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Synthesis of Plastoquinoneanalogs and Inhibition of Photosynthetic And Proc. Nat. Acad. Sci. USA Vol. 69, No. 12, pp. 3713-3717, December 1972 Synthesis of Plastoquinone Analogs and Inhibition of Photosynthetic and Mammalian Enzyme Systems* (chloroplasts/beef heart/coenzyme Q/electron transport) JAN BQLERt, RONALD PARDINIt, HANNA T. M\IUSTAFAt, KARL FOLKERSt, RICHARD A. DILLEYT, AND FREDERICK L. CRANE: t Stanford Research Institute, Menlo Park, California and Institute for Biomedical Research, The University of Texas at Austin, Texas 78712; and $ Purdue University, Lafayette, Indiana 47907 Contributed by Karl Folkers, September 25, 1972 ABSTRACT New 5-hydroxy- and 5-chloro-6-alkyl-1,4- energy (ATP) and the utilization of the hydrogen of water as benzoquinones with one or two methyl groups on the a source of reducing power (NADPH2). Data from nucleus were synthesized as potential antimetabolites of several plastoquinones for biological research on photosynthetic experimental approaches indicate that plastoquinone does and mammalian enzyme systems; the primary emphasis function in the electron transport system of chloroplasts. was on photosynthesis. The data stem from extraction-restoration studies, spectro- 2,3-Dimethyl-5 - hydroxy - 6 - phytyl - 1,4 - benzoquinone photometric and chemical determination of redox changes, completely inhibited in chloroplasts the water-de- pendent electron transport, but photosystem I was in- correlation with metabolic function, and changes in electron sensitive to this analog. The data are consistent with the transport in mutants, as well as its universal distribution in all interpretation that this analog inhibits electron transport oxygen-producing photosynthetic organisms (3). in the chain prior to the site of electron donation from the To study relationships between structure and activity in ascorbate-dichlorophenolindophenol couple. Concentra- respiration, Redfearn and Whittaker (4) evaluated the tions of 70 MM and 120 ,M of this analog caused about 50 and 100% inhibition, respectively, of cyclic photophos- inhibition of succinoxidase from a heart muscle preparation by phorylation. both coenzyme Q and l)lastoquinone analogs. They found that 2,3-Dimethyl-5-hydroxy-6-phytyl-1 ,4-benzoquinone is a inhibitory quinones have at least one unsubstituted position new type of inhibitor of photosynthetic electron transport in the nucleus, that the nature of the substituent ortho to the that specifically inhibits the rate-limiting step between photosystems I and II. Structurally related analogs caused free position was important, and that the inhibitory effect inhibitions in the range of 50-100% in chloroplasts. disappeared in the presence of sulfhydryl compounds. Qui- These analogs showed marginal inhibition in mito- nones with one unsubstituted position in the nucleus are known chondrial coenzyme Qjo-oxidase systems from beef heart. to react by a 1,4-addition with molecules containing a thiol group. This reaction might be involved in the mechanism of Plastoquinone-9 (Plastoquinone-9,I), a compound found inhibition by the compounds studied by these investigators. 0 Catlin et al. (5) showed in studies of new inhibitors of co- enzyme Q enzyme systems that some of the analogs CH-3 H 5-hydroxy of coenzyme Q strongly inhibit succinoxidase and DPNH- Il Il CH:l oxidase in intact mitochondria or in mitochondria extracted CHEW (CH2CH= C-CH.,)9H to remove coenzyme Qio. Specifically, 2,3-dimethoxy-5- hydroxy-6-phytyl-1,4-benzoquinone completely inhibited suc- 0 I cinoxidase activity, and reduced by 80% DPNH-oxidase activity in the absence of supplementary coenzyme Qjo in CH30 CH3 intact enzyme preparations. These data of Catlin et al. (5) predict that similar analogs Il Il CH3 of plastoquinone might inhibit photosynthetic enzyme CH3O (CH29CH C-CH,)nH systems; consequently, synthesis and testing of such analogs of plastoquinone were undertaken. II The effort was focused on two main groups of potential predominately in the chloroplasts of higher plants, is a plastoquinone antagonists, namely, hydroxy (IIIa-e) and naturally occurring benzoquinone with an isoprenoid side chloro (IVa-c) analogs. The starting quinones without the chain. Its structure was determined (1), and it was synthesized hydroxy or chloro functions were known compounds. They (2). It was proposed that it has an important role in the elec- were converted to the corresponding new hydroxy- and tron transport system of photosynthesis that is analogous to chloroquinones by use of the Thiele-Winter acetylation that of coenzyme Q (II) in mitochondria (3) by acting as an procedure (6). In most cases, boron trifluoride etherate was oxidation-reduction carrier in the electron transport system used inl place of the more conventional sulfuric acid as the associated with the transformation of light into chemical acid catalyst; good yields of the triacetates were obtained. These acetates were hydrolyzed in refluxing acidic methanol * This is paper no. CLVI in the series, "Coenzyme Q." under nitrogen to the corresponding hydroxybeuizohydro- 3713 Downloaded by guest on October 2, 2021 3714 Chemistry: Boler et al. Proc. Nat. Acad. Sci. USA 69 (1972) TABLE 1. Hydroxy- and chloroalkylquinones none. The decomposition produced free radicals, which reacted with the quinone (9). Generally, the hydroxyquinones were formed in low yields and were extremely unstable and difficult to purify. They partly decomposed on thin-layer plates, but the chloro com- pounds were formed in fairly good yields and were relatively stable. The structures of the products were assigned by their R2 R3 R5 R6 nuclear magnetic resonance (NMR) spectra (Table 2). For R6 = phytyl, the sidechain protons that were distinguished III a CH3 CH3 OH Phytyl were of the ring methylene (-6.9r) group, and the vinylic b OH CH3 CH3 Phytyl (-5.OT) and the protons of the methyl group at the double c OH Phytyl CH3 CH3 bond (-8.3T). For R6 = 8'-cyclohexyloctyl, the ring methyl- d H CH3 OH Phytyl enes were found at -7.66Tr remaining e CH3 CH3 OH 8'-Cyclohexyloctyl and the sidechain protons had two broad absorptions between 8.3r and 8.8T. IV a CH3 CH3 Cl Phytyl b Cl CH3 CH3 Phytyl EXPERIMENTAL c CH3 Cl CH3 Phytyl 2,3-Dimethyl-1 ,4,5-triacetoxybenzene. To a solution of o-xylo- quinone (5 g, 37 mmol) in acetic anhydride (20 ml), was added redistilled boron trifluoride- etherate (1 ml). The quinones, which were characterized by oxidation with silver mixture was stirred at room temperature for 24 hr and poured oxide to the corresponding hydroxyquinones. The chloro- into ice water; the crude triacetate was collected by filtra- quinones were prepared by a 1,4-addition of hydrogen chloride tion. Recrystallization from ether-hexane gave 7.2 g (70%) to the starting quinone to yield the chlorohydroquinone, of pure product, mp 87°-88° (mp 87°-88°, ref. 10). which was oxidized by dichromate in sulfuric acid. The following triacetates were prepared in a similar man- The alkyl quinones (III and IV, Table 1) were prepared ner in good yields: 2,5-dimethyl-1,3,4-triacetoxybenzene, from the hydroquinones or quinones by two alkylation mp 1070 (mp 106°-107°, ref. 10; 1080, ref. 11); 2,6-dimethyl- procedures. The phytyl derivatives were prepared by the 1,3,4-triacetoxybenzene, mp 102°-103° (mp 103°-104°, ref. conditions of acid-catalyzed alkylation of Daves et al. (7) of 12), sulfuric acid was used as the acid catalyst; and 2-methyl- the appropriate hydroquinone with an allylic alcohol (phytol). 1,4,5-triacetoxybenzene, mp 112°-114° (mp 114°-115°, ref. 6). The resulting phytyl hydroxyhydroquinones become air- oxidized during purification by thin-layer chromatography. 2,3-Dimethyl-1,4,5-trihydroxybenzene. To a solution of 2,3- The phytyl chlorohydroxyquinones are relatively stable, and dimethyl-1,4,5-triacetoxybenzene (5 g, 18 mmol) in methanol were treated with an oxidizing agent to obtain the quinones (30 ml), was added hydrochloric acid (5 ml). The mixture was before final purification by thin-layer chromatography. refluxed in an atmosphere of nitrogen. The hydrolysis was The 8'-cyclohexyloctylbenzoquinone (IIIe) was synthesized followed by thin-layer chromatography and was stopped by thermal decomposition of the appropriate diacyl peroxide when no more triacetate was detected in the reaction mixture (8) in the presence of 2,3-dimethyl-5-hydroxy-1,4-benzoqui- (after about 1 hr). The cold solution was then evaporated TABLE 2. Spectral data for analogs of plastoquinone NMRt Compound xEtOH CH3 (1,4-benzoquinone) No. nm Ring-CH3 Ring- H Ring-CH2 Viny 1 R Alkyl 2,3-Dimethyl-5-hydroxy-6- IIa 276 8.05 (s) 6.96 (d) 4.98 (t) 8.32 (s) 8.5-9.3 (m) phytyl- 2,.5-Dimethyl-3-hydroxy-6- IIIb 276 8.05 (s) - 6.86 (d) - 8.32 (s) 8.6-9.3 (m) phytyl- 271 (sh) 8.15 (s) 2,6-Dimethyl-3-hydroxy-5- IIc 276 7.98 (s) 6.85 (d) 5.09 (t) 8.30 (s) 8.4-9.2 (m) phytyl- 8.12 (s) 2-Methyl-5-hydroxy-6-phytyl- IIId - 7.98 (d) 3.57 (q) 6.97 (d) 5.03 (t) 8.34 (s) 8.6-9.3 (m) 2,3-Dimethyl-5-hydroxy-6-(8'- IIMe 276 8.02 (s) - 7.66 (t) 8.2-9.0 (m) cyclohexyloctyl)- 269 (sh) 8.07 (s) 2,3-Dimethyl-5-chloro-6-phytyl- IVa 272 7.95 (s) - 6.73 (d) 5.02 (t) 8.29 (s) 8.6-9.3 (i) 265 (sh) 8.00 (s) 2,5-Dimethyl-3-chloro-6-phytyl- IVb 272 7.85 (s) 6.85 (d) 5.16 (t) 8.29 (s) 8.5-9.3 (m) 263 (sh) 7.94 (s) 2,6-Dimethyl-3-chloro-5-phytyl- IVc 272 7.85 (s) - 6.81 (d) 5.12 (t) 8.27 (s) 8.5-9.2 (i) 263 (sh) 7.97 (s) * sh = shoulder. t Spectra were obtained on carbon tetrachloride solutions with a Varian Associates HR-100 spectrometer.
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