Naphthodianthrones

The most researched class of compounds isolated from H. perforatum is the naphthodianthrones, which occur in concentrations ranging from 0.1% up to 0.3% (9,25,26). The most common naphthodianthrones include hypericin, pseudohypericin, isohypericin, and protohypericin (10,21). Of those, hyper-icin is the best known and, to date, the most studied. Hypericin is a reddish pigment that is responsible for the red color of St. John's wort oils. Hypericin is found in greatest abundance in the flowers, particularly in the black dots that are located on the petals of St. John's wort flowers (3).

OH O OH

OH O OH

Figure 2 Structures of chemically active constituents of St. John's wort, Hypericum perforatum, (a) Hypericin — a naphtodianthrone found primarily in the black dots on the flower petals. (b) Hyperforin — an acylated phloroglucinol typically derived from buds and flowers.

(a) Hypericin (b) Hyperforin

Figure 2 Structures of chemically active constituents of St. John's wort, Hypericum perforatum, (a) Hypericin — a naphtodianthrone found primarily in the black dots on the flower petals. (b) Hyperforin — an acylated phloroglucinol typically derived from buds and flowers.

Hypericin is highly photoreactive, owing to its chemical structure. Biochemically, hypericin is a polycyclic quinone, having four hydroxyl groups that lie adjacent to two carbonyl groups (Fig. 2a). Owing to resonance of the molecule and the relatively short distance between oxygens (-2.5 angstroms), the hydroxyl hydrogen is able to transfer back and forth from the hydroxyl oxygen to the carbonyl oxygen when in the presence of fluorescent light (27). Therefore, the hydrogen is in constant flux between both oxygens when under fluorescent light (28). Studies examining the fluorescence spectrum of hypericin and its analogs have demonstrated the existence of a "protonated'' carbonyl group, therefore proving the H-atom transition (27). This hydrogen transfer causes acidification of the surrounding environment (29,30).

B. Flavonoids

Flavonoids found in H. perforatum range from 7% in stems to 12% in flowers (21) and leaves (9). Flavonoids include flavonols (kaempferol, quercetin), flavones (luteolin), glycosides (hyperside, isoquercitrin, rutin), biflavonoids (biapigenin), amentoflavone, myricetin, hyperin, proanthocyanidins, and miquelianin (10,20). Rutin concentration is reported at 1.6% (10).

C. Lipophilic Compounds

Extracts of St. John's wort contain several classes of lipophilic compounds, including phloroglucinol derivatives and oils, that have potential or demonstrated therapeutic value. Acylated phloroglucinols are typically derived from buds and flowers of H. perforatum (16). Hyperforin, found in concentrations of 2-4.5% (9,31), consists of phloroglucinol expanded into a bicyclo (2,1) nonaendionol, substituted with several lipophilic isoprene chains. Other phloroglucinols include adhyperforin (0.2-1.9%), furohyperforin, and other hyperforin analogs (9,10,21,32).

Essential oils are found in concentrations ranging from 0.05% to 0.9% (9). They consist mainly of mono and sesquiterpines, mainly 2-methyl-octane, n-nonane, a- and h-pinene, a-terpineol, geranil, and trace amounts of myrecene, limonene, caryophyllene, and others (20,32).

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