Abstract
6alpha,7beta-Dihydroxyvouacapan-17beta-oic acid (1) was isolated from Pterodon polygalaeflorus Benth. Modification of 1 yielded 6alpha-hydroxyvouacapan-7beta,17beta-lactone (2) and then 6-oxovouacapan-7beta,17beta-lactone (3). Photosynthesis inhibition by 3 was evaluated in spinach chloroplasts. The uncoupled non-cyclic electron transport rate and ATP synthesis were inhibited by 3, which behaved as a Hill reaction inhibitor. Furthermore, 3 acted as an uncoupler because it enhanced the basal and phosphorylating electron transport rate on thylakoids. This last property of 3 was corroborated when it was observed that it enhances the Mg2+-ATPase activity. In contrast, 3 did not affect photosystem I (PSI) activity. Analysis of the partial photosystem II (PSII) reactions from water to DCPIPOX and water to silicomolybdate allowed to locate the inhibition sites at the redox components of PSII. The OJIP test of the chlorophyll a fluorescence transient confirmed that the inhibition sites were 1.) the oxygen-evolving complex (OEC) and 2.) by the formation of silent centers in the non-QA reducing centers.
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