Further determination of spectral and chemical properties indicated that it was a tri or tetra substituted benzoquinone with a long isoprenoid side chain. A negative Cravens2 test for an unsubstituted site on the quinone ring indicated a tetra substituted benzoquinone. Later examination indicated a positive test for a tri-substituted quinone (Kofler et al. 1959; Isler et al. 1961).The structure

of the prenyl side chain remained unclear. At first, it was thought to be 10 prenyl or 50 carbon-long because solensol, which was used as the long chain compound in the synthesis, was thought to have 50 carbons. However, after the synthesis of coenzyme Q9, instead of coenzyme Q10, and when solanosol was used 4EGI-1 as a side chain, it was discovered that PQ with a solanosol side chain was identical with a natural PQ and, therefore, had a 45 carbon chain made up of nine isoprene units (Folkers et

al. 1961; also see Trenner et al. 1959). This was in agreement with Isler et al. (1961). These studies defined Kofler’s SRT2104 ic50 quinone or Q254 (see below) or PQ as 2,3 di-methyl 5 solanosyl benzoquinone (Fig. 1). In his original work on PQ, Kofler (1946) had measured the PQ content in leaves of six plants for which Dam–Karrer reaction had established Vitamin K content by his biological assay of blood clotting time in chickens. Kofler (1946) found that PQ in leaves ranged from 150 mg/kg dry weight for alfalfa (or oats) to 400 mg/kg for fir needles and a maximum of 1,000 mg/kg in horse chestnut. In comparison with the content of Vitamin K, established by the biological assay, the PQ was 3–5 times greater in amount. When PQ was tested in chickens for Vitamin Methane monooxygenase K activity, 1 mg Vitamin K was more effective than 500 mg PQ. (The clotting time for Vitamin K was 2.3 min vs. 30 min for PQ.) With the lack of Vitamin K activity, work on PQ was stopped until the discovery of coenzyme Q. Fig. 1 Structure of plastoquinone A (top), Vitamin K (Vitamin K1) (middle), and α tocopherylquinone (bottom). Vitamin K

functions in photosystem I, tocopherylquinone is in chloroplasts but has no known function The re-discovery of the plastoquinone The rediscovery of PQ came about as a direct result of the discovery of coenzyme Q (Crane et al. 1957; Morton 1959) and the study of coenzyme Q distribution in diverse species. We had relatively simple procedures for the analysis of coenzyme Q. We started either with its direct extraction with a solvent mixture or by saponification in the presence of pyrogallol followed by extraction with hydrocarbon solvents and chromatography on sodium aluminum AZD2171 molecular weight silicate (Decalso). The solvent was evaporated and the yellow oil taken up in ethanol to run the absorption spectrum followed by the addition of borohydride to reduce the quinone to the hydroquinone.