There are two known crystalline polymorphs of psilocybin, as well as reported hydrated phases. The compound is reported to have a melting point between , and a slightly ammonia-like taste. In 2020, a second-generation synthesis of psilocybin has been developed. Psilocybin (O-phosphoryl-4-hydroxy-N,N-dimethyltryptamine, 4-PO-Psilocin, or 4-PO-HO-DMT) is a prodrug that is converted into the pharmacologically active compound psilocin in the body by a dephosphorylation reaction. This chemical reaction takes place under strongly acidic conditions, or under physiological conditions in the body, through the action of enzymes called alkaline phosphatases. Biosynthesis In terms of its biosynthesis, the transformation from tryptophan to psilocybin involves several reactions: decarboxylation, methylation at the N9 position, 4-hydroxylation, and O-phosphorylation. Isotopic labeling experiments from the 1960s suggested that the biosynthesis begins with decarboxylation of tryptophan and ends with O-phosphorylation. Recent analyses using purified enzymes demonstrate that O-phosphorylation is in fact the third step, at least in P. cubensis. The sequence of the intermediate steps has been shown to involve four