Abstract
α,7β-Dihydroxyvouacapan-17β-oic acid ( 1) was isolated from Pterodon polygalaeflorus Benth. Then, ( 1) was modified to obtain 6α-hydroxyvouacapan-7β,17β-lactone ( 2). Inhibition properties of both ( 1) and ( 2) were evaluated in spinach chloroplasts. Only compound ( 2) inhibited non-cyclic electron transport and ATP synthesis, i.e., ( 2) behaved as a Hill reaction inhibitor. Compound ( 2) did not affect photosystem I (PSI) activity but it did inhibit electron flow through PSII. Analysis of the partial PSII reactions from water to DCPIP ox, water to silicomolybdate, and diphenylcarbazide to DCPIP ox allowed locating inhibition sites at (i) the oxygen-evolving complex (OEC) and (ii) the redox enzymes of the electron transport chain, in the span of P 680 to Q A. Chlorophyll a fluorescence measurements confirmed the action site for ( 2).
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